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It has been estimated that there are somewhere between 13,000 and 30,000 different lichens in the World, but no one really knows. Up to 100 new lichens are found in the UK each year, some new to science.

Lichens and Symbioses between algae & fungi
Lichens are a symbiosis between two different organisms belonging to differing kingdoms, and as such, they very often have very unusual, but probably not unique, chemistry. However, the majority of chemical structures found within Lichens are confined to lichens and are not found elsewhere in the natural world. Lichens are adapted fungi that have, over the aeons of evolutionary past, acquired algae in order to photosynthesize. The algal partner(s) are either green algae or, less frequently, cyanobacteria (previously known as blue-green algae). The cyanobacteria are mostly from the Nostoc family whereas the green algae are commonly from the Trebouxia genus and less often from the Trentopohlia genus. Both green algae and cyanobacteria are photosynthetic. Forty percent of the lichenised fungal symbionts usually come from the Ascomycota fungal order, the other orders being Graphidales, Gyalectales, Peltigerales, Pertusariales or Teloschistales.

The algal part usually consists of just one algae, but sometimes there are two or even three different algae within the lichen. All the algal partners are probably capable of living an independent existence without the fungi, but this is not so for the fungal partner. Only in very few lichens are two fungal partners involved.

But, that was all before the year 2016...

Lately (in 2016) it was discovered, to the amazement of lichenologists, that the 'skin' of lichens (technically called the cortex) across six Continents also contain Basidiomycete yeasts, which are single-celled fungi. These basidomycetes fungi occupy the thin outer layer of the crust of the lichen and are really hard to spot even through a microscope, being embedded within a matrix of sugar molecules. These basidomycetes fungi also produce chemicals which help defend the lichen from microbes and other predators and are the main third symbiotic partners, in addition to those symbionts mentioned in the preceding paragraph. Being in the cortex they are better able to defend the lichen from external attack. This newly-discovered extra yeast also explains why many genetically similar lichens exhibit numerous differing physical features and chemistry. It also accounts for why scientists have been unable to cultivate lichens in the laboratory when combining species which partner successfully in nature. Specifically, the scientist who discovered the third symbionts were puzzled as to why the lichen Bryoria tortuosa (Tortured Horsehair Lichen) is yellow and produces Vulpinic Acid whilst that of the genetically identical lichen Bryoria fremontii (Black Tree Lichen aka Tree-hair Lichen) which contains the same fungus and algal symbiont partners is dark brown and does not produce this secondary metabolite. It is because of this previously undiscovered yeast - a second fungal symbiont which is present in just one of these (the first symbiont enables the second to produce the toxic Vulpinic Acid - to help it defend itself against other potential invading microbes).

Since that unexpected discovery, traces of similar yeast symbionts have been found in another 52 genera of Lichens which indicates a prolonged and shared evolutionary history between these three symbiont partners.

The above suggests that the relationship between the two differing organisms is symbiotic, but it appears to be much more one-sided than that; the lichen needs the fungus to grow with more vigour, but the fungus doesn't need the lichen, it grows faster without it. However, when under stress, the lichen survives when either on their own may die. Rather than 'symbiotic' the term  'helotism' may more accurately describe the master/slave relationship of lichens/fungi. However, to think that lichens grow faster than their algal partners is totally wrong; algal blooms in the sea grow at a phenomenal rate, but lichens grow so slowly that those on gravestones a few inches across may have taken decades to grow that large.

Lichens' tolerance to extreme environmental conditions
Most lichens are also able to withstand and recover from extreme desiccation. It seems that lichens, as long as they are dry, can survive in temperatures as low as that of liquid nitrogen (-196°) and when warmed up again and re-watered they recover almost completely from the ordeal. This severe treatment can be repeated time and again with few consequences for the lichen. The symbiotic partners themselves are sensitive to differing stresses and when one fares badly the other can help the combined organism survive.

This begs the question why it has this extreme tolerance to cold. Could it perhaps have come from outer space as in Fred Hoyles Panspermia theory. But then, lichens would have to be tested for their tolerance to the high radiation levels present in outer space. It is known that the algal symbiont is the more sensitive of the symbionts to high-dose ionizing radiation, but they fare very well considering. The lichen with the highest known resistance to the conditions in outer space is Circinia gyrosa which shows no significant changes to increasing doses of X-rays nor to the ions of helium or iron. They do suffer some damage, but none that the lichens would be incapable of repair themselves when under their usual conditions (but then, what is 'usual' for lichens...).

Maybe lichens can travel beneath the surface of rocky interstellar objects which would offer them some protection from the other hazards of outer space. This is the litho-pansperia theory whereby some lifeforms could be transported across outer space to other worlds on some of the many trillions of meteorites under the protection of the rocky surface of such objects.

Growth & Growth Rate
The growth rate of lichens is restricted by availability of water. They do not possess a waxy outer cuticle like plants to conserve water, so readily dry out in hot sunny weather. They cannot grow when dry, just survive, perhaps for several months. Dry lichens are metabolically inactive and impervious to pollution; lichens transplanted from the countryside to a city when dry may die as soon as it rains. Lichens obtain their water from moisture in the air, such as from a heavy dew or when it rains, when they are able to start photosynthesizing within minutes providing the light is sufficient. Lichens containing green algae are able to absorb between 2.5 to 4 times their dry weight in water, whereas those containing cyanobacteria can absorb up to 16 or 20 times their own dry weight. However, conversely, lichens cannot photosynthesize if they are too wet (as well as when too dry), unlike mosses they will not grow in very wet environments.

Habitat & Chemistry
As symbionts of two or three organisms, individual fungi and algae are quite different from the free-growing algae or fungi from which they once came, in form, physiology and biochemistry. Lichens are able to grow on very different substrates ranging from bark, rock, soil, leaves, grass, man-made artefacts, animals, sand, hot deserts and toxic wastes, but particular species favour a specific habitat. The main criteria being the acidity of the substrate. They produce some unique chemical compounds unknown elsewhere in the natural world, amongst them Depsides, Depsidones and Depsones. Other products belong to chemical classes also produced by plants, such as xanthones, anthraquinones and carotenoids, but even in these categories there are some compounds unique to lichens. Some lichens are easily poisoned by pollution, whilst others are extremely tolerant of what would normally be noxious substrates. In some lichens it is the fungal partner which dominates the biochemistry and in others the algal partner.

Colours available from lichen dyes
  Many Lichens contain highly coloured pigments and have been used for dying fabric throughout the ages. Mauve, purple, beetroot red, cyan, fawn, lilac, yellow, cream, and various shades of brown are readily achievable, given the right mordant. However, lichens are getting scarcer and many are now protected from being harvested for dyestuffs even assuming that it is still economical to do so.

Secondary metabolite properties
Like flowers, lichens produce some very colourful compounds, but many differ from the pigments and dyes that plants produce, although some do belong to the same categories such as the carotenoids and anthraquinones. Altogether over 700 secondary metabolites of lichens are known, with only 60 to 80 also occurring in other organisms. Therefore only a very few can be shown below. Those known are mainly complex organic acids, usually phenolic, and are present outside the cells in lichens in large quantities, usually between 1% and 5% by dry weight, but it can be up to 36%.

Lichens contain a huge variety of unusual acids called 'Lichen Acids' ranging from Lecanoric Acid, Gyrophoric Acid, Thamnolic Acid, Usnic Acid, Salazinic Acid, Stictic Acid, Picrolichnic Acid, Baeomcesic Acid, Fumarprotocetraric Acid, Hypoprotocetraric Acid, Protocetraric Acid, Pulvinic Acid, Vulpinic Acid, Perlatonic Acid, Chlorophaeic Acid, Mevalonic Acid, Shikimic Acid, Alectorialic Acid, as well as other compounds such as Atranorin, Parietin and Anthraquinones, Naphthoquinones, Xanthones and Chromones, most of which are highly coloured. Steroidal Triterpenes, Orcinols, dihydroxydibenzofurans, and m-dihydroxyphenols and Depsones also abound. The Spot Tests are designed to test for the presence or absence of these characteristic chemicals, usually by a colour change (a positive result), and by which means identification can be ascertained.

It is thought that some lichen substances, having antibiotic properties, may inhibit the growth of other nearby and more vigorously growing plants. Compounds in lichens having a bitter taste probably deter animals, slugs and snails from eating the lichen. These substances are usually poisonous too.

Vulnerability and Hardiness
Some lichens are able to photosynthesize at temperatures as low as -20C but then are unable to survive at 25C for longer than a few months.

Lichens are highly tolerant to nuclear radiation and can survive a flux of 1000 rads every day for two years whereas a single dose of 400 rads will kill a human. However, although some lichens are tolerant of sulfur dioxide pollution, others, notably Usnea species, are very sensitive of SO2 air quality and make good long-term monitors of air quality. The Hypnogymnia physodes lichen is, however, the most tolerant of airborne sulfur dioxide, incorporating it into their structure over time which makes it a valuable integrating SO2 monitor.

Growth Forms
There are eight physical types of lichens, depending upon their growth forms:

  • Leprose Lichens (Dust Lichens)
    Lacking both an upper & lower cortex the medulla is attached directly to the substrate. Upper has loose overall covering of fine powder.
  • Crustose Lichens (Crust Lichens)
    Like Dust Lichens, but possess a hard protective upper cortex, often appearing stain-like.
  • Squamose Lichens (Scale Lichens)
    Lack lower cortex like dust & crust lichens, but thallus often raised, like small flakes, above the substrate and usually over-lapping. Some scale lichens possess an erect hollow fruiting structure like a toothpick, matchstick or golf-tee, and less often, a branching shrub (see club and shrub lichens below).
  • Foliose Lichens (Leaf Lichens)
    Thalli are flat and paper like, often rising above the substrate in narrow or broad paper-like lobes, either short or long. Leaf-like.
  • Fruticose Lichens are split into 3 sub-groups:
    • (Club Lichens)
      Radially symmetric with no lower cortex nor rhizines. Most have a thickened, upright, un-branched (or just sparsely branched) stems. If hollow the stems are called 'podieta' and usually also have basal scales (tiny flakes).
    • (Shrub Lichens)
      Like Club lichens with ±radial symmetry and thickened stems, Shrub lichens are usually upright and have highly branched and usually solid stems.
    • (Hair Lichens)
      Unlike Shrub lichens these have much finer and longer branches, often pendulant.
  • Basidiomycete Lichens
    Differ from Club lichens in that they look like either toadstool mushrooms or club-like truncheons.

Contact allergens
Lichens are also used as model shrubs, bushes and vegetation in model railway layouts. Some lichens have also been harvested for use as cosmetics or medicines.

Those lichens containing Usnic Acid, Evernic Acid, Stictic Acid, Atranorin or FumarProtoCetraric Acid are capable of photo-sensitizing human skin as well as being contact allergens. Foresters have long been aware of this problem with lichens. Atranorin and β-Orcinol absorb UV light entering an excited molecular state and may be especially capable of causing photo-sensitization.

Lichens as food
Some moth species thrive on eating some lichens, indeed, many of these moths are named accordingly, such as Pale Lichen Moth (Crambidia pallida) or Powdered Lichen Moth (Bruceia pulverina). Butterflies, on the other hand, seem to avoid eating lichens, probably being distasteful or poisonous to them, although several lichen substances, such as Usnic Acid, Atranorin and Zeorin have been detected in some (few) butterflies just after they emerged from the pupae stage.

 Lichens are indeed unusual organisms.

[Note: all colours displayed on this page are approximate only, and will vary with both your monitor and your computer operating system anyway].


Oxalic Acid is the simplest di-carboxylic acid and is a fairly potent acid, stronger than other lichen acids. It is soluble in water and will attack alkaline rocks such as limestone (calcium carbonate) to produce the barely soluble calcium oxalate.

Oxalic Acid is quite widespread in certain lichens where it is thought it helps to dissolve essential nutrients from the substrate the lichen is living on. To a lesser extent, other lichen acids may also help to release nutrients from the substrate. Lichens also obtain nutrients from rain water and others from bird droppings.

 The calcium salt of Oxalic Acid forms on the lichen Physicia aipola growing an calcium bearing rocks which forms a thin layer of bluish-grey calcium oxalate crystals on the surface of the fruiting body.

 The same substance, Calcium Oxalate, appears a brilliant white on the surface of the very common crustose lichen Aspicilia mashiginensis which is especially frequent on limestone walls and disused limestone quarries. The brilliant whiteness of this coating is thought to be due to high reflection of sunlight falling onto this lichen affording it good protection from extremely sunny environments.

Since crystals of the very same substance are bluish-grey in one lichen whilst brilliant white on another it is probably the size and shape of the crystals which vary and determine the amount of light reflected. But the amount of hydration of the crystal may also have a bearing: The mineral Weddelite (Calcium Oxalate DiHydrate, (COOCa)2•2H2O, which has two molecules of water of crystallization) is one form of calcium oxalate which forms in certain lichens. It crystallizes in the tetragonal system and is typically an 8-faceted bi-pyramid in shape, but other shapes are possible. The mineral Whewhellite (COOCa)2•H2O) is another form of calcium oxalate crystals which have just one molecule of water of crystallization and crystallizes in the monoclinic system. Both forms are found in lichens. The shape and refractive index could well have a bearing on whether the surface of a lichen, having many thousands of these minute crystals, appears grey or dazzlingly white.

Other salts of Oxalic Acid in lichens are possible, depending upon the minerals in the basic rock.

Lichens play an important role in the weathering of rocks for which lichen acids such as Oxalic Acid and others are probably partly responsible. The physical expansion of rock by the hyphae filaments from lichens that permeate the semi-permeable rock, especially along grain boundaries or cracks, may also help to break the rock apart. In sandstones the hyphae can penetrate up to 16mm into the rock. Rocks weathered by lichens typically have brown stains due to the release of iron, as rust, which can also stain the lichen brown in places.

It should be remembered that rocks are not the only substrate for lichens - tree bark, twigs, branches, sheltered mossy trunks, sand and cacti - all provide certain habitats for other kinds of lichen.


(→Air→) Orcinol, or 5-MethylResorcinol, is similar to the ubiquitous Phloroglucinol which has the Methyl CH3 group replaced by yet another -OH group and to Resorcinol where the methyl group (top dead centre) is absent altogether leaving just two -OH groups. Orcinol is a volatile fragrant compound which occurs in many species of lichens especially Lecanora species. Orcinol crystallizes into colourless prisms with one molecule of water of crystallization but reddens when exposed to air.

Phloroglucinol exists in a resonance hybrid equilibrium of two tautomers, one 1,3,5-Cyclohexanetrione which has ketone-like (=O) characteristics and the other 1,3,5-Trihydroxybenzene which exhibits phenol-like (-OH) characteristics. Hence the term keto-enol tautomerism given to this form of resonance. In this instance the structure has Benzoid-Quinoid tautomerism. Here the hydrogen atoms of the -OH are shared between the oxygen atoms and the carbon ring. This is probably a direct result of the alternate structure, and possibly would not occur at all (or not be as strong) for 1,2,5- or 1,2,3- arrangements of the -OH groups. As a result phloroglucinol is polyfunctional and assumes characteristics of both forms.

Orcinol (and Orcinol derivatives as shown below) sub-units are present in many lichen compounds, including the Orceins, Depsidones, Depsones and Depsides.

Oak Moss lichen (Evernia prunastri)
Perfumes can be extracted from some lichens, most frequently from 'Oak Moss' (Evernia prunastri) but others have also been used such as Lobaria pulmonaria, Evernia furfuracea and Pseudevernia furfuracea. The fragrances, all light volatile components comprising single units of Orcinols, are shown below. Not all volatiles have an odour detectable by homosapiens, but these do.

Orcinol-MonoMethylEther (3-Methoxy-5-MethylPhenol)
MethylOrcinol-MonoMethylEther (3-Methoxy-2,5-DiMethylPhenol)
Orsellinic Acid (2,4-DiHydroxy-3,6-MethylBenzoate)
MethylOrsellinic Acid (2,4-DiHydroxy-6-MethylBenzoate) has an extremely penetrating odour which may be due to Benzoid-Quinoid tautomerism (a form of  keto-enol tautomerism) where the position of atoms and/or double-bonds/single-bonds rapidly flips between two possible positions (see text for PhloroGlucinol above).

All the above small Orcinol derivatives are present in the aroma emanating from the essential oils derived from the lichen Oak Moss (Evernia prunastri). Oak Moss is a fruticose lichen which grows on Oak trees in un-polluted atmospheres and has a woody, sharp and slightly sweet fragrance but it is also a known and potent dermal sensitizer which can cause allergic dermatitis. Vacuum distillation of the extract yields a pale yellowish-green highly toxic essential oil containing all the above Orcinol derivatives plus the toxic and odorous α-Thujone and β-Thujone. The essential oil is a restricted material - the total amounts added to consumer products such as perfumes should not exceed 0.1% - probably due mostly to the poisonous Thujone content. The essential oil also contains heavier, non-volatile (therefore odourless) compounds such as Usnic Acid.

Tree Moss lichen (Evernia furfuracea)
Tree Moss lichen (Evernia furfuracea) also contains the toxic but odorous α-Thujone and β-Thujone plus the non-volatiles Atranorin, ChloroAtranorin, Evernic Acid, Furfuracinic Acid and Physodic Acid. The lichen Pseudevernia furfuracea, confusingly also commonly called Tree Moss, contains another fragrant Orcinol derivative 2-Hydroxy-4-Methoxy-3,6-DiMethylBenzoic Acid as well as non-volatile compounds such as OxyPhysodic Acid, Physodic Acid, Virensic Acid and Atranorin plus the steroidal triterpenoids Ergosterol, Ergosterol Peroxide and Lichosterol.


Depsidones are cyclic ethers with two benzene rings attached either side of a 7-membered cyclic ether ring (-O-), which is also a lactone (in the diagram - the arrangement of the bottom two oxygen atoms).

Stictic Acid is found in the Lichens Parmotrema reticulatum, Parmotrema chinense, Menegazzia caesiopruinosa, Menegazzia subpertusa and Menegazzia platytrema. The K spot-test yields positive in the presence of Norstictic Acid turning a blood-red, the same as for Salazinic Acid. The K spot-test yields positive in the presence of Stictic Acid turning yellow, as does Atranorin and Thamnolic Acid whereas the P spot-test yields positive turning orange-red in the presence of Stictic Acid.

Norstictic Acid has a methoxy group substituting an -OH group on Stictic Acid. Un-like Stictic Acid, Norstictic Acid is able to chelate metals (see 'Metal Sequestration' box below) and is found in many lichens including Aspicilia mastrucata. Norstictic Acid tests positive in the K test turning deep yellow whilst the P spot-test yields positive in the presence of Norstictic Acid and Salazinic Acid also turning deep yellow.

Menegazziaic Acid (sometimes mis-spelled Menegaziaic Acid) is found in the Lichens Parmotrema reticulatum, Parmotrema chinense and in Lichens belonging to the Menegazzia genus.

Salazinic Acid (not to be confused with another lichen acid Salacinic Acid which is found in Parmelia sulcata) is one of the more common lichen acids present in the lichens Parmelia sulcata, Parmelia saxatilis, Lobartia pulmonia, Ramalina subbreviuscula and many others. The K spot-test yields positive in the presence of Salazinic Acid turning a blood-red, the same as for Norstictic Acid whereas the P spot-test yields positive in the presence of either Salazinic Acid and Norstictic Acid turning deep yellow.

Unlike the more ubiquitous Stictic, NorStictic and Menegazziaic Acid, Psoromic Acid lacks the fused 5-membered lactone ring, as do the other Depsidones below here. The P spot-test yields positive in the presence of Psoromic Acid turning light-yellow.

ProtoCetraric Acid, found in the lichen Usnea albopunctata, would make an excellent broad-spectrum anti-microbiotic compound active against medically important pathogenic microbes affecting humans.

Gangaleoidin and Diploicin (sometimes mis-spelled Diploieiin) are two rare chlorine-containing depsidones the first found within the lichen Lecanora gangaleoides, the second in Diploica canescens (aka Buillia canescens), both of which are found in Ireland.

Lobaric Acid is found in lichens of the genus Stereocaulon, testing positive in the K test turning slightly yellow, and a deeper shade of orange on the KC test.

Loxodin is another depsidone like Lobaric Acid with aliphatic chains, the only difference between them being the swapping of one of the -OH and -OCH3 groups. Thus Loxodin is an isomer of Lobaric Acid and no longer an acid but a methyl ester.

FumarProtoCentraric Acid (sometimes mis-spelled FumaroProtoCentraric Acid) is found in the Lichen Xanthoparmelia semiviridis, Bryonia fuscesens, and in Cladonia species. The P spot-test yields positive in the presence of FumarProtoCentraric Acid turning dull red.

Other depsidones are Physodalic Acid (in the lichen Hypnogymnia occidentalis), Physodic Acid, Gangaleoidin, Glomellonic Acid Salazinic Acid, ConSalazinic Acid, ConStictic Acid, CryptoStictic Acid, PeriStictic Acid, ConnorStictic Acid, Galbanic Acid, DiVaricatic Acid, DiVaronic Acid, Virensic Acid, Variolaric Acid, ConFumarProtoCetraric Acid, SubLobaric Acid, OxoLobaric Acid, Alectoronic Acid, SubPsoromic Acid, Excelsione, Diploicin and Collotolic Acid.


Depsides are polypeptide-like small molecules consisting of a series of linked phenol carboxylic acids esters and were first discovered in the early part of the 20th Century. They are derived from Orsellinic Acid, which is itself derived from Orcinol.

Orsellinic Acid is ortho-Orsellinic Acid (o-Orsellinic Acid) and it can be extracted from certain lichens and is the pre-cursor of the Depsides, Depsidones and DiBenzoFuranes. There is also a β-Orsellinic Acid which has a second methyl group attached diametrically opposite the methyl group of Orsellinic Acid.


Thamnolic Acid, itself a depside, shows how the depsides below are related to the depsidones above. Just twisting the molecule the other way reveals the relationship. Thamnolic Acid is contained in lichens of genus Thamnolia, such as Thamnolia vervimularis (which fluoresces blue under UV) and Thamnolia subuliformis (which fluoresces pale-green). The K spot-test yields positive in the presence of Thamnolic Acid turning yellow, as does Atranorin and Stictic Acid, and positive with the P test turning a deeper yellow.


Lecanoric Acid is found in the lichens Punctelia borreri belonging to the Canoparmelia species and in Ochrolechia subpallescens. Gives a positive under the C spot test turning orange-red with either Lecanoric or Gyrophoric Acids.

Diffractaic Acid, found in Usnea longissima, may be useful medicinally as an analgesic and anti-pyretic, as is Usnic Acid.

  (UV→)   Atranorin is a depside derived from Orcinol present in the Lichens Cetrelaia cetrarioides, Canoparmelia species, Flavoparmelia caperatula, Hypogymnia subhysodes, Physica jackii, Parmelia cunninghamii, Parmotrema reticulatum, Parmotrema chinense and Punctelia borreri. It is a colourless pigment that fluoresces yellow in UV sunlight and thus acts as a sunscreen in the lichen Heterpdermia ciliatomarginata. The K spot-test yields positive in the presence of Atranorin turning yellow, as does Stictic Acid and Thamnolic Acid.

ChloroAtranorin is a chlorinated depside present in the lichen Heterodermia appendiculata and some other Heterodermia species, Hypotrachyna ikomae and other Hypotrrachyna species, Parmotrema clavuliferum and other Parmotrema species.

Evernic Acid (not to be confused with Everninic Acid, the methyl ester of Orsellinic Acid) is one of the three commonest allergenic compounds within lichens, and is found in Oak Moss (Evernia prunastri). Oak Moss injects Evernic Acid into its host substrate, usually branches and twigs, which inhibits respiration by the leaves thereby retarding leaf formation. Certain other lichens have an inhibitory effect on the growth of their hosts, such as the lichen Cladina alpestris on the Scots Pine tree (Pinus sylvestris). Also certain acids from the lichen Usnea longissa also inhibit growth.


Thamnolic Acid (head of section on Depsides), Alectorialic Acid and Sekikaic Acid (below) are also Depsides, but more specifically are meta-depsides where the linkage is from the meta-position, whereas Lecanoric Acid and the like are para-depsides, with linkages at the para-position.

Alectorialic Acid is a Benzyl Ester found in the lichen Alectoria nigricans. Some sources seem to incorrectly call it Alectoric Acid instead.

Sekikaic Acid is found in the Thysanothecium scutellatum lichen.


Gyrophoric Acid is a tri-depside and is found in the lichen Punctelia borrerei. The KC spot-test yields positive in the presence of Gyrophoric Acid, Electoronic Acid and Merochlorophaeic Acid turning red.

Other di-Depsides not shown are: Squamatic Acid, Barbatic Acid (aka Coccelic Acid which was found in Cladoonia coccifera is found in most Usnea species of lichens), Obtusatic Acid, NorObtusatic Acid, HomoSekikaic Acid, HyperHomoSekikaic Acid, Stenosporic Acid, Divaricatic Acid, Miriquidic Acid, Prasinic Acid Tumidulin, Olivetoric Acid, Cryptochlorophaeic Acid, Merochlorophaeic Acid and Everninic Acid.

Tri-depsides are not as numerous, but others include MethylGyrophorate, Tenuiorin, Crustinic Acid, Deliseic Acid, Hiascic Acid, Lasallic Acid, Ovoic Acid, Papulosic Acid and Umbillicaric Acid.

There are even fewer Tetra-Depsides such as Prunastrin from the Oak Moss lichen (Evernia prunastrin) and Apthosin from the lichen Peltigera apthosa. When the number of Gallic Acid (aka Pyrogallol) units exceeds about three then they become known as Pyrogallon Tannins, (depending upon the arrangements of -OH groups).


  PicroLichenic Acid is the bitter principal with a taste similar to quinine occurring in the crustose lichen Pertusa amara, which is widespread throughout Europe growing on the bark of broadleaf trees such as Oak or Beech. It is colourless, but turns an intense violet colour with Ferric Chloride and Ethanol. The K test yields positive turning tangerine, whilst the KC test shows positive by turning red.

Other Depsones include:
HyperPicroLichenic Acid (R1 = pentyl, R2 = heptyl)
IsoHyperPicroLichenic acid (R1 = heptyl, R2 = pentyl)
MegaPicroLichenic acid (R1 = heptyl, R2 = nonyl)
IsoMegaPicroLichenic acid (R1 = nonyl, R2 = heptyl)
IsoSubPicroLichenic acid (R1 = pentyl, R2 = Pr)
PicroLichenic acid (R1 = R2 = pentyl)
(where R1 and R2 replace the two aliphatic chains). These variations on PicroLichenic Acid, as well as its namesake, all occur with within the lichen Pertusaria truncata.


Diphenyl-butenolides are mostly yellow or orange pigments which afford the lichens possessing them protection from UV light. But their propensity to complex with metals may be another reason why some lichens possess them.

  Vulpinic Acid, which is found in several lichens amongst them the Wolf Lichen (Letharia vulpina) where it was first discovered and in Scandinavia used to poison wolves and foxes. Vulpinic Acid is bright yellow in colour and toxic. Both Vulpinic Acid and Usnic Acid inhibit the growth of the fungal symbiont Terbouxia irregularis in the Lichen Cladina mitis, which seems to be a regulatory effect within the Lichen. It is the methyl ester of the related Pulvinic Acid.

 Pulvinic Acid is related to Vulpinic Acid which can be converted to it by being saponified into a di-acid. Pulvinic Acid is related to Pulvinone, which has no natural sources but the hydroxylated derivatives of which are found in some mushrooms such as Larch Bolete (Boletus elegans) where they, being yellow pigments, colour the cap yellow.

 Pinastric Acid and Vulpinic Acid, both yellow pigments, are contained in the lichen Powdered Sunshine (Vulpicida penastri). It is structurally similar to Vulpinic Acid but with an additional methoxy group on one of the phenyl groups.

Pulvic Acid Lactone (sometimes referred to as Pulvic Lactone and Pulvinic DiLactone), is the dimer of PhenylFuranone.

 Calycin (not to be confused with a set of large proteins with the same name) occurs in the lichens Lecanora fulvastra and Candelariella vitellina. Indeed, the latter contains Vulpinic Acid, Pulvinic Acid, Pulvic Lactone as well as Calycin. Calycin is an egg-yolk yellow pigment found in the crustose lichen Candelariella rosulans.

  Rhizocarpic Acid is another bright yellow pigment and is found in Xanthothallia species of lichens and in the lime-green Map Lichen (Rhizocarpon geographicum. Unfortunately, there is no reliable spot-test for identifying Rhizocarpon geographicum and it is possible many sub-species are being ignorantly grouped as Rhizocarpon geographicum.

Other diphenyl-butenolides are Epanorin, Leprapinic Acid and Pinastric Acid.


Butenolides are based upon 2-Furanone, a 5-membered ring lactone ring. The butenolides shown here have carboxylic acid group and a long aliphatic hydrocarbon chain attached.

ProtoLichenesterinic Acid is found in the American populations of the lichen Cetraria aculeata from which a red-brown dye can be obtained. This same acid is also found in Cetraria islandica which was used, after processing to remove the poisonous and bitter lichen acids, to make bread, gruel, porridge, jelly and salads. The actual constituents of lichens can vary with where they are growing; in Europe Cetraria islandica contains mainly FumaroProtoCetraric Acid and AlloProtoCesteric Acid. Protolichenesterinic Acid has been found to be anti-proliferative and cytotoxic to breast cell carcinomas.

Lichesterinic Acid may also be present in American populations of Cetraria islandica.

ProtoConstipatic Acid is found in various Xanthoparmelia species lichens from Australia.

Constipatic Acid was found in the lichen Parmelia constipa and other Permelia genus lichens, which also contains Usnic Acid and Loxodin.

Other Aliphatic Butenolides are NephroSterinic Acid and IsoNephroSterinic Acid.


MethylPhloroAcetophenone is very similar to the chemical structures of Phloroglucinol, Orcinol and Orsellinic Acid, all shown on this page somewhere. The DiBenzoFurans are all derived from MethylPhloroAcetophenone.

Pannaric Acid and Usnic Acid are DiPhenylEthers, derivatives of DibenzoFuran.

 Usnic Acid is MethylPhloroAcetophone derivative and a bitter pale yellow-green substance almost exclusively found only in lichens. It is found in many lichens in the genera Alectoria, Cladonia, Evernia, Lecanora, Parmelia and specifically in the lichens Flavoparmelia caperatulla, Flavoparmelia soredians, Ramalina, Teleoschites, Thysanothecium scutellatum, Xanthoparmelia semiviridis and in Usnea oncodeoides from which it was first isolated in 1844. Usnic Acid is believed to safeguard the lichen from the adverse effects of strong sunlight. The unpleasant bitterness probably deters consumption by grazing animals. In high concentrations it is probably toxic. Usnic Acid, and to a lesser extent Vulpinic Acid, is active against the pathogen Staphyloccocus aureus regardless of the strains resistance to the antibiotics methicillin or mupirocin, so may prove useful against MRSA. Unfortunately, Usnic Acid is not also a bacteriocide. The KC spot-test yields positive in the presence of Usnic Acid turning pale yellow (the K test alone hardly changes the colour).

Other DiBenzoFurans include Porphyrillic Acid, Haemophaein, Didymic Acid, ConDidymic Acid and Placodiolic Acid.


Anthraquinones are highly coloured compounds found in many plants and useful as dyes. Anthraquinone dyes are to be found in many yellow lichens.

  Emodin is a purgative resin and orange pigment found also in a Himalayan Rhubarb Rheum emodi, and also in Buckthorn (Rhamnus cathartica) and Japanese Knotweed (Fallopia japonica). In mice it limits the effect of glucocorticoids and may be useful against insulin resistance in type II diabetes. It also has an anti-cancer effect on several human cancers such as pancreatic cancer and can block certain infections such as herpes simplex and cytomegalovirus. Emodin is also found in lichens such as Nephroma laevigatum where it acts as a UV protectant. Emodin can also offer protection to the effects of alcohol-induced toxicity to liver cells (as can Chrysophanol but to a lesser extent).

  7-ChloroEmodin, an orange pigment found in the lichens Nephroma laevigatum, Heterodermia flabellata and Heterodermia obscurata, probably has similar properties, but will probably be very poisonous, as most organochlorides are. 5,7-DiChloroEmodin (and Emodin itself) is also found in Heterodermia obscurata.

  Chrysophanol is a yellow pigment found in the Rheum genus of plants such as Rhubarb from where it is extracted and also in some lichens. It has anti-microbial and anti-inflammatory properties.

  Parietin derived from polyaromatic ring polyketides and is present in lichen of genera Xanthoria and Teloschites in particular in the lichens Teloschites chrysothalmus, Teloschites spinosus and Xannthoria parietina. In lichens it acts as a UV protectant and is responsible for the bright orange colouring of the Xanthoria species of lichens, and was found in Golden Shield Xanthoria paretina from which it was extracted for use as a dye. Parietin and Emodin are structurally related and have a similar but not identical colour. It is a common orange pigment in lichens. Parietin tests positive under the K test turning reddish-purple.

  Parietinic Acid is also an orange pigment but is less common than Parietin.

  Fragilin is a yellow anthraquinone with a substituted chlorine atom and is the major pigmented component of the yellow thalli in the Caloplaca allocroa lichen, but the apothecia also contain the golden-coloured parietin. Compounds containing chlorine atoms are common in algae, but quite rare in plants.

  Fallacinol, not to be confused with Falcarinol, is a yellow pigment.

  Averythrin is a bright-red anthraquinone with an aliphatic side-chain attached. Averythrin occurs in the lichen Solorina crocea, in the fungus Aspergillus versicolor (Streptomyces) as does the related Averantin. Averantin (not shown) is a similar anthraquinone with a shorter and slightly different side-chain.

  Russulone, a bright-red pigment found in the bright-red apothecia of Ramboldia (which belongs to the so-called 'Russula-group' of about 18 species) and in the lichen Lecida russula, Pyrrhospora and Haematomma species of lichens, is an anthraquinone with a fused lactone ring.

  Other Anthraquinones include NorSolorinic Acid which is red, Solorinin, Solorinic Acid (orange, found in the lichen Solorina crocea), Asahinin (red, found in the lichen Asahinea chrysantha), Nemetzone (orange-red), Bellidiflorin (reddish-brown, from the lichen Cladonia bellidiflora).


   FlavoObscurin A and FlavoObscurin B are chlorinated anthraquinone dimers, the former with three atoms of chlorine, the latter more symmetrical with four atoms of chlorine. Both are found in the lichen Heterodermia flabellata and Heterodermia obscurata. FlavoObscurin A and B are a lemon yellow pigments

 Skyrin is another symmetrical anthraquinone dimer (BiAnthraquinone) but is joined not in the centre as are the FlavoObscurins above, but at each end. It is orange-red pigment and ubiquitous, being widely present in many differing lichens species including Hypotrachyna toiana and Parmotrema rampoddense.

Rugulosin and the deep-red pigment RubroSkyrin (neither shown) are related compounds. Both are found in fungi and are cytotoxic. Rugulosin (and LuteoSkyrin and FlavoSkyrin), unlike Skyrin and RubroSkyrin, are Dioxanes, which imparts an extra toxicity to them especially by LuteoSkyrin, but as far as your author can ascertain are produced by fungal moulds rather than by lichens.

  2,2',7,7'-Tetra- Chloro-Hypericin is an anthraquinone dimer (Bianthrone).

  There is also the 7,7'-DiChloroHypericin with the two right-hand chlorine atoms an number 2 and 2' positions absent, replaced by hydrogen atoms.

Both are purple pigments found in the lichens Nephroma laevigatum and Heterodermia obscurata.

  Hypericin is an un-chlorinated version of the above compounds, but is a yellow pigment rather than a purple pigment and is found in all St John's-worts.


  RhodoCladonic Acid is a NaphthoQuinoneFuran and is the scarlet-red pigment found in the red or pink tips of the apothecia of many Cladonia species of lichen such as British Soldier (Cladonia cristatella), Cladonia floerkeana, Cladonia diversa, Cladonia digitata, Cladonia macilenta, Cladonia polydactyla and others. Determining the proper structure of this compound has posed many problems for chemists, but a structure with a Furano third ring (rather than an AnthroQuinone with a second phenyl group) was declared correct (although other structures can be found on the internet that have the anthraquinone skeleton).

  Canarione is a yellow-coloured novel naphthoquinonepyrone found in Usnea canariensis and in the orange lichens from species of Lethariella subgenus Chlorea which are in the Parmeliaceae family. Most also contain Atranorin, another yellow pigment.

  Haemoventosin is a blood-red coloured novel naphthoquinonepyrone found in the blood-red apothecaria Ophioparma ventosa. This structure too was once thought to be NaphthoQuinoneFuran with a 5-membered furan ring, but the latest methodology employed by Dr David S. Rycroft determined the above structure in 1995, which once again puts the NaphthoQuinoneFuran structure of RhodoCladonic Acid (above) once again in doubt.


  Lichexanthone is a bright-yellow xanthone (sometimes mis-spelled 'lichenxanthone') found in the non-native tree Cupanea cinerea (Sapindaceae family), in the fungus Laurera benguelensis and in the lichens Parmelia formosana and Parmotrema lichexanthonicum.

  Arthothelin (2,4,5-Trichloro-LicheXanthone) is a trichloro substituted Lichexanthone and is present in the lichen Lecidella elaeochroma along with many other differing chlorinated lichexanthones, one the fully chlorinated 2,4,5,7-tetra-chloro-lichexanthone plus ChloroAtranorin. Arthothelin together with Thiophanic Acid occur in the yellowish-green to yellow thallus of the lichen Lecanora pseudodecorata.

2,4,5,7-TetraChloroLichexanthone is a tetrachlorinated xanthone present in the same lichen as is Arthothelin, namely Lecidella elaeochroma, along with perhaps a dozen other chlorinated Lichexanthones.


Eugenitin is a chromenone found in some lichens, in the fungal species such as Lecanora rupicola and Cylindrocarpon and is also found in Cloves (Eugenia caryophyllata). Sordinone is another similar chromenone found in lichens.


  VioXanthin is an ErgoChrome and a naphthopyrone derivative (not to be confused with the orange carotenoid Violaxanthin), and is present in the lichen Hypotrachyna toiana. Despite its name it is a yellow-green coloured pigment and was first found in the mycellium Trichophyton violaceus and is the major pigment in the lichen Hypotrachyna osseoalba. It has more recently been found in the lichens Buellia vioxanthina and Megalospora galactocarpa.

  A similar compound called VioPurpurin, an insoluble dark-red pigment is also found in the mycellium Trichophyton violaceus and in the lichen Calopadia circumlutosa.

  Pigmentosin A is an isomer of VioXanthin (not to be confused with Violaxanthin, a carotenoid) )with much the same yellow-green colour, the only difference being in the position on the NaphthoPyrone by which the two naphthopyrone units are joined. It is pigment. It too is present in the lichen Hypotrachyna toiana.


Contortin is a bi-phenyl found in the lichen Psoroma contortum and is the only known biphenyl contained within lichens and is also a symmetrical dimer containing 2 units of C-MethylPhloroAcetophenone - aka two PhloroGlucinol units (with two additional side-groups apiece).


 Polyporic Acid, a diphenyl-benzoquinone, is a toxic bronze-coloured pigment found in some Polypore mushrooms such as Polyporus nidulans and Polyporus rutilans and in the lichens belonging to the Stricta genus. The colour of it varies with pH. It is not a planar molecule, the outer two phenyl rings are twisted at an angle of 45 degrees to the plane of the central benzoquinone ring.

  Another Terphenylquinone is a benzo-furanone that is related to Polyporic Acid, Thelephoric Acid, is found in Lobaria genus lichens is a bluish-greenish pigment found in mushrooms such as Clitocybe subilludens and Polyozellus multiplex and in the lichen Lobaria retigera. It is a dimer.


Scabrosin DiAcetate (from the lichen Xanthoparmelia scabrosa) has a similar skeleton to that of Thelephoric Acid, but with several substitutions and omissions, thus it is not a benzoquinone. Highly unusual in lichens Scabrosin has a disulfide linkage across the piperazine-2,5-dione core, which has replaced the xanthone core of the Terphenylquinones above. It also has two additional epoxide rings which do not seem to contribute to its observed biological properties. Scabrosin itself forms several esters in lichens, only the diacetate is shown (above). Epipolythiopiperazines are widely known amongst the Ascomycete genera (which includes Aspergillus and Penicillium fungi) but only one occurrence has so far been found in Lichens, Scabrosin, and it is highly likely that the ascomycetous fungal symbiont is responsible. Nitrogenous and/or sulfurous compounds are unusual in lichens and it is pondered whether Xanthoria scabrosa incorporates atmospheric pollutants such as Sulfur Dioxide (SO2) and Nitrogen Oxides (NOx) into Scabrosin thus rendering them harmless to itself.

[Far fewer lichens are tolerant to Sulfur Dioxide. The most tolerant lichen to SO2 is Leconora conizaeoides. It was found that the super-hydrophobic (water repellent) nature of the thalli of this species was responsible for its tolerance to sulfur dioxide].

Several other esters of Scabrosin are also found in the lichen Xanthoparmelia scabrosa and in an unidentified species of Usnea lichen in Ceylon. Ambewelamide A (aka Scabrosin DiButyrate) and Ambewelamide B, found in Usnea species of lichen from Ambewela in Sri Lanka, are two compounds very similar to Scabrosin, the only difference being the two terminal moieties on the outer two phenyl groups, everything else is the same, including the di-sulfide bridge, the nitrogen atoms and the epoxy groups. They all exhibit potent cytotoxicity.


(+)-Aspicilin (not to be confused with Ampicillin) is an 18-membered lichen macrolide/macrolactone which is present in the Lecananoracea family of lichens (it is also synthesized in the laboratory and used as an antibiotic).


Retigeranic Acid, with four fused 5-membered rings in a rare penta-cyclic arrangement with a cyclohexane ring, is the only sesquiterpene isolated from lichens, and is found in the lichen Lobaria retigera. Indeed four Retigeranic Acids from Retigeranic Acid A through to Retigeranic Acid D have been found in this lichen, with all but the B version being in the minority. Retigeranic Acid has not been found in any other lichen species.


16-α-HydroxyKaurane is a diterpenoid - one of only about 70 diterpenoids found in lichens of the Ramalina species. Hopane diterpenoids, of which HydroxyKaurane is one, are to be found in the Lecanorineae (Physiscia and Heterdermia species), the Cladoniineae (some Cladonia species) and the Peltigerineae (Pseudocyphellaria and Peltigera species). A few diterpenoid Kaurenes such as ent-Kaurene are also in other lichens such as Cladonia portentosa.


 Zeorin (aka Hopane-6α,22-diol) is a colourless steroidal and triterpenoid compound with the hopane skeleton found in many lichens. Many other steroidal compounds are to be found in many lichens, such as Nephrin, Taraxerene, Friedelin and Ursolic Acid which is also found in many plants. Friedelin is also found in species of trees non-native to the UK such as Cork Oak (Quercus suber), Salix japonica, Rhododendron westlandii and Ceratopetalum trichotum.

Other Hopane Steroidal compounds present in lichens related to Zeorin are 16β-AcetoxyHopane-6α,22-diol and 16α-AcetoxyHopane-6β,22-diol which just have an extra Acetoxy (OCOMe) group attached at just one of two differing positions. There is also the hopane triterpenoid Pyxinic Acid (3β,22-dihydrohopan-29-oic Acid) with a novel 3β-hydroxyl function. They are present in Heterdermia species of lichen such as Heterdermia flabellata, Heterdermia japonica, Heterdermia obscurata and Heterdermia appendiculata.


  Zeaxanthin, a symmetric compound and dimer, is a yellow carotenoid and antioxidant (isomeric with Lutein) which is white and found in lichens (and in egg yolk and within many dark-green leafy plants such as Marigolds and vegetables) and is the dominant part of the thermal energy dissipation mechanism of dehydrated foliose lichens when subjected to strong sunlight. In these circumstances it fluoresces to dissipate the light which would otherwise be degraded into heat, which the lichen is trying to avoid. The Zeaxanthin thus affords protection to the lichen from light-induced heat-stress when it is short of water.


None of the Orceins or other structures described in the Orcein section are actually found natively within any lichen species, but are rather derived by the chemical processing of compounds that are present in (some) lichens.

The Orceins (aka 'archil', 'lacmus', 'Lichen Purple' and 'C.I. Natural Red') are man-made pigments that do not occur naturally, but are prepared by processing certain lichens (commonly known as 'orchella weeds'), those which contain ample quantities of Depsides and/or Depsidones. The 'arcil lichen' (Roccella tinctoria), which in Europe was used to produce the dye known as Orseille, is a major source of Depsidones. The Depsidones and Depsides within the lichen are first hydrolysed by ammonia, NH3 to Orcinol, which then oxidizes in the presence of air and ammonia for a fortnight to form the purple orcein pigments, which consists of several slightly different but related compounds.

        Orcein forms dark-brown crystals and is a mixture of (at least) eight Phenoxazone derivatives consisting of three HydroxyOrceins, three AminoOceins and two AminoOrceinImines. It is used as a dye and also as a food permitted colouring, E121, and to stain biological specimens in the laboratory. It is a reddish-brown dye once extensively used for dying textiles extant since classical Roman or Greek times and up to the 19th Century. Nowadays it is used as a specialist dye only, the colours are muted and subtle compared to modern synthetic dyestuffs. It is still used for some specialist Harris Tweeds in Scotland using the lichen Parmelia omphalodes and in Ireland. Cloth so dyed has the added advantage that it repels moths because the lichen acids are bitter. The exact colour can be varied by altering the pH with the addition of either vinegar (Acetic Acid - an acid from the brewing industry) or washing soda (alkaline).

The alpha-Orceins have only one orcinol moiety attached to the phenoxazone core and are quite different to the beta- and gamma-orceins which have two orcinol moieties, one attached to each end.

The beta- and gamma-orceins are rotational isomers; each Orcinol moiety at each end can adopt one of only two possible rotational orientations; they are not free to rotate willy nilly. All are non-planar, with the Orcinol moiety set at an angle relative to the plane of the Phenoxazone core. Your Author thinks it is entirely possible that the colour changes mediated by changes in acidity/alkalinity are a reflection of the changes in the preferred rotational orientation of these orcinol moieties on the ends.





To possibly confuse the issue further, Orchil is another dye, a purple-blue dye (rather than the reddish-brown of Orcein) this time produced from 'orchil lichens' (rather than 'orchella weeds'). The rather arcane language of these dyes testifies to the antiquity of some of the processes and ingredients used in their making, when analytical chemistry would not be invented for centuries and there was no way of determining the uniqueness or purity of any product. Most products then were mixtures of various unknown and mainly inseparable substances. Think of tar. We can analyse products today, but the main problem is analysing the same product that the ancients made, its constitution would probably vary a great deal depending upon the exact method of processing, which was not well documented, some being valuable and therefore secret processes. Some dyes were as treasured as gold dust and kept shrouded in alchemical stealth even into the 19th century. Because of secrecy and continual improvement in process, methods varied, producing differing mixtures of compounds.

The lichens used for Orchil, Archil, Orseille (French) and Cudbear type dyes are Rochella tinctoria, Ocrolechia tartarea, Lecanora tartatea and Stag's Horn Lichen (Evarina prunastri) and some species of Parmelia Umbilicaria and Lasallia lichens.


'Cudbear' and 'French Purple' (which is a redder-purple with less blueness), two other very similar pigments both producing purple dyes, are extracted from the same 'orchil lichens' using differing processes. They are fast dyes and do not require the use of a mordant. The starting lichen for Cudbear dye is the lichen Cudbear Ochrolechia tartarea which contains Gyrophoric Acid (above, in the Depsides box). This decomposes on processing to Orcein and further treatment yields a purple dye.

pH4.3   pH8.3 LITMUS

'Litmus' is yet another purple dye obtained from lichens, especially the same lichen (Roccella tinctoria) from which Orcein is produced. Other Lichens can also be used to produce 'Litmus' such as Roccella fuciformis, Roccella pygmaea, Roccella phycopsis, Lecanora tartarea, Variolaria dealbata, Ochrolechia parella, Parmotrema tinctorum and, Roccella montagnei, Dendrographa leucophoea and lichens from the genus Parmelia such as Parmelia sulcata. Again, litmus is not a single compound, but a mixture of 10 to 15 slightly different Orceins and in differing proportions to that of the dye 'Orcein'. The mixture obtained from the above process used in obtaining Orcein for the dying industry is subject to further processing by the addition of Potash and Lime to the ammoniacal solution to yield 'Litmus'. The chromophore present in litmus is 7-HydroxyPhenoxazone. Litmus is usually soaked into filter paper (similar to blotting paper) when it is then known as 'Litmus Paper' which is sold in 'books' of strips. Litmus is used as an acid/alkali indicator, whose colours vary from pink/red (pH<4.3, acid) through purple (pH 7.0, neutral) to blue (pH >8.3, alkaline).

Litmus can be split up into separate fractions, each having slightly differing properties which were given various names including Erythrolitmin (aka Erythrolein), Spaniolitmin, Leucorcein, Leucazolitmin and Azolitmin, the latter having much the same characteristics as 'Litmus' itself. Some, apart from Azolitmin shown below, are still mixtures (as is Litmus) rather than individual compounds.

pH4.5   pH8.3 AzoLitmin, mentioned above, is either a dark-red powder or has dark-violet scales and is a more reliable acid-base indicator than is Litmus, varying from red at pH 4.5 and below through to blue at pH 8.3 and higher. Azolitmin, however, misses out the first 0.2Δ pH, missing out the pH 4.3 to 4.5 range, a small blind-spot. AzoLitmin has a phenyl group attached to the nitrogen atom of the five-membered ring Imidazolidine to which additional side-groups are attached. It is usable with most mineral acids, but only with some organic acids, and not at all with hydroxy acids. Somewhat surprisingly it is also used pharmaceutically to treat Coronary Heart Disease.


   Crottal (aka Crottle) was the name of a brown, reddish or purple pigment used by the Romans and was extracted from species of Parmelia, Ochrolechia and Evernia lichens. It too is likely to be a mixture of differing compounds. Since it was made from differing lichens, it probably also comprised differing mixtures of compounds, and that 'Crottal' was not a single mixture of compounds, but several separate mixtures, depending who made it from what using which process. It is not well documented. In Scotland Crottle was obtained from Parmelia saxatilis harvested in August when the concentration of the orange-red dye compounds were thought to be at their highest.


Many Lichen compounds (in particular the dibenzofurans such as Usnic Acid, the anthraquinones such as Parietin and the diphenyl-butenolides such as Pulvinic Acid) commonly form chemical complexes with metals. Complexes with metals form under acidic conditions (substrates) for Usnic Acid, and under alkaline conditions for anthraquinones and for most of the diphenyl-butenolides apart from Rhizocarpic Acid, which forms metal complexes on acidic or alkaline substrates. There is a strong preference for lichens with Usnic or Rhizocarpic Acids for acidic substrates, whereas there is an equally strong preference for alkaline substrates (calcareous substrata) in lichens possessing Parietin (an anthraquinone) or diphenyl-butenolides such as Pulvinic Acid. It is almost as if lichens have a pre-requisite for metals, however that is mostly a false assumption. Many metals are toxic to lichens, but they employ certain compounds capable of sequestering the metals thus locking them safely from harms way. Lichen species which possess these sequestering agents are better able to survive on toxic spoil heaps, those that lack the agents just die.

The metals involved in complexing are, in rough order of concentration within lichens, Aluminium, Iron, Manganese, Zinc, Copper, Titanium, Magnesium, Manganese, Nickel and Chromium. Others can be found such as Cadmium and Mercury. The Tree Moss lichen Pseudevernia furfuracea absorbs and accumulates heavy metals such as copper and nickel as well as many of those aforementioned in proportion to their concentration in airborne particles and thus makes a good atmospheric pollution monitor. The same lichen has also been used to measure the levels of the radioactive Caesium-137 released into the atmosphere in large amounts as   radioactive nuclear fallout to be carried thousands of miles on the wind and precipitated from the sky during rain. The source of the volatile   Cs-137 was the explosion of the   Chernoble Nuclear Reactor in April 1986 and subsequent meltdown of the core.

Norstictic Acid has rather special properties amongst Lichen acids which the closely related Stictic Acid lacks: it can (and within lichen species possessing it, does) sequester metals. That single change from the -O- group of Stictic Acid to the HO- group makes all the difference to Norstictic Acid. If the lichen happens to be growing on a copper mineral, then copper can be sequestered by the lichen, changing its colour (in the case of copper, to green). The colour change is so strikingly distinctive that lichenologists have previously mis-identified them as different species of lichen. Previously, the colour of all lichens was attributed to the normal colourful organic lichen pigments, but now several species of lichen have been found where the primary colouring agent is from heavy metal sequestration. Green copper-rich specimens of the lichens Acoraspora smaradula and Lecidea lactea form a Cu-NorStictic Acid complex on copper-rich substrata. Other metals can be similarly sequestered. The flower Thrift also has mechanisms for heavy metal sequestration.

Some lichens, such as Lecanora cascadensis, can accumulate up to 5% of copper (dry weight). The colour varies from light green (1% copper) to a darker malachite green at 4% copper. Another lichen, Acorospora rugulosa was found to contain 16% dry weight in copper, which was affixed interstitially outside and between the cells rather than within them. A recently discovered, and very rare, copper sequestering lichen (Lecidea inops) has been found growing on the extensive copper ore tailings from both ancient and more modern copper mining in the Coniston Copper mining valley. It is very rare and is characteristic only of more basic and copper-rich rocks. The lichen Psilolechia leprosa, which is a light-green colour, has only recently been classified but is found throughout Britain on church walls growing near to copper lightning conductors.

Many lichens grow on a diverse range of toxic heavy-metal containing substrates, embodying ores of lead, copper, uranium and arsenic. In some lichens, the oxalates of metals such as zinc, manganese, copper, lead and magnesium are to be found within them.

It is thought that Norstictic Acid is not alone in being able to form metal complexes and that other lichen acid complexes may also occur in lichens. Psoromic Acid (aka Parellic Acid), which is far less common than NorStictic Acid, might also form copper complexes which colour the thalli of the lichen green. Not all lichens actually like metals, many lichens find them toxic and die, incapable of locking them safely out of harms way.

Much more information on Metal Sequestration (in plants) can be found on the Thrift page.


Lichens can be hard to identify positively from the numerous similar species. To help identify lichens, lichenologists usually carry out some chemical tests called 'Spot Tests' on the lichen specimens using various, often dangerous, substances.


The K test
The so called 'potassium' test, utilising a 10% solution of potassium hydroxide, KOH. [In reality this is rather a misnomer, for it is not the potassium that is doing the testing, but rather a strong alkali - and Sodium Hydroxide, NaOH from caustic soda crystals, can be used almost as effectively]. The K-test takes some time for the colour change, if any, to develop.

The C test
Another misnomer, this test has nothing to do with carbon, C, but rather with chlorine, Cl2, for which a solution of Sodium Hypochlorite, NaOCl, provides a ready source. Low-cost domestic bleaches usually contain NaOCl - but check the label - up-market bleaches contain several other ingredients as well that will thwart the result).

The KC test
The K test followed by the C test, in that order.

The Pd test
Again, another potential misnomer, for this test has nothing whatsoever to do with palladium, but rather with the dangerously dermato-toxic para-Phenylenediamine. When exposed to bare flesh para-Phenylenediamine photo-sensitizes the skin, resulting in serious burns for up to 2 years after the initial contact with the chemical if the person exposes that skin to sunlight (it has been used as a 'black henna' hair dye in some commercial preparations, but this use is highly discouraged - for it has been the subject of many lawsuits made by ladies with damaged scalps). This reagent is best avoided by amateur lichenologists for it is mutagenic, allergenic and possibly carcinogenic. The Pd-test also takes some time for the colour-change, if any, to develop, and may be faint and any results often spurious. Best to leave at the lab. Substitutes (perhaps less effective at differentiating between lichens) are Steiner's Solution, which is safer to use and consists of a mixture of 10g Sodium Sulfite, 1g para-Phenylenediamine plus 0.5ml of detergent and 100ml of water.

The HCl test
This time lichenologists have got the chemistry correct - this is the Hydrochloric Acid test to test whether the rock upon which the lichen is growing is acidic or basic. If basic (is calcareous), it will effervesce CO2. A suitable less dangerous but slower substitute is Jiffy Lemon juice in a plastic squeezy 'lemon', the active acid being Citric Acid.

The I test
This involves the use of a solution of iodine (usually Potassium Iodide), to test for the presence of a starch-like linear polysaccharide in lichens called IsoLichenin when it will turn a dark-blue; it is best left for use in the laboratory. Cetraria islandica and Evernia prunastri are lichens containing IsoLichenin. A variation is to use Meizers solution, a mixture of 1g Iodine, 1.5g Potassium Iodide, 50ml water and 50g Chloral Hydrate - which has the advantage that the lichen structures are more transparent.

The cN test
This uses concentrated Nitric Acid, HNO3, is best left and used in the laboratory. Its only use is to distinguish between Melanella (which turns the lichen red?) and Neofuscelia lichens by testing for specific pigments.

An source of ultraviolet light (at 365nm wavelength and between 1W and 3W in power) is also useful for identifying different classes of compounds according to their colour of fluorescence, such as yellow for Xanthones. The UV source should be used in the dark and eyes should be dark-adapted. Do NOT shine this light into eyes - it is very damaging!

Quite frequently a concatenation of these tests will be required for positive identification, with each test returning a positive or negative result. Several KEYS are published that detail the result of each test on any one particular lichen specimen.

As well as many lichen compounds being highly coloured, some change colour when reacted on with chemical agents. These agents form the basis of the Spot Tests for identifying some lichen species, or in many cases, narrowing down the possibilities. To further help identification a number of other tests have been devised, amongst them illuminating the lichen with UV light to observe the colour of any fluorescence.

K+ purple → orange Anthraquinones (e.g. Caloplaca, Xanthoria & Teloschistes species)
K+ orange-red crystals → Depsidones (e.g. NorStictic Acid)
K+ deep yellow → para-Depsides (e.g. Atranorin)
K+ violet-red → anthraquinones, commonly Parietin
K- → yellow Pulvinic Acids (Candelaria Candelariella, Candelina, Acoraspora & Letharia species)
KC+ orange → several substances, esp. Barbatic & Physodic Acid
KC+ violet → Picrolichenic Acid
C+ red → (Xanthones, Chromenones, meta-Depsides) if two free -OH groups in meta-position
P+ orange → usually Thamnolic Acid
P+ red → usually Fumarprotocetraric Acid
Pd yellow orange red → Depsidones & Depsides containing aldehyde groups
C pink → Depsides & Xanthones with 2 free -OH groups

LicheXanthone → UV + yellow
Squamatic Acid → UV + white
Barbatic Acid (2-Dodecyl-3-MethylSuccinic Acid) → UV + whitish blue
Arthothelin (2,4,5-Trichloro-LicheXanthone) → UV + orange

This involves dissolving the extracts from the lichen in a solvent and putting a drop on a thin layer of filter paper. As the colour stain spreads down the layer by capillary action, different compounds travel at differing rates, to leave a colour trail. The positions of various colours reveals the identity of the components in the extract. Different combinations of solvents are used, each assigned a code letter:

A - Toluene + Dioxane + Acetic Acid in the ratios 180 : 45 : 5
Solution deteriorates rapidly due to hygroscopic nature of Dioxane

B - Hexane + DiethylEther + Formic Acid in the ratios 130 : 80 : 20
Solvent deteriorates in less that 6 hours, largely replaced by solvent B'

B' - Hexane + Methyl tert-Butyl Ether + Formic Acid in the ratios 140 : 72 : 18
This solvent has largely replaced solvent B which deteriorates in less than 6 hours.

C - Toluene + Acetic Acid in the ratios 170 : 30
This provides the best discrimination between compounds for most lichens, but the separation of the colours is not very great for certain compounds, where solvent G will provide a more sensitive discrimination.

E - Cyclohexane + Ethyl Acetate in ratios 75 : 25
Needs to be prepared freshly daily but is capable of resolving non-polar compounds which have very high Rf-values in solvents A, B, B' and C.

G - Toluene + Ethyl Acetate + Formic Acid in ratios 139 : 83 : 8
A very stable solution which resolves components well between substances possessing very low Rf-values in solvents A, B, B' and C.

There are computer programs available which will analyse the answers provided by the various TLC, SPOT and UV tests to hopefully pin-point the chemical(s) found and thence to identify the lichen.


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