LICHEN COMPOUNDS AND DYES
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
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 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
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.
Growth & Growth Rate
Habitat & Chemistry
Colours available from lichen dyes
Secondary metabolite properties
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, Napthaquinones, 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 more vigorously growing plants. Other compounds having a bitter taste probably deter animals and slugs and snails from eating the lichen. These substances are usually poisonous too.
Vulnerability and Hardiness
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
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
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 / SALTS
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
The same substance, Calcium Oxalate, appears a brilliant white on the surface of the very common crustose lichen
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.
ORCINOL & FRAGRANT COMPOUNDS(→Air→)
5-MethylResorcinol, is similar to the ubiquitous Phloroglucinol which has the Methyl CH3 group replaced by yet another -OH group and to
Resorcinolwhere 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
Lecanoraspecies. Orcinol crystallizes into colourless prisms with one molecule of water of crystallization but reddens when exposed to air.
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 (
All the above small Orcinol derivatives are present in the aroma emanating from the essential oils derived from the lichen
Tree Moss lichen (
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
Unlike the more ubiquitous Stictic, NorStictic and Menegazziaic Acid,
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.
Other depsidones are
Depsidesare 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.
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.
Tri-depsides are not as numerous, but others include
There are even fewer Tetra-Depsides such as
Other Depsones include:
DIPHENYL BUTENOLIDESDiphenyl-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.
Other diphenyl-butenolides are
ALIPHATIC ACID BUTENOLIDESButenolides 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.
Other Aliphatic Butenolides are
METHYL-PHLORO-ACETOPHONES / DIBENZOFURANS / DI-PHENYL-ETHERS
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.
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
Other DiBenzoFurans include
ANTHRAQUINONESAnthraquinones 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
7-ChloroEmodin, an orange pigment found in the lichens
BIANTHRONES / BIANTHRAQUINONE
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
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
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 NapthoQuinoneFuran and is the scarlet-red pigment found in the red or pink tips of the apothecia of many Cladonia species of lichen such as
XANTHONES and ERGOCHROMES
A similar compound called VioPurpurin, an insoluble dark-red pigment is also found in the mycellium Trichophyton violaceus and in the lichen
DIPHENYL-BENZOQUINONES / TERPHENYLQUINONES
EPITHIO-PIPERAZINE-DIONES / DIKETOPIPERAZINE-EPIDISULFIDES
Several other esters of Scabrosin are also found in the lichen
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
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
Marigoldsand 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.
ORCEINSNone 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' (
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.
ORCHILTo 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
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
Ochrolechia tartareawhich 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
Dendrographa leucophoeaand 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
pH4.5 →→ pH8.3
CROTTALCrottal (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 saxatilisharvested in August when the concentration of the orange-red dye compounds were thought to be at their highest.
METAL SEQUESTRATIONMany 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
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
Some lichens, such as
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.
LICHEN TESTSLichens 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 C test
The KC test
The Pd test
The HCl test
The I test
The cN test
An source of ultraviolet light is also useful for identifying different classes of compounds according to their colour of fluorescence, such as yellow for Xanthones.
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.
INTERPRETING THE RESULTS FROM THE SPOT-TESTS
THIN-LAYER CHROMATOGRAPHY (TLC)
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.
LICHEN COMPOUNDS AND DYES