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Lichens

What is a Lichen?

A lichen is a complex community having a fungus and a photobiont as its main components.  The photobiont (a green alga or a cyanobacterium) provides the energy source for the lichen and the fungus the structured habitat with its the absorptive and protective structures. Some cyanobacteria also provide the nitrogen for the lichen. Some lichens have both green algae and cyanobacteria in the one thallus, in this case the cyanobacteria are usually in structures called cephalodia.

Lichens are capable of inhabiting extreme environments, and it may be true that a lichen can only survive in an environment that would support neither of the separate components.  The close mutuality of the lichen thallus may confer great stress tolerance on a lichen, but the delicate balance between the components makes lichens very susceptible to damage.  Lichens are generally intolerant of all but the most chemically infertile environments, and are therefore destroyed by air pollution.

This delicate balance maintained by adversity means that lichens are usually extremely slow growing and very long lived.  It is easy to destroy a decade of growth by carelessly walking on a lichen or collecting it. It may take several years for a new thallus to reach the size of a pinhead.

Lichens come in many shapes, sizes colours.  The larger lichens, the shrubby and leafy lichens, are visually striking in many parts of subtropical Queensland.  They occur on tree trunks and branches, on rocks and on soil.  They occur in rainforests, in eucalypt woodlands and out into the driest parts of the state. Lichens are sensitive indicators of microclimate and of air quality. 

History

The lichens of subtropical Queensland have been the subject of study for over one hundred years. John Shirley (an Inspector of Schools) published the first taxonomic account of the lichens of Queensland in 1888-89. Little more published information about Queensland lichens then appeared for about 80 years, when work on lichen biology was taken up in the University of Queensland. This was coincident with a burst of activity around the country. In the last thirty years Australian lichens have attracted attention within Australia and around the world.  As a result we have seen the production of four lichen volumes in the Flora of Australia series, with several more yet to come. Because of this history, it is now possible to produce an account of the larger lichens in subtropical Queensland.

The larger lichens, those that are shrubby (fruticose) and leafy (foliose) are the most obvious and the easiest to determine to species.  The crustose lichens are still relatively poorly understood in Queensland, and their identification requires great patience, considerable skill and a good microscope.  Identifying the shrubby and leafy lichens does require patience too, especially as the set of new terms that must be understood in identification is learned. Like all fields of studies, lichenology has its own language that, once learned, makes the task easier.


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Parts of a Lichen

The Lichen Body

The lichen body is termed a thallus.  The thallus contains two main components, a fungus and a photosynthetic organism, the photobiont.   Green algal photobionts are usually present as single, bright green cells in a layer just below the cortex. Cyanobacterial photobionts are usually present as clumps of blue-black cells immediately below the cortex.

The name of a lichen applies only to its fungal component.  The photobiont has its own name. Almost all lichens have a fungal component from the class Ascomycota.

The thallus of a leafy (foliose) or shrubby (fruticose) lichen usually has three distinct tissues.  The outer layer is a cortex composed of tightly packed fungal cells that forms a protective layer around the thallus.  In leafy lichens the upper and lower cortex are usually sharply differentiated.  In shrubby lichens there is usually no such differentiation.  Inside the cortex (upper only) is found a layer of less dense fungal tissue (the medulla) with the photobionts incorporated into it (see illustrations below)

The colour of the lower cortex varies from white to jet-black: colour variation is taxonomically significant in some genera.  Colour of the upper surface is also taxonomically significant because of the link between colour and chemical constituents.


Peculiar Lichen Structures

Lichens have some distinctive asexual reproductive structures known as soredia and isidia.  These are easily detached from the lichen, especially by raindrops, and when scattered may grow into a new thallus.

Soredia are made up of groups of algal (photobiont) cells loosely entangled with fungal hyphae. Soredia are gathered into restricted areas known as soralia. Soralia can be pin points, small globular structures, or linear structures along the edge of lobes.

Isidia are more complex structures that have a well-developed outer wall that encloses a central core of algae and fungi. Isidia are often finger-like, and a about 1mm tall, but they do vary considerably. Isidia have an abscission zone at their base, making their detachment from the thallus easy.  In some case the isidia are hollow and swollen at the tip, and may even burst and become pustulate.

Rhizines are commonly present on the lower surface of foliose lichens. Rhizines may take a variety of forms (see below) that are taxonomically significant.  In some cases the lower surface is quite naked, in others it is covered by a dense carpet-like tomentum, and in yet others by a light tomentum of hypha-like hairs.  The distribution of rhizines on the lower surface can be significant - always note whether they come right to the lobe margins or leave a distinct bare zone around the edge.

Cyphellae and pseudocyphellae are pores occur in both upper and lower surfaces, more commonly on the lower surface.  When these pores have a well-developed margin with a concave pore behind the lower cortex the structures are cyphellae. If the pore is formed simply by a thinning of the lower cortex or by the development of deep fissures through the upper cortex while the lobe is young these are called pseudocyphellae.  When pseudocyphellae occur in the lower cortex they are usually plugged with hyphae from the medulla, although they may not be the same colour as medullary hyphae.  Take care not to confuse either the cracking that comes with age in the upper cortex or the fine, regular, reticulate cracking present in the upper cortex of some species with pseudocyphellae.

Cilia are hair like structures that occur on the margin of lobes and also sometimes on apothecia, arsing from the line where the upper and lower cortex meet. Care must be taken not to confuse cilia with rhizines that protrude from the lower surface. In a few genera cilia have a distinct swollen bulb at their base: these are bulbate cilia.

Dactyls are finger like structures that may be simple or branched, they my resemble isidia but that do not detach easily from the thallus.  Dactyls may become sorediate.


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Lichen chemicals

Most shrubby and leafy lichens produce large quantities of chemicals that are deposited around the hyphae.  In most cases these chemicals are not water-soluble. The role of the compounds varies.  Some are apparently to do with management of the light regime, some have anti-biotic properties, and some may be anti-herbivore compounds.  The metabolic pathways that produce these chemicals are genetically based, which makes the chemicals themselves taxonomically useful.

The chemicals are deposited specifically in some tissues.  A few chemicals are found only in the cortex, and these may influence the colour of the upper surface.  Most chemicals are found in the medulla, and a few of these do colour the medulla.  Melanin derived pigments are found mostly in the lower cortex, but do occur in the upper cortex of some genera. Apothecial tissues may also accumulate specific chemicals.

Fortunately, it has been discovered that some of the chemicals show colour reactions with a variety of reagents.  This makes chemical tests easy, and, since clear-cut taxonomic characters are few on a lichen, chemical tests are very useful indeed.  How to carry out these tests is discussed below.

 

Structures on the Upper Cortex

Structures on the upper cortex

Cilia are on the margin of the thallus.

 

Structures on the Lower Surface

Structures on the upper cortex


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Lichen Identification

Identification of lichens has the reputation of being very difficult.  True, getting started can be a challenge!

Lichen identification works on both visible (morphological and anatomical) characters and on invisible chemical characters. Lichens are small and many of the visible characters are barely visible to the naked eye. This makes lichen identification rather different from flowering plant identification.

The materials you will need

  1. A dissecting microscope to view details of lichen structure (you could scrape by with a hand lens)
  2. A scalpel or razor blade to cut lichen thalli.
  3. A bottle of Potassium Hydroxide solution (10%) for the K test. (This is caustic, so take care).
  4. Some domestic bleach as a source of Calcium hypochlorite for the C test. (this bleaches clothes and burns skin so take care).
  5. Access to paraphenylinediamine that can be dissolved in alcohol for the P test. The colouring used for men's beards is a useful substitute, but lacks the delicacy of the real thing. (Beware! Paraphenylinediamine stains everything it touches a permanent deep brown).  This solution or gel must only be used while fresh and light coloured.
  6. Glass dotting rods to add the test chemicals to the lichen.  Straightened paper clips work too.
  7. A compound microscope may be necessary at times to look at spores.  If you ever plan to identify crustose lichens such a microscope is essential.

Performing colour tests

Lichens produce a wide range of chemicals that are taxonomically significant.  In this respect lichens are similar to the fungi that produce antibiotics. The chemical variation is usually associated with visible variations, even if these are sometimes subtle. These chemicals are best detected by such techniques as Thin Layer Chromatography or Nuclear Magnetic Resonance.  These techniques are beyond all but a few of us.  Fortunately a range of chemical colour tests provide a lot of the information needed.

Using colour tests is simple and safe if care is taken.

Tests are performed on either the upper surface (upper cortex) of the lichen, or on the medulla, a cottony layer beneath the cortex. The medulla is usually white, but is sometimes buff or even red. 

To carry out a test use a dropping rod or straightened paper clip to put a small drop of liquid on the cortex, and watch for colour changes. If you need to test the medulla, the most common test, then cut away a small area of cortex to expose the medulla, then place a drop of fluid (or gel if using beard colour for P tests) and watch for colour change.  A colour change may take 20 seconds or so - and the colour may first be one colour (eg yellow) and then turn another (eg red).  Sometimes the colour appears then vanishes after short time. Watch for changes!

For a KC test first add a drop of K solution then a drop of C solution.

Some lichen chemicals are fluorescent under ultra-violet light.  UV reactions are rarely mentioned in these keys, but are taxonomically significant in a number of genera.


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The Colour of Lichens

Lichens commonly have quite distinctive colours when they are air dry.  All colour references in this account are for air-dry thalli.

The common colours of lichens can be attributed to the accumulation of complex organic chemicals in the upper and lower cortex. The colour of most lichens is therefore much the same whether living or dead. Some however, have no chemicals in their cortex or have radically different chemicals that may change with time after death of the lichen.

Some common chemicals in the cortex of lichens and the resultant colours and tests:

Thallus colour

Chemical

Test

Grey

Atranorin

K+ yellow

Grey

Lichexanthone

UV+ gold

Yellow-green

Usnic acid

K-, KC+ yellow

Gold

Parietin

K+ wine-red

Olive-brown

Melanins

 


Lichen Ecology

Lichens are very widespread organisms.  They occur in the darkest rainforests and on rocks in full sun. Lichens grow on rainforest leaves, on tree trunks and twigs, on rocks in creeks and on the beach rock of Heron Island. Lichens are common on the soil on roadsides and inside eucalypt forests as well as on undisturbed soil in arid lands.

Lichens have some peculiar sensitivities.  By and large, lichens are sensitive to nutrient levels.  City air is often too dirty for them, and lichens on tree trunks are killed by the nutrients that flow down tree trunks in inner city areas and near main roads. By contrast, lichens that grow on rainforest leaves benefit from the extra nutrients in the dust near footpaths.

Lichens are resistant to prolonged drought and very high temperatures when dry.  However, many lichens are very sensitive to heat when they are wet. Lichens can use the water deposited in dew to restart their photosynthesis and respiration.  Some lichens grow only in such locations that liquid water does not touch them, but they absorb all their water from mists and the air.

Some lichens have a cyanobacterial photobiont that can fix nitrogen. This means that the photobiont provides not only the carbohydrate for the lichen, but nitrogenous compounds too.  In some genera species with a predominantly green algal photobiont also have special structures that house a cyanobacterium as well.

The conditions that permit the formation of a new lichen thallus from spores and algal cells appear to be highly specific, and not often met. Survival of young thalli to the stage where identifiable tissues start to develop is probably low.  Growth rates are slow.  Reproduction by soredia and isidia seems easier, but many species do not have such clonal structures.  When collecting always remember the years of struggle that have passed before a thallus is big enough to collect.

 


When collecting lichens collect as little as is necessary


Less than this is a waste


More than this is destructive



 

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