Which organisms form lichens

When looked at microscopically, the fungal partner is seen to be composed of filamentous cells and each such filament is called a hypha. These hyphae grow by extension and may branch but keep a constant diameter.

Amongst the photobionts there are those that are also filamentous in structure while others are composed of chains or clusters of more-or-less globose cells. Given that they contain chlorophyll, algae and cyanobacteria can manufacture carbohydrates with the help of light via the process of photosynthesis. By contrast, fungi do not make their own carbohydrates. Every fungus needs existing organic matter from which to obtain carbon. In a lichen some of the carbohydrate produced by the photobiont is of course used by the photobiont but some is 'harvested' by the mycobiont.

Worldwide, over 20, species are known and over 3, are known from Australia. There has been scientific study of Australian lichens over the past two centuries, though with somewhat erratic progress during the first century and a half since European settlement. Lichens can be found growing in almost all parts of the terrestrial world, from the ice-free polar areas to the tropics, from tropical rainforests to those desert areas free of mobile sand dunes.

While generally terrestrial a few aquatic lichens are known. The surfaces or substrates on which lichens grow vary from the natural such as soil, rock, wood, bone to the man-made bitumen, concrete, glass, canvas, metal - to give just a few examples. Lichens possess structures not formed by either of the partners and produce chemicals usually absent when the fungus or the photobiont are cultivated separately and so lichens are more than a sum of their parts. In fact, lichens synthesize over substances, many of them not found elsewhere in nature.

Though the fungi that form lichens do not occur in nature as independent organisms, a number of the photobionts can be found in free-living forms. It is possible to separately culture the two partners in the laboratory but it is difficult to resynthesize the lichen. Success has been achieved when the fungus and photobiont are placed under stress e. As well as having important ecological roles lichens have also been used by humans as food, medicine and for the dyeing of cloth.

For example, traditionally lichens were used to produce the colours of Harris tweed. Lichens are classified with the fungi being sometimes referred to as lichenized fungi. The fungi incorporated into lichens are largely ascomycetes, with very few basidiomycetes involved. Though a number of lichen species can be readily identified in the field the precise identification of many lichens demands examination of their macroscopic and microscopic structures such as reproductive structures, spores and cellular features as well as chemical tests.

The usefulness of the chemical tests lies in the fact that the chemical substances are often species specific. Lichens show a variety of growth forms and there are terms used to name these forms. The following are three very commonly seen types:. Fruticose lichens are erect or pendulous and markedly three-dimensional. The genus Usnea right is an example. Crustose lichens are markedly two dimensional and firmly attached to the substrate by their entire lower surfaces, making it impossible to see a crustose lichen's undersurface.

A crustose lichen looks very much like a thin crust on the substrate. The bright orange Caloplaca is an example. Foliose lichens could be thought of as halfway between crustose and fruticose. Though obviously three dimensional they grow in a more-or-less sheet-like form, but often with a lobed appearance.

They are not attached by their entire lower surfaces to their substrates. Indeed, some foliose lichens are just centrally attached to their substrates with the rest loose, so making it possible to see both the lower and upper surfaces very easily. Xanthoparmelia substrigosa below is an example. Those three growth forms will account for the majority of genera that most people are likely to see.

It is also worth mentioning the concept of a squamulose lichen since the genus Cladonia is very widespread and often shows a squamulose growth form. But the species in the genus also produce upright fruticose structures called podetia - sometimes with the appearance of fairly simple stalks, sometimes flared at the apex and so presenting a somewhat trumpet-like form.

In this photo you can see a Cladonia colony growing on soil. There are numerous squamules on the soil but you can also see a number of the upright podetia with broader apices.

Moreover, at the margins of some of the broader apices you can see additional podetia developing. Here is a closer view, showing an enlargement of part of the previous photo. You can also see that the podetia themselves also have flake-like squamules. Cladonia is not the only squamulose lichen genus, just a very commonly seen one. A colony of a squamulose lichen looks like a scattering of small flakes or scales on the substrate. A byssoid lichen has a somewhat wispy appearance, like cotton-wool teased out to some degree.

Leprose lichens have a powdery or granular appearance. You can find combinations of growth forms in some lichens. For example, some species are crustose centrally but somewhat foliose at the margins. Just in case you're interested, such a lichen is called placodioid or placoid and Placopsis perrugosa is an example. In most cases a species will always have the same gross morphology but a number of species are known to show some plasticity.

This box gives one example. Usually Siphula coriacea is characterised by erect, bluish grey lobes, shown here. The species is known from mainland Australia and New Zealand, in heathlands and grasslands. In moister, sheltered areas the lobes may be over a centimetre in length but in the drier rangelands of inland Australia the lobes are markedly shorter, often only a few millimetres in length.

An unusual form has been found in Idalia National Park, western Queensland. The thalli consist of more or less circular disks, rather than of erect lobes. The discoid thalli, shown here , are from two to eight millimetres in diameter and, though different in gross morphology, match the usual forms of this species in both chemistry and micro-morphology. Typical forms of Siphula coriacea were found elsewhere in the same general area.

All of these expressions may be called usefully imprecise descriptive terms. They are useful in the same way that expressions such as shrub and tree are useful when talking about plants.

They are imprecise in that sometimes it may be difficult to place a particular specimen in a particular growth form 'pigeon-hole'. See Figure 2. Fruticose lichens have no distinct top and bottom and are often round in cross-section. Their thalli may be upright, shrubby, or of pendulous strands. See Figure 3. Lichens can survive severe conditions because they can withstand drying. The lack of water interrupts photosynthesis. In this suspended state, some lichens are able to withstand great extremes in temperature.

The slow growth in lichens could possibly be attributed to their slow rate of photosynthesis since they are usually dry. A lichen absorbs most of its mineral nutrients from the air and rainfall.

Pollution in the atmosphere can be especially dangerous to lichens because they retain, and can accumulate, deadly amounts of heavy metals, sulfur, radioactive elements, NO 2 , and ozone. Sulfur dioxide SO 2 is especially lethal to lichens because it lowers pH and deteriorates chlorophyll, which causes photosynthesis to cease. Anti-sulfur dioxide legislation in the last 25 years is allowing lichens to return to formerly polluted areas.

Lichens have been used to monitor the amount of pollutants in an environment. This is done by observing the condition of lichens as well as their chemical composition. Reproduction in lichens may be either sexual or vegetative asexual. In asexual reproduction, fragments of the thallus containing both the photobiont and the mycobiont separate and form into a new lichen.

Fungi are classified in part by the type of spore-producing structures they produce, with the cup fungi ascomycetes named for the open, cup-shaped structures which often bear the sexual spores of the fungi. Not all ascomycetes have these cup-shaped structures, however, and, easily observed morphological characteristics like fruit type cup-like apothecia versus flask-shaped perithecia, for example cannot always be used to assess relationships. Unfortunately, this means that not all fungi sharing a single characteristic are likely to be related.

However, some order can be distilled. The bulk of lichen diversity belongs to the class including the well-known genera Lecanora , Cladonia , Parmelia and Peltigera Lecanoromycetes, or the Lecanora -group , where spores are borne mostly in open or cup-shaped fruits apothecia. This group of fungi is very old, estimated to have evolved during the Carboniferous period.

The very first lichens probably date back to before the origin of land plants, when most of the biodiversity of Earth was in the sea. Many Arthonia relatives also have open cup type fruits, but their development is quite different, giving a clue that they are not closely related to the Lecanora -group. Instead, they are more closely related to other ascomycetes that have flask-shaped spore-bearing structures perithecia. Similarly, for still other lichen groups, morphological similarities have been confirmed by molecular evidence to point to their widely disparate origins in the ascomycete tree of life.

For examples of these, students would be advised to visit the tropics, where the members of the Arthonia -, Trypethelium - and Pyrenula - groups form conspicuous and sometimes colourful crusts. In Britain, the smooth barked trees of the western districts are good places to see some of our Arthonia and Pyrenula species.

Students of lichenology will probably not be surprised to read that lichen fungi can be difficult to identify, partly due to the paucity of morphological characters to go on, but also due to the repeated and independent evolution of such characters. For example, the fruticose habit has evolved repeatedly within the Lecanora-group, but also within the distantly related Arthonia -group.

Unrelated fungi repeatedly evolve similar morphologies to succeed under similar conditions, making morphological identification especially difficult in some groups.

Fungi are heterotrophic, meaning that, like animals, they require a carbon source to survive. The lichen fungi share a common ecological strategy of hosting an internal population of photosynthetic cells, from which they obtain their carbon source in the form of simple sugars.

These photosynthetic cells can either be green algae Chlorophyta or cyanobacteria or sometimes both, in which case the cyanobacteria are localised in distinct areas of the thallus. As the photosynthetic partners come from divergent parts of the tree of life green plants vs bacteria , the term photobiont is used as collective term for any of them.

The role of the photobiont in lichens is clear — to provide carbon in the form of simple sugars. These sugars are used by the fungi to maintain physiological functions, to grow, and reproduce. However, in the case of lichens with both green algae and cyanobacteria, the lichen gets an added nutrient input from the cyanobacteria in the form of fixed nitrogen.

Although lichens can probably access inorganic nitrogen from the atmosphere directly, it can be a limiting nutrient, so having an internal source can be an advantage especially in heavily leached environments. Only about species of photobionts are commonly found across all known lichens, representing 4 main genera.