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LICHEN2

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FUNGAL BIOLOGY
A Textbook by JIM DEACON
Blackwell Publishing 2005

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BIOLOGY OF LICHENS, continued

The range of forms of lichens

In terms of their morphology, we can group lichens into four broad categories, illustrated in Figs 8 - 13 below.

  • Foliose lichens, which have a flat, leaf-like structure.

  • Fruticose lichens that have an erect or pendulous structure.

  • Squamulose lichens that produce small, scale-like plates

  • Crustose lichens that produce a flat crust on rock or tree surfaces.

FOLIOSE LICHENS

Fig 8. Lobaria pulmonaria (the lungwort), which grows on tree trunks in unpolluted parts of Britain. The lobes are bright green, about 1-2 cm diameter, and can have brown fungal fruiting bodies (apothecia) on the ridges. [© Jim Deacon]

Fig 9. Peltigera canina (the ‘dog lichen’) is a common lichen that produces flat, grey lobes about 2-3 cm diameter on mossy banks. Note the conspicuous root-like projections (rhizinae) on the lower surface of this lichen. [© Jim Deacon]

FRUTICOSE LICHENS

Fig 10. Cladonia portentosa, which commonly grows among heather in heathland habitats. This species is similar to Cladonia rangiferina (‘reindeer moss’) which produces a grey-green, brittle, multiple-branched thallus. C. rangiferina provides a major source of winter food for reindeer in northern Scandinavia. [© Jim Deacon]

SQUAMULOSE LICHENS

Fig 11. Squamulose lichens, commonly found on peaty soils. The squamules of Cladonia spp, shown here, are small, green, scale-like structures, about 1-2 mm diameter, like flakes of skin (hence the name squamulose).

Fig 12. Some further examples of Cladonia spp.which produce squamules but also can produce goblet-shaped and candle-shaped structures. [© Jim Deacon]

CRUSTOSE LICHENS

Fig 13. Crustose lichens. The image shows the face of a rock, about 15 x 10 cm, colonised by three or four different types of lichen, including the green-coloured lichen, Rhizocarpon geographicum (the ‘map lichen’). [© Jim Deacon]

The fact that lichens can be formed by more than one type of fungus (ascomycota or basidiomycota) and more than one type of photosynthetic partner (green algae and cyanobacteria) clearly shows that the lichen symbiosis must have evolved independently on several occasions.  But it is impossible to trace the evolutionary history of lichens, because they are not single organisms.

PHYSIOLOGY OF LICHENS

The separate organisms that constitute a lichen play different roles in the symbiosis. The principal roles of the fungus are two-fold:

  1. to protect the photobiont from exposure to intense sunlight and desiccation;

  2. to absorb mineral nutrients from the underlying surface or from minute traces of compounds in the atmosphere or rain water.

The photosynthetic partner similarly has two important roles:

  1. to synthesise organic compounds that provide a carbon and energy source for the fungus;

  2. in the case of cyanobacteria, to fix atmospheric nitrogen in the form of ammonium in environments where fixed nitrogen is not available.

Because of their symbiosis, lichens are able to colonise a wide range of environments where few other organisms could survive. So, lichens are often the pioneer colonisers of inhospitable environments such as bare rock surfaces and even cooled lava flows. Furthermore, in some ecosystems such as desert soils, tundra heaths, and Douglas-fir forests of the Pacific Northwest of the USA, lichens have been shown to provide the major input of "fixed" nitrogen for the whole ecosystem, supporting the development of other life forms.

Water relations of lichens

Lichens are remarkable for their ability to withstand prolonged drying and to resume activity rapidly after rewetting. Most lichens that contain green algae can recover from drought by absorbing water from humid air and then begin to photosynthesise. However, the lichens that contain cyanobacteria can only resume photosynthesis after absorbing free (liquid) water.

The drought-tolerance of lichens is likely to be conferred by a water-repellent (hydrophobic) coating on the hyphal walls of the medulla. Small peptides that are rich in sulphur-containing amino acids have been found on the hyphal walls of many free-living fungi. They are termed hydrophobins and they probably also occur in lichen fungi. The presence of these compounds would ensure that the medulla around the photosynthetic cells does not become waterlogged, allowing the diffusion of gaseous carbon dioxide for photosynthesis. Of interest, the hydrophobic materials seem to be produced only by the fungus, because cells of Trebouxia, which have a naturally hyrophilic surface, become covered with a hydrophobic material when grown in the presence of a lichen fungus.

The rewetting of dried lichens is thought to start by the absorption of water in the gelatinous matrix of the cortex, after which water might move through the medulla in the wall space of the fungal hyphae or perhaps by capillarity in regions where the hyphae have a hydrophilic surface.

Nutrient exchange between the lichen partners

Most attention to nutrient exchange in lichens has centred on the mechanisms of carbon flow (supply of organic nutrients) from the photosynthetic partner to the fungus, because radioactive labelling studies have shown that both green algae and cyanobacteria can release up to 90% of their photosynthate to the fungal partner.

In lichens that have Trebouxia as the photobiont, and presumably also with other green algae, the fungal hyphae can produce short branches that penetrate through the algal wall to act as nutrient-absorbing structures, termed haustoria. In contrast, in lichens with cyanobacteria (e.g. Peltigera) the fungal hyphae do not penetrate the photosynthetic cells. Instead, the hyphae produce thin-walled protrusions that penetrate the hydrophilic gelatinous sheaths that surround the cyanobacterial cells. These interfacial zones are shown in Fig 1 below.

Sites of potential nutrient exchange in lichens of a soil crust community in semi-arid desert soils. Left: three green algal cells (labelled "a") firmly attached to a hypha in the medulla of the desert lichen Peltula. The arrowhead shows a hyphal projection into an algal cell. Right: detail of the interface between fungal hyphae and cyanobacteria in the lichen, Collema sp. Note the hyphal peg closely associated with the cyanobacterial cell. Collema is a lichen with a gelatinous sheath. [© Jim Deacon]

The major soluble carbohydrates in lichens are sugar alcohols (polyols). In the fungal partner these compounds are present as mannitol and, to a lesser degree, arabitol. In these respects lichen fungi are no different from other fungi, which characteristically have sugar alcohols as the main soluble carbohydrates. The green algae also produce sugar alcohols as their main photosynthetic products - e.g. the sugar alcohol ribitol is produced by Trebouxia. However, the cyanobacteria seem to release glucose to the fungal partner, and this seems to occur passively through a glucose carrier in the cell membrane, after enzymic degradation of an intracellular glucan (glucose polymer) in the cyanobacteria.

Of interest, the maximum rates of nutrient release from the photosynthetic partner occur in optimal moisture conditions, whereas the photosynthetic cells retain most of their carbohydrate in conditions of water stress. So, it is suggested that cycles of wetting and drying may be advantageous in maintaining a lichen symbiosis, because both partners could gain sufficient carbohydrate at different stages of this cycle.

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