Of the 70,000 or so species of fungus that are known, approximately 20% are 'lichenized' (Palmqvist, 2000). Lichens are nutritionally specialized fungi (the so-called 'mycobiont' component) that have escaped from their normal way of life into a mutualistic association with a 'photo-biont'. In around 90% of lichen species the photobiont is an alga, which provides carbon compounds to the mycobiont through photosynthesis. In some cases, the photobiont is a cyanobacterium, which may also provide fixed nitrogen to the association. In a relatively few, 'tripartite' lichen species (around 500) both an alga and a cyanobacterium are involved. Lichenized fungi belong to diverse taxonomic groups and the mutualistic algae to 27 different genera. Presumably, the lichen habit has evolved many times.
The photobionts are located extracellularly between the fungal hyphae, in a thin layer near the upper surface. Together, the two components form an integrated 'thallus' but the photo-biont makes up only about 3-10% by weight. The advantage to the photobiont in the association, if any, has not been established clearly. All lichenized algal species, for example, can also occur free-living outside their association with their mycobiont. It may be that they are 'captured' by the fungus and exploited without any recompense. However, some of the species (e.g. of algal genus Trebouxia) are rare in their free-living form but very common in lichens, suggesting that there is something special about life in their mycobiont that they need. Moreover, since minerals, including nitrogen, are largely 'captured' from what is deposited it depends on the species mycobionts and phytobionts
directly onto the lichen, often in rainwater and from the flow and drip down the branches of trees, and since the surface and biomass are largely fungal, the mycobiont must contribute the vast bulk of these minerals.
Hence, the mutualistic pairs (and threesomes) in lichens provide two striking parallels with higher plants. There is a structural parallel: in plants, the photosynthetic chloroplasts (see also Section 13.12) are similarly concentrated close to light-facing surfaces. There is also a functional parallel. The economy of a plant relies on carbon produced largely in the leaves and nitrogen absorbed principally through the roots, with a relative shortage of carbon giving rise to shoot growth at the expense of roots, and a shortage of nitrogen leading to root growth at the expense of shoots. Likewise, in lichens, the synthesis of carbon-fixing photobiont cells is inhibited by a relative shortage of nitrogen in the mycobiont, but synthesis is stimulated when the carbon supply is limiting (Palmqvist, 2000).
Lichenization, then, gives the mycobiont and the photobiont, between them, the functional role of higher plants, but in so doing it extends the ecological range of both partners onto substrata (rock surfaces, tree trunks) and into regions (arid, arctic and alpine) that are largely barred to higher plants. Indeed, it has been claimed that lichens dominate 8% of terrestrial communities, both in terms of abundance and species diversity. However, all lichens grow slowly: the colonizers of rock surfaces rarely extend faster than 1-5 mm year-1. They are, though, very efficient accumulators of the mineral cations that fall or drip onto them, and this makes them particularly sensitive to environmental contamination by heavy metals and fluoride. Hence, they are amongst the most sensitive indicators of environmental pollution. The 'quality' of an environment in humid regions can be judged rather accurately from the presence or absence of lichen growth on tombstones and tree trunks.
One remarkable feature in the life of the lichenized fungi is that the growth form of the fungus is usually profoundly changed when the alga is present. When the fungi are cultured in isolation from the algae, they grow slowly in compact colonies, much like related free-living fungi; but in the presence of the algal symbionts they take on a variety of morphologies (Figure 13.18) that are characteristic of specific algal-fungal partnerships. In fact, the algae stimulate morphological responses in the fungi that are so precise that the lichens are classified as distinct species, and a cyanobacterium and an alga, for example, may elicit quite different morphologies from the same fungus.
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