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FOCUS TOPIC: THE BIOLOGY OF LICHENS There are estimated to be between 13,500 and 17,000 species of lichen, extending from the tropics to the polar regions. They can be found in some of the most inhospitable environments on Earth, including some of the major deserts and rock surfaces, as well as some of the rainforests and other moist, lush environments. However, all lichens have one important feature in common - they are "composite organisms" made up of at least two components: a fungus and a photosynthetic partner, which live in an intimate symbiotic association. And, at some stage in their lives, many lichens break down into their component parts and then need to reassemble by finding their appropriate partners. Despite this, lichens can live for many years and they function as single organisms, so they are assigned proper Latin names. For example, the very common yellow-coloured lichen that is often seen on rocks near the sea is named Xanthoria parietina (see Fig. 1). We will begin by looking at this lichen in some detail, because it illustrates all the major features of lichen biology
Fig 1. The lichen Xanthoria
parietina, which commonly grows on rocky shores
exposed to salt spray, but also can be found on
substrates such as roofing felt. Left: The lichen
thallus. Right: close-up of part of the thallus,
showing the yellow, disc-shaped apothecia which release
ascospores. At the edge of this colony (Fig. 1, left) the lichen is growing as flat, plate-like lobes which grow slowly (perhaps 3-5 mm each year). But behind the advancing front the lichen produces circular, yellow disk-like structures (Fig. 1, right) that are the fungal fruitbodies termed apothecia. These apothecia produce ascospores - the sexual spores of the fungus. When we look at a cross-section of one of the lichen lobes (Fig. 2.) we see a clear zonation of the tissues. There is an upper cortex of tightly packed fungal cells, a zone beneath this containing algae (of the genus Trebouxia), then a more loosely arranged medulla with air spaces, a lower cortex and a structure termed a rhizina, which senves to attach the lichen to a rock surface.
Fig 2. Cross-section of Xanthoria parietina, showing the zonation of tissues. [© Jim Deacon] A similar arrangement of the tissues is found in most lichens and is functionally significant. The photosyntheic cells are protected from exposure to severe sunlight by the surface tissues, and the air spaces (resulting from the surrounding hydrophobic fungal hyphae) allow gaseous exhange. If we take a cross-section through one of the apothecia (the ascospore-producing structures), we see that the upper surface (Fig. 3, left) is rich in yellow-orange carotenoid pigments. Beneath this is a layer of club-shaped asci (containing the ascospores) and beneath these is the normal arrangement of the lichen tissues. Fig. 3, right) shows some of the asci in higher magnification, including two asci marked with arrowheads. These are nearly-mature asci, each containing 8 ascospores, which will be released into the air and serve to disperse the fungal spores.
Fig 3. Left: Section through part of
an apothecium of the lichen Xanthoria parietina.
Beneath the orange-coloured zone there are many
tightly-packed asci containing ascospores. Right:
higher magnification, showing two asci (arrowheads), each
containing 8 ascospores. The lichen symbionts All lichens consist of at least two organisms - a fungus, termed the mycobiont (the fungal symbiont) and a photosynthetic organism, termed the photobiont. This can be either a green alga or a cyanobacterium, and there are a few lichens that contain both a green alga and a cyanobacterium. In these relatively few cases it is interesting to note that the symbiosis involves 3 kingdoms of organisms! Most of the lichen fungi are members of the Ascomycota (ascus-forming fungi), and they seem to be specially adapted because they do not seem to have an independent role in nature. They grow very slowly in laboratory culture and generally lack the enzyme systems for degrading complex polymers. They usually produce disk-shaped fruiting bodies (apothecia) to disperse their spores. When these spores germinate they must find a suitable photosynthetic partner in order to re-establish the lichen symbiosis. A few of the lichen fungi are members of the Basidiomycota (the group that includes the mushrooms and toadstools), and these also must find a suitable partner to continue the lichen association. One of the interesting features of all these lichen fungi is that they grow as normal hyphae in culture (but very slowly) but when they find a suitable photosynthetic partner they drastically change their shape, giving rise to the many different forms of lichens such as those shown above. In other words, the fungus is responsible for producing the shape of a lichen, but only when the fungus finds a suitable photosynthetic partner. In contrast to the many thousands of different lichen fungi, there are only about 100 photosynthetic partners. By far the most common of these are the single-celled green algae of the genus Trebouxia, which are found in many lichens of temperate and arctic/alpine regions. Trebouxia species seldom grow as free-living cells in nature; instead they seem to be specialised lichen symbionts (see Fig. 4).
Fig 4. Single cells of Trebouxia, the most common green alga found in lichens. Another green alga, Trentepohlia, is commonly found in lichens in Mediterranean and tropical regions. But, unlike Trebouxia, the algae of tropical lichens can often grow independently in nature. The most common cyanobacterium found in lichens is the genus Nostoc which grows as beaded chains (Fig. 5). But some of the desert lichens, described later, are associated with mats of the cyanobacterium Scytonema. About 10% of lichens have cyanobacteria (usually Nostoc) as their main or only photosynthetic partner, instead of a green alga. For example, lichens of the genus Peltigera (see Fig. 6) have Nostoc as their only photosynthetic partner. However, some lichens that contain green algae can also have cyanobacteria in special wart-like structures (called cephalodia) on the lichen surface. These structures are found in about 3-4% of lichens and their role is probably to exploit the nitrogen-fixing ability of cyanobacteria. Most of the lichens that contain cyanobacteria as their main photobiont are recognisable by their brown, blue-green or olive green colour. A good example is the common heathland lichen, Peltigera polydactyla (Fig. 6).
Fig 5. Chains of Nostoc cells released from a desert lichen
Fig 6. Peltigera polydactyla, a common lichen on dry heathland soils. The leaf-like lobes of this lichen are bluish-grey in colour, but the undersides of the lobes are white and have hair-like projections (termed rhizinae) for attachment to the substrate (extreme right in the image above). The ends of some of the lobes also have shield-shaped fruiting bodies of the fungal partner (apothecia, shown by arrowheads) which release the fungal ascospores. Many lichens tend to be slow-growing organisms, living for perhaps hundreds of years in a stable association, and they have evolved mechanisms for propagating themselves by fragmentation or by producing vegetative propagules. The dry lichen thallus is brittle, so fragments can be broken off easily and transported by wind or by animals. In addition, several lichens produce stalk-like or pillar-shaped structures termed isidia, which are easily broken off and dispersed. All these fragments can resume growth in a new environment after they are rewetted. A further means of propagation is by the production of specialised dispersal units termed soredia (Fig. 7). These consist of a few photosynthetic cells enveloped in fungal hyphae, and they are formed as a powdery mass near the centre of a lichen thallus or at the tips of some of the thallus lobes. Soredia are readily dispersed by wind.
Fig 7. Soredia: small packets of algal cells surrounded by fungal hyphae. |
