Symbiosis in lichens - Wikipedia
Symbiosis in lichens is the mutually helpful symbiotic relationship of green algae and/or blue-green algae (cyanobacteria) living among filaments of a fungus. Discusses parasitic and mutualistic relationships of fungi. This fungus is a lichen, providing nutrients to the tree. The lichen gets sugars from. Lichens are a mutualism formed between fungi and various groups of algae. The photobiont and its relationship with the phytobiont are the same as in M.
However, there are also good arguments in favour of the controlled parasitism camp. Up to half of the carbon fixed by algae is immediately converted to fungal sugars which are inaccessible to the alga itself. In fact, it is thought that many early stages of developing lichen spores may survive using such a parasitic or saprophytic strategy.
Lastly, there are many lineages of lichen fungi that are parasitic on other lichens — the so-called lichenicolous lichens! In some cases, non-lichen fungi have evolved from lichenised forms.
What is a Lichen?
These can be specialised opportunistic parasites or saprophytes or even symbionts, competing for nutrients with other fungi in the lichen thallus. The symbiosis may be more complex than this.
Recent work by Spribille et al has found yeasts embedded in the cortex of ascomycete macrolichens, and their abundance correlates with previously unexplained variations in phenotype.
There is also convincing evidence for a consistent presence of non-photosynthetic bacteria within the thalli of all lichens, although the role of these bacteria is as yet unknown. Interestingly, a role for non-photosynthetic bacteria was suspected for many years, as the relichenization of separately cultured fungi and algae in the lab was facilitated by the presence of bacteria. In fact, a legacy of exclusion from accepted mycological research persisted until the s, despite their obvious affinities with non-lichen fungi.
With the advent of molecular biology, the shared history of lichens and non-lichens has been elucidated and acceptedand we now know that the fungi that form lichens have evolved from many only distantly related lineages across the fungal tree of life, uniting them and their non-lichen relatives in the Kingdom Fungi.
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Lichen fungi are a heterogeneous group; they are similar only ecologically, in that they share the nutritional strategy of gaining carbon from an internal symbiotic photosynthetic partner, the photobiont. In the study of lichens, the name and classification belongs to the fungal partner, which in most cases is the dominant member of the association, at least in terms of biomass.
Lichen fungi have evolved independently several times within the mushroom-forming fungi and relatives the basidiomycetesbut much more commonly, from within the cup fungi the ascomycetes. Probably more than ten distinct major lineages of fungi within the ascomycetes are lichenised. Current estimates suggest that one fifth of all known fungi and half of all ascomycetes are lichenised, with about 28, species worldwide.
As with most organisms, lichen fungi are most diverse and least studied in the tropics. For example, the genus Arthonia is comprised of a mix of lichenised and non-lichenised species and includes many which are specialist parasites, only found on one or a few closely-related host lichens. In a single genus, then, we have a case of lichen parasites evolving from lichen fungi! Other non-lichen fungi arose from lichenised ancestors, such as Stictis and Ostropa.
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. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand.
In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, vascular higher plant seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.
Pine forest with lichen ground-cover When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil.
Mutualisms between fungi and algae
While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument.
Lichens may be eaten by some animals, such as reindeerliving in arctic regions. The larvae of a surprising number of Lepidoptera species feed exclusively on lichens. These include Common Footman and Marbled Beauty. However, lichens are very low in protein and high in carbohydratesmaking them unsuitable for some animals.
Lichens are also used by the Northern Flying Squirrel for nesting, food, and a water source during winter. Although lichens typically grow in naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured pollutants and to air quality. Hence, they have been widely used as pollution indicator organisms. Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory or kill bacteria.
These compounds are very useful for lichen identification, and have or had economic importance as dyes or primitive antibiotics. Extracts from many Usnea species were used to treat wounds in Russia in the mid-twentieth century Kane Orcein and other lichen dyes have largely been replaced by synthetic versions Armstrong In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen —Rhizocarpon geographicum and Xanthoria elegans—were sealed in a capsule and launched on a Russian Soyuz rocket on May 31, Once in orbit, the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation.
After 15 days, the lichens were brought back to earth and were found to be in full health with no discernible damage from their time in orbit. Gallery A foliose lichen on basalt. Usnea australis, a fruticose form, growing on a tree branch Map lichen Rhizocarpon geographicum on rock The cyanobacterium Hyella caespitosa with fungal hyphae in the lichen Pyrenocollema halodytes Foliose lichen grows on a fallen log.
Notice the red tips. One type of growth habit. Lichen dyes and perfumes. Retrieved October 5, Insight into sex life of lichens. Lichens of North America. Lichens survive in space. The Book Guild Ltd. Lichenology in the British Isles The Richmond Publishing Co.
Tuscon Clinic of Botanical Medicine Newsletter 4 4. Assembling the fungal tree of life: Progress, classification, and evolution of subcellular traits.
Amer J Bot The lichen flora of Great Britain and Ireland. The Lichenologist 26 2: Other lichen fungi occur in only five orders in which all members are engaged in this habit Orders GraphidalesGyalectalesPeltigeralesPertusarialesand Teloschistales. Lichenized and nonlichenized fungi can even be found in the same genus or species.
TrebouxiophyceaePhaeophyceaeChlorophyceae have been found to associate with the lichen-forming fungi. One fungus, for example, can form lichens with a variety of different algae. The thalli produced by a given fungal symbiont with its differing partners will be similar, and the secondary metabolites identical, indicating that the fungus has the dominant role in determining the morphology of the lichen. Further, the same algal species can occur in association with different fungal partners.
Lichens are known in which there is one fungus associated with two or even three algal species. Rarely, the reverse can occur, and two or more fungal species can interact to form the same lichen.