Lichen anatomy and physiology

Lichen

Lichen anatomy and physiology is very different from the anatomy and physiology of the fungus and/or algae and/or cyanobacteria that make up the lichen when growing apart from the lichen, either naturally, or in culture. The fungal partner is called the mycobiont. The photosynthetic partner, algae or cyanobacteria, is called the photobiont. The body of a lichens that does not contain reproductive parts of the fungus is called the thallus. The thallus is different from those of either the fungus or alga growing separately. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi.[1][2] Lichens are capable of surviving extremely low levels of water content (poikilohydric).[3] However, the re-configuration of membranes following a period of dehydration requires several minutes at least.

The algal or cyanobacterial cells are photosynthetic, and as in plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.

Although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains.[4] The lichen association is a close symbiosis. It extends the ecological range of both partners but is not always obligatory for their growth and reproduction in natural environments, since many of the algal symbionts can live independently. A prominent example is the alga Trentepohlia which forms orange-coloured populations on tree trunks and suitable rock faces. Lichen propagules (diaspores) typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the “core species.”[5]

Lichen associations may be examples of mutualism, commensalism or even parasitism,[citation needed] depending on the species. Cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen.

In tests, lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).[6][7]

  1. ^ F.S. Dobson (2000) Lichens, an illustrated guide to the British and Irish species. Richmond Publishing Co. Ltd., Slough, UK
  2. ^ R. Honegger (1988) Mycobionts. Chapter 3 in T.H. Nash (ed.) (1996) Lichen Biology. Cambridge University Press. ISBN 0-521-45368-2
  3. ^ Nash, Thomas H., ed. (2008). Lichen Biology (2nd ed.). Cambridge University Press. pp. 5–6. ISBN 978-0-521-69216-8.
  4. ^ Sciencemag.org
  5. ^ A.N. Rai; B. Bergman; Ulla Rasmussen (31 July 2002). Cyanobacteria in Symbiosis. Springer. p. 59. ISBN 978-1-4020-0777-4. Retrieved 2 June 2013.
  6. ^ Baldwin, Emily (26 April 2012). "Lichen survives harsh Mars environment". Skymania News. Archived from the original on 28 May 2012. Retrieved 27 April 2012.
  7. ^ de Vera, J.-P.; Kohler, Ulrich (26 April 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars" (PDF). European Geosciences Union. Archived from the original (PDF) on 4 May 2012. Retrieved 27 April 2012.