Lichens do not attract much attention. Though often encountered daily, on rocky shores, walls and trees for example, they are generally overlooked in the public consciousness. It is certainly true that other plants, fungi and seaweeds are much more familiar: algae, mushrooms, and numerous land plants for example. And yet it is specifically lichen’s unique ability to branch across these species – in particular its relationship with algae and fungi – that make it of primary interest to scientists examining how organic life first evolved on land.
We know that land plants likely evolved from marine life, in particular from green algae, and that plants and green algae remain very closely related at a genetic level (Niklas and Kutschera, 2009). We also know that fungi are essential to at least 90% of all plant life on Earth (Bonofante, 2003), and recent studies suggest that without fungi, land plants could not have developed at all (Field et al., 2012). With lichens containing both algae and fungi, it is perhaps not surprising that some researchers believe lichens played a key role in the transfer of genetic information from marine organisms to the precursors of plant life on land (Heckman et al., 2001, Atsatt, 1988).
Lichens are formed through a symbiotic association between fungi and algae (Skaloud and Peksa, 2010), and are made up of a fungal element (mycobiont) that survives by associating itself with a green algae or cyanbacteria (photobiont) and consuming the sugars and nutrients the algae produces through photosynthesis (Ahmadjian, 1993). While this sounds predatory, the relationship between mycobiont and photobiont is in fact mutually beneficial. Not only does the fungus thrive, it encases and protects the algae, which receives further water and nutrients from the fungus in turn (Taylor et al., 1995). The fossil record shows that lichens have been fostering this symbiosis between algae and fungi for more than 400 million years (Taylor et al., 1995), back to the time around which it is thought plants evolved from green algae (Becker and Marin, 2009).
Scientists are still not completely certain though what organisms predicated and evolved during the “bio-invasion” of life onto land, though the consensus is that bacteria, fungi and algae were initially present, and these lead to the development of lichens, mosses and ferns (Masuda and Eszaki, 2009). So while the fossil record shows how old lichens are, it is not as easy to make the claim that they were crucially present during the millennia when algae’s genetic code was passed on to the precursors of life on land. The fossil record from the time is limited, probably due to the harshness of the environment, and because any primitive organisms present will have found it difficult to leave rock impressions (Gray and Shear, 1992).
Whatever life was able to survive those harsh physical realities will have to been versatile, and been able to endure fluctuating salination, dessication and heat (Gray and Shear, 1992). Lichens have been shown to be able to do this, in dry areas high in temperature and radiation (Langhans et al., 2009), in areas of repetitive tidal changes and wave action (Fletcher, 1973), and even in space (Sancho, 2007). As well as this, some lichen have been shown to be adept at switching between symbionts over time, an attribute which would likely aid survival in changing environmental conditions (Nelsen and Gargas, 2008).
Algae themselves are also capable of airborne transportation (Sharma et al., 2007), a fact that may have aided their initial dispersal onto land. Research has shown that once dispersed, algae are able to colonise areas known as algae soil crusts, a phenomena which is likely to have close developmental links with lichen soil colonisation (Wu et al., 2011). In fact, Taylor et al. (1995) note that not only were early lichen responsible for new land colonisations, they very likely contributed to the formation of the soil itself by eroding rocks, a further physical factor central to land plant evolution.
So while scientists have known for some time that lichen are formed from two species essential to the biological transfer of life to land from the sea, research has also shown that lichen do possess the qualities necessary to survive the extreme environments thought to have been present at that time. Though the fossil record on the matter is incomplete and further research required, it is likely that lichen provided a key stepping stone used by marine life in the colonisation of land.
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