Australia: The Land Where Time Began

A biography of the Australian continent 

Lichen

Moore suggests it is probable that the most ancient known mutualism is that of lichen in which filamentous fungi and photosynthetic unicellular algae form a symbiosis. The earliest example of these involved cyanobacteria, photosynthetic organisms that were the first to release oxygen into the atmosphere. According to Moore the most ancient fossil that appears similar to lichen that has been discovered so far was found in South China. These fossils dating from between 635 and 551 Ma are filamentous hyphae in close association with cyanobacteria. This fossil shows that fungi were forming partnerships with photosynthetic organisms long before vascular plants evolved (Yuan, Xiao & Taylor, 2005). Fossil lichens have also been described from the Rhynie Chert dating to 400 Ma (Taylor, Hass & Kerp, 1997). Fungi, mostly of the ascomycete moulds of the present, developed partnerships that are now lichens as the eukaryotic algae evolved, usually an association between a fungus and a green alga, though there are still some involving cyanobacteria that are known of.

According to Moore the basis for the association is that the alga is a photobiont, i.e. it produces carbohydrate photosynthetically by using sunlight, the carbohydrate is then shared by both partners that comprise the lichen. Some lichens contain both green algae and cyanobacteria as photobionts. In such cases the cyanobacterium may focus on fixing atmospheric nitrogen that all 3 partners then use for the joint metabolism. Most of the thallus is a fungus, the algal partner(s) constitute(s) only 5% to 10 % of the total lichen biomass, which must be considered to be the dominant partner that forms a plant-like body within which it ‘cultivates’ photosynthetic algal symbionts (Sanders, 2001). This clearly suggests the fungus is ‘farming’ its photobionts. About 20 % of all known extant fungi are involved in lichens.

The lichen thallus, which the tissue formed by the fungus, differs greatly from either a fungal mycelium or an independent alga. The algal cells are surrounded by the fungal hyphae, which often incorporates them into multicellular fungal tissues that are unique to fungal associations. At the present there are about 20,000 known species of these unique composite organisms that vary in size, shape and colour. Some are flat and attached firmly to the surfaces they grow on, such as the discs that are often yellow/orange/brown that can often be seen on walls and roofs and old bridges, though there are others that are scaly, leafy or bushy, or in some cases hang down in strands from their support; the growth of some lichen thalli have a similar appearance to simple plants. The lichen association, which is an intimate symbiosis, extends the ecological range of its partners, with some extant lichens dominating a larger area of the land surface of the Earth than do tropical rainforests, even being able to survive in places that are inaccessible to other organisms. A feature that is characteristic of lichens is that they can tolerate severe desiccation, when dry lichens are able to survive such severe environments such as extremes of temperature, radiation and other harsh environments. As lichens are pioneers they invade places that are no more than bare rocks and mist, which is all they require to live. The alga is protected and supported by the fungus, both physically and physiologically, absorbing water and extracting nutrients from whatever soil it is in contact with, as well as the rocks, by the use of its exuded enzymes which even allow it to extract minerals from rocks, which results in mobilisation and absorption of minerals such as magnesium, manganese, iron, aluminium and silicon. As the algal and/or cyanobacterial cells in symbiosis are able to photosynthesise, photolysing water and reducing carbon dioxide from the atmosphere to produce carbohydrates for use by all the partners, and atmospheric nitrogen can also be fixed by cyanobacteria for use by all the partners. Moore suggests that the degree to which the mutual benefits might be is in doubt. It has been found by microscopic examination that the fungal cells might even penetrate the cells of the algae in a similar manner to what occurs in pathogenic fungi, though for the most part the ease with which the nutrients leak from the algal cells means that it is not necessary for the fungal hyphae to penetrate the algal cells. Photobiont cells are destroyed routinely in the course of nutrient exchange, though the stability of the association depends on the rate of photobiont reproduction being higher than the rate of destruction. It is suggested by such observations that the fungal part of the lichen is in effect parasitising the algae and using for fungal nutrition the products of algal photosynthesis.

The method of reproduction of lichens is the production of small flakes, called soredia, which are composed of fungal filaments surrounding small groups of algal cells. Lichens also produce another type of reproductive structure, called insidia, which are fragile elongated upright outgrowths from the thallus that break off and are then dispersed. There are also many other types of lichens that fragment when they are dry and are dispersed by the wind, then resuming growth in the presence of moisture. The fragments in any of these cases include fungus and alga which means the composite organism is reproduced. There are also fungal sexual structures that are often produced abundantly, though the fungal spores that result need to find an algal partner that is compatible in order to form functional lichen.

Sources & Further reading

1. Moore, David, 2013, Fungal Biology in the Origin and Emergence of Life, Cambridge University Press.