Lichens & Mycorrhizae

by Michael Colebrook

The soil is teeming with life (1). I want to focus on just two elements of this life, both of which involve that that rather odd group of organisms, the Fungi. By tradition fungi are included in the botanical curriculum, although it is now believed that the group are more closely related to the animals than to the plants. The Fungi now have the status of a kingdom all to themselves.

Like animals, fungi need complex organic nutrients, and all fungi are saprophytic, (living off dead organic matter) or parasitic (living off living organic matter). The fungi involved in the two systems I want to describe have found ways of associating themselves with green (photosynthetic) organisms and by co-evolving have established mutually beneficial, symbiotic, arrangements with their green partners.

Historically there has been a marked reluctance on the part of evolutionary biologists to accept the existence of intimate, mutually beneficial relationships between different species. In evolutionary theory, competition rules, at least until recently. In his Analysis of Biological Populations (1972) Williamson stated ‘[mutualism] is a fascinating biological topic, but its importance in populations is generally small’ (2). This echoes Beatrix Potter’s dismissive reception by the Linnnean Society to her suggestion that Lichens exist as permanent associations between Algae and Fungi (3).

It is not altogether surprising that Beatrix Potter had a problem persuading the Linnean Society (and Kew Gardens) that lichens were symbionts. They are emergent entities and exist in forms that are sufficiently consistent to be classified as if they were individual species. It is estimated that there are about 20,000 different forms. The photosynthetic components, green algae or cyanobacteria, are capable of independent existence, the fungi are not, they are obligate symbionts.

   
Lichens on Rocks Cumbria UK.
Photo Dave & Lynne Slater
Lichen crust on tundra, Iceland
Photo Erwan Balança

Lichens play a vital pioneering role in soil formation. Thin surface crusts of lichens are found in most seemingly barren sites from deserts to arctic tundra. They form the first layer of organic matter and, where the conditions are suitable they provide the basis for the subsequent formation of soil. Lynn Margulis describes the process:Algae growing under the protective cover of fungi cling to sheer rock, extend over its face, and ultimately break it down into soil that can be penetrated by roots of plants and fungal hyphal networks. The hard rock of this spinning planet has been crumbling for hundreds of millions of years into rich, nutritive soil as a result of the fungal-algal partnerships.(4)

Without the Lichens there would be no soil. Without the soil there would be no complex life on land. We are most aware of Fungi in the form of mushrooms (edible) and toadstools (some edible, some not). But the real body of an individual soil fungus consists of long and very fine (microscopic) tubular cells, called hyphae, forming a more of less extensive branching and sometimes networking system known as a mycelium. These are not insignificant or transient entities, the mycelium of a specimen of Armillaria ostoyae (the honey mushroom) in a national forest in Oregon is believed to cover an area of nearly 9 sqkm (2,200 acres) and is estimated to be 2,400 years old! This is exceptional, the familiar fairy rings, which indicate the presence of an active mycelium underground, are usually only a few metres in diameter.

Some Fungi have developed symbiotic relationships with land plants known as mycorrhizae, literally fungusroot, which is a good name, it describes exactly what they are. The fungal element in the partnership merges with and extends the root system of the host plant. The host benefits from the extended root system. The very fine fungal hyphae can penetrate into smaller interstices in the soil than even the fine root hairs of the plant and they are very good at extracting nutrients, especially phosphates, from the soil. The fungus benefits by receiving a share in the energy rich, photosynthetic, materials made by the host.

There are two distinct forms of mycorrhizae. In Endomycorrhizae the hyphae penetrate the cell walls of the host plant but the cell membranes remain intact. The hyphae spread out into the surrounding soil for relatively short distances. The fungal species cannot exist independently. Here are about 130 species of endomycorrhizal fungi and all belong to the phylum Glomeromycota. The number of host species is not known but comprises a significant proportion of the total number of plant species.

In Ectomycorrhizae the hyphae form layers around the roots of the host plant and do not penetrate the cell walls. The hyphae radiate out into the surrounding soil for up to several meters.

The fungi are nearly all from the phylum Basidiomycota (toadstool forming). There are about 5,000 ectomycorrhizal species and they form alliances with about 2000 species, mostly conifers and nearly all are trees.

One of the ectomycorrhizal fungi is the quintessential toadstool, the Fly Agaric (Amanita muscaria). This produces the well known, bright red capped toadstool flecked with white which clearly signals that it is not one of the edible kinds. It also forms fairy rings thatmight encircle several trees. The single mycelium may be associated with several host trees. Most of the host and fungal species can exist independently but do not flourish nearly as well as when part of a symbiotic association. It is interesting to speculate on the marked difference in the numbers of species of endomycorrhizal (c 130) and ectomycorrhizal (c 5000) fungi.

Taxonomists place the endo- species in four orders, and all the members of these orders are exclusively mycorrhizal. It would seem likely that the habit evolved once only and the existing species are all descended from a common ancestor through differentiation involved in forming relationships with a enormous variety of host species. There is fossil evidence for the existence of endomycorrhizal species in the early Devonian period (c 400million years ago), long before the emergence of flowering plants.

The ectomycorrhizal species are found in three orders but the species involved are not all mycorrhizal. It is suggested that ectomycorrhizal species emerged together with the appearance of Conifers in the late Mesozoic (c 150 million years ago). The taxonomy would also suggest that the habit emerged more than once, through parallel evolution.

It is impossible to overestimate the significance of mycorrhizal associations for the flourishing of plant life of all forms and in all locations, they are a key element in the life of the soil. Along with Charles Darwin’s beloved earthworms, mycorrhizae are a vital part of the nearly invisible and often discounted infrastructure of life on Earth.

References

1. James Nardi. Life in the Soil (University of Chicago Press, 2007).

2.MarkWilliamson. The Analysis of Biological Populations (Academic Press, 1972).

3. Michael Colebrook. ‘What have Lichens to do with Peter Rabbit’ (GreenSpirit, Summer 2002), p. 7

4. Lynn Margulis. The Symbiotic Planet (Weidenfeld & Nicolson,1998) p. 109.

Michael Allen. The Ecology of Mycorrhizae (Cambridge University Press, 1991

A classification of the whole Kingdom can be found at:

http://www.britannica.com/EBchecked/topic/222357/fungus/57975/Outline-ofclassification-of-fungi