Gnarly

Lichens are truly fascinating, being made up of two or more closely interacting organisms, a fungus, and one or more partners, called photobionts. The photobiont may be an alga and/or cyanobacteria, both of which can produce simple sugars by photosynthesis. In contrast, fungi are 'heterotrophic' and require an external source of food. The fungi build the structure of the lichen thallus, within which they provide conditions for a long term, stable association with their photobionts, the basis of the lichen symbiosis. There is also convincing evidence for a consistent presence of non-photosynthetic bacteria within the thalli of all lichens so far examined, although the role of these bacteria is as yet unknown.

There is some debate about the exact nature of the symbiotic association between the lichen-fungi and their photobionts. Are the fungi ‘farming’ the photobionts in a controlled parasitism, or are the photobionts gaining some benefit also? There is good evidence for the lichen symbiosis as a mutualism, in which both partners benefit from the relationship. It is clear that fungi obtain their carbon-source in the form of simple sugars, but the photobionts seem also to be provided with optimal living conditions, in which their populations are often much larger than outside lichens. The photobiont probably also benefits from improved access to mineral nutrients which are provided because of fungal digestion outside their cells. Last, but not least, the interior of lichens is often a place richly infused with complex secondary fungal chemicals found nowhere else in nature, and these compounds are likely to play a role in protection from UV radiation, desiccation, and grazing by herbivores as well.

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. Secondly, some lichens which can form stable associations with their ‘usual’ host algae, form parasitic-type interactions with non-host algae when grown in the lab. 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. These can be specialised opportunistic parasites or saprophytes or even symbionts, competing for nutrients with other fungi in the lichen thallus.

This winter I'm going to try and get to grips with identifying lichens, starting with those that grow on tree bark. I think this is Punctelia subrudecta, a common species in southern Britain, which lives on well-lit trees. I found it on the branches of a fallen oak - originally it would have been high up in the canopy..