Lichens - fungi that have learned the art of gardening

Lichens are a familiar sight on walls and trees, especially in areas free of pollution. They are the pioneers of the natural world, living in often hostile conditions, and converting stone and wood into soil. The following image is a wonderful example of this pioneering process:

The fencepost which forms the basis of the eco-system we see in the image was put in position around 50 years ago. A series of lichens would have appeared very quickly in succession, each one preparing the post for the next. As the lichens died they would leave dead fungal matter, dead algae and rotted wood. Sooner or later a moss spore will have found this mixture suitable as a substrate and the moss began to live (and die) there. This decomposed vegetation eventually provided sufficient soil for the Bilberry to take root. So in a very short number of years, we have a sterile fencepost being turned into a habitat suitable for fruiting plants. It should be noted that the post still hosts a number of lichens and mosses in addition to the Bilberry. The process on stone takes slightly longer.

If you read any literature on lichens, you will be told in no uncertain terms that "lichens are a symbiotic relationship between a fungus and one or more algae or cyanobacterium". Whilst it's true that each lichen contains a fungus and one or more algae, with the combination being named after the fungal partner, I think the relationship is rather one-sided:

• Firstly, the algal partner can - and often does - survive on its own, whereas the fungus cannot. (Typical examples are Trebouxia and Nostoc, both of which I have found growing freely in a number of places.)
• Secondly, the reproductive system of a lichen is either purely fungal, or a combination of a fungal component with a captive parcel of algae.

We need to look at what each partner adds to the relationship. Clearly, the alga produces energy from sunlight - something the fungus cannot do, so the fungus is the major beneficiary here. The fungus provides the physical structure of the lichen - basically a house for the alga, which is constrained by the lichen, and is only able to reproduce within the confines of that structure, and is unable to free any part of itself as independent offspring. It seems to me that the alga is trapped and imprisoned by the fungus largely for the benefit of the fungus. Hardly a fully symbiotic relationship.

The following image shows the fruitbodies of a lichen that clearly demonstrates the fungal nature of the reproductive system. Lichenomphalia umbelliferae is a lichen that grows in association with mosses. The fruitbodies are about 10 mm across the cap.


The actual situation is much more complex, of course, with some fungi that can be part-time or opportunistic lichens. A couple of years ago, I took this picture of Boletus erythropus with a golfball-sized lump of algae on its stipe. Notice the lichen-like appearance of the surface of the lump.
This 'skill' of harnessing the photosynthetic properties of algae must be deeply embedded in fungi.