All you have to do is look

Lichens are always described as a fungus and one or more algae (or cyanobacteria) living in symbiosis. The fungus provides structure and anchoring for the partnership and the algae transform sunlight into food via photosynthesis. I'm not so convinced that the partnership is quite as symbiotic as we're led to believe: in a symbiotic relationship, the partners cooperate to mutually benefit each other, but I think the fungus has the upper hand in a lichen for a number of reasons:

1) The methods of reproduction are all either purely fungal (via fungal fruitbodies that produce fungal spores), or are controlled by the fungal partner (broken or ejected fragments of lichen contain the fungus with a convenient bundle of algae already confined, ready for the next specimen).

2) The algae that are found in lichens can happily exist independently of the partnership: the fungal component cannot.

3) The fungal partner benefits from the food produced via photosynthesis which it cannot perform by itself, but what does the alga receive in return, apart from a place to live?

4) The alga can only reproduce within the confines of the lichen body: as the lichen grows, there is more space for the alga to grow.

I rather think the partnership is heavily weighted in favour of the fungal partner. 

This is a specimen of Graphis scripta (so-called because the spore-producing fruitbodies closely resemble handwriting):

Thallus of the lichen Graphis scripta

Notice the shape of the lichen: it is much wider than it is tall, and for a while I wondered why this characteristic shape is the norm for so many of our crustose lichens. The answer is quite simple: in percentage terms, a tree expands in girth much more quickly than it expands in height. The lichen grows at a common rate (perhaps a millimetre per year) in all directions from its starting point, but the bark of the tree expands more quickly horizontally than it does vertically. Over the years the lichen takes on this characteristic shape in response to the growth of the tree.

Here's another example:

Thallus of the lichen Pertusaria hymenea

I have actually been photographing the above specimen for perhaps 8 years now, and it's beginning to show its age: The thallus has been eaten away in a number of areas, most likely by slugs or snails. The bark of the tree has also split on the left, and this has almost split the original lichen into multiple specimens. Finally, Ivy has been growing on the tree and some of its tendrils have been pulled away, taking some of the thallus with them (you can see traces of the Ivy tendrils at the lower edge of the lichen, to the left of centre, looking like a fossil, and in the pale line to the right of the image).

(The dark blotches to the upper right and lower centre are the liverwort Frullania dilatata.)

I took a close-up of a much younger specimen of the Pertusaria, and you can see that it is much more rounded at this stage, but the vertical cracks that will spread it horizontally have started to appear:

I suppose the spaces get filled in with new thallus body as the lichen grows. So here we have a fungus that not only knows how to garden (it cultivates the alga), but has also learned a bit of DIY to plaster over the cracks.

(Click here for an image of the Pertusaria from 2004)

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.

Glenullin Bog

Some of you know that I have been awarded the contract to take the photographs for a wildlife booklet based around Glenullin bog in Northern Ireland. Glenullin, which was rescued from peat development, is an important raised bog with a 13m high dome, shown in the following shot:

Most of the red colouration comes from the leaves of Cotton grass, but there's another source of red in the fruit-bodies of this Cladonia lichen:


I'm not sure of the exact species, but I think it's somewhere close to Cladonia macilenta or Cladonia diversa.


I'll be showing more photographs from Glenullin as the season progresses.

This is the view south-west from my bedroom window at 8 o'clock this morning: