Structurally, lichens are among the most bizarre of all forms of life. Lichens are a complex life form that is a symbiotic partnership of two separate organisms, a fungus and an alga. The dominant partner is the fungus, which gives the lichen the majority of its characteristics, from its thallus shape to its fruiting bodies. The alga can be either a green alga or a blue-green alga, otherwise known as cyanobacteria. Many lichens will have both types of algae.
In this amazing association the fungus benefits from the algae because fungi, having no chlorophyll, can't photosynthesize their own food. A lichen's fungal part is thus "fed" by its photosynthesizing algal part. The algae benefit from the association because the fungus is better able to find, soak up, and retain water and nutrients than the algae. Also, the fungus gives the resulting lichen shape, and provides the reproductive structures. This kind of relationship between two or more organisms, where both organisms benefit, is known as mutualism.
Lichens colonize some of the most inhospitable habitats on earth. They can survive in extremely cold areas such as on high mountains and in regions such as the arctic. They may be virtually the only plant form surviving in some of these areas and can be vitally important sources of food for animals. They are also found throughout less extreme climates, inhabiting just about any solid surface. This can range from rocks on sea shores, to walls, trees and concrete. A few are unattached and blow about freely.
There is uncertainty over the exact nature of the relationship between the fungus and the alga. Some people think the fungus may be a type of weak parasite, which doesn't kill all of the algal cells or, that it keeps the alga imprisoned as a kind of slave. Alternatively, it may be a type of relationship called a 'symbiosis' where both partners benefit. Whatever the exact nature of the relationship, it undoubtedly results in an 'organism' which is capable of surviving in places where neither the fungal partner, nor the algal one, could survive on their own.
Lichens grow relatively slowly. The actual growth rate depends both on the species and on the environmental conditions around it. The smaller encrusting lichens may grow as little as 1mm a year! Larger forms may grow up to 1cm per year.
This slow growth rate has been used to develop a method of dating surfaces on which lichens are growing. The method, known as lichenometry, has been used in places such as the arctic, where lichens grow very slowly and can live for very long times. The method works by using a series of photographs over a period of time, to work out the growth rate of the particular lichen. From the size of the lichen, it is then possible to calculate how long it has been growing there. Using this method, some individual lichen colonies have been estimated to be 9000 years old. If this is so, then these particular lichens may well have been alive while people were still in the Stone Age and woolly mammoths roamed.
Lichens absorb water and minerals from rainwater and directly from the atmosphere, over their entire surface area. This makes them extremely sensitive to atmospheric pollution. As a result, there are usually very few lichens around industrial centres and towns. Different lichen species vary in their tolerance to pollution and therefore make very good biological indicators of levels of atmospheric pollution.
A walk around your local churchyard can often reveal a lot about air quality in your area. Churchyards are usually relatively undisturbed areas, with stone headstones which provide a good substrate for lichens. A good look at these lichens will give an indication of how good the air quality is locally.
Lichens have had a wide variety of uses over the ages. Before the advent of modern dyes they were extremely important sources of dyes for clothing. Different lichens yielded different dye colours and they could be mixed to produce a wide variety of colours. Lichens also have an interesting chemistry and produce a large number of acids, many of them found only in lichens. The litmus dye used so widely as an acid/alkaline indicator in chemistry comes from lichens. Some species also have antibiotic properties. Some of the lichen acids are utilized in drugs that can be more effective than penicillin. One of the more bizarre uses of lichens from the past is as packing material for ancient Egyptian mummies.
This image was taken using flash lighting underneath and to the right, almost at 90 degrees to the subject in order to make sure that the background doesn't intrude on the lichen. I used a 100mm Pentax SMC-A f4 macro lens mounted on bellows.