For my final year Ecology degree project, I chose the unlikely subject of using lichens as pollution monitors. Whilst the subject sounds as dry as an old ball of tumbleweed, I have to say it turned out to be a very interesting and worthwhile study.
The first thing to realise is that lichens are not a species of plant, but actually an agreement between an algae (a phycobiont) and a fungus (a mycobiont). Together, they live symbiotically. That is to say, they support each other. The fungus allows the algae to do the photosynthesis and provide the food; and the algae benefit from being housed securely in the body of the lichen (the thallus), where water can accumulate and prevent the free living algae drying out (water being needed for photosynthesis). So lichens are a kind of "super organism" being a collection of more than one species. And this allows them to eek out a living in otherwise harsh and exposed environments. Often, lichens are the first (or only) species on bare rocks or in tundra. NASA have even been able to grow lichens in space for several months at a time; that is, actually outside of the shutle and in the vacuum of space itself. They are hardy primitive symbionts!
The second thing to note about lichens is; they are sensitive to pollution. Whilst a lichen can endure quite extreme hardships of insolation (harsh sunlight and ultra violet radiation), extreme water stress, and freezing temperatures for many months if not years, even the smallest amount of air pollution can be their undoing.
The third thing to note is that not all lichens are built the same, and thus they have different responses to air pollution. Both fungi and algae are quite a varied evolutionary group, and are capable of quite diverse morphology (shape), so it follows they can sometimes change their spots to suit the circumstances. This has meant some lichens with favourable shapes and design (for example flat, simple water resistant varieties) are less susceptible to air pollution and, with no competitors, have been able to colonise urban areas with impunity. As air quality is a continuum, we get a predictable spread of lichen species throughout all different levels of air quality. So much so, that in 1970 scentists Hawksworth and Rose were able to draw up an index of lichen indicator species which could quite accurately predict the level of air pollution, (particularly sulphur dioxide concentrations) just from seeing which lichens were around. It is still in use today by meterologists.
So my project was to examine three particular species of lichen and monitor their distribution along a pollution gradient (a transect) from urbal Liverpool to the pure air forest of Clocaenog in North Wales. I looked at various factors which may account for the distribution I found, using literature reviews, experimental data, and field observations. I paid particular attention to the shape (morphology) of the three lichens and examined their gross (macroscopic) features, as well as their microscopic anatomy using a scanning electron microscope. I transplanted some of the pure air lichens to the top of John Moores University roof and studied them over a six week period of time to see if there was any degeneration in their structures. I also performed dehydration experiments on all the lichens to ascertain their "wettability" to see if this explained their different capacity to absorb dissolved sulphur dioxide in polouted rainwater.
My experiments kept me entertained for several months, and I visited the three sites in Liverpool (Sefton Park), the mountainous Moel Famau County Park, and the beautiful lush Clocaenog Forest with its ancient branches festooned with old man's beard lichen. Whilst I couldn't find any association between "wettability" and absorption of sulphur dioxide, or indeed elicit any decline in the thallus structure of the pure air lichens when they were transplanted to Liverpool, I was able to postulate that the more branched (or foliose) a lichen was, the more likely it was to hold water, and the less likely it was to withstand life in polluted urban areas. By contrast, flat lichens which kept themselves out of trouble and encouraged water run off, prospered in the big smoke.