Thursday, 31 May 2012

The Spots on the Rocks

Have you ever seen speckled rocks? Rocks with gray or colored patches thinly covering the surface, patches that are not actually parts of the rock itself. Such spots are usually lichens. They are not just stains on the rock, but actual living organisms adapted to living in hostile, extreme environments that cannot be occupied by other living things, at least other living things that we can see. They are most frequent on rocks in open sunny locales, such as those on the seashore or on exposed mountain crests. In such places only organisms adapted to extremes such  as heat, cold, desiccation, or salt spray can survive, and these kinds of lichens are masters of the extreme. Such thin lichens are known as crust lichens. Rock surfaces in shaded forested sites are moister, and less severe, and usually covered by mosses.

Look closely at some of these patches and you will see that there are different kinds of patches, and that they often have little spots and structures upon them. The different kinds of patches are different species, and the little spots are reproductive bodies, where spores are produced. On a surface that has been undisturbed for a long period of time the lichens grow outwards until they make contact, and no actual rock surface is visible. What you are seeing is a living skin which follows the shape of its stony substrate. It has, however, taken many years for this thin covering to reach that stage, as these coatings are very slow growing. One of them, the map lichen (Rhizocarpon geographicum) grows at such a steady rate that geologists use it to date such events as landslides and retreat of glaciers. A map lichen increases its diameter at less than a millimetre a year. The species is called map lichen because it looks like a map. It covers large areas of surface, is bright green in color and has little black fruiting bodies. This lichen of granite-type rockfaces is very common and due to its bright color is easily identified.

 Map Lichen (Rhizocarpon geographicum)
Photo by Kelly Sekhon

The crust lichens are the most common lichen group, but because of their simple structure are very difficult to identify. Their more complex relatives which have shrub and leaf-like growth patterns are much more easily distinguished from each other. To really be able to identify crust lichens you need to be an expert on crust lichens, not just an expert on lichens, and there are very few scientists with such expertise. Many of these organisms are so nondescript that  they cannot be told from each other without microscopic study, and special chemical tests. No wonder we know so little about them. Over the years a number of European crust lichen specialists have come to British Columbia and discovered species not previously known from the province, and even species that are completely new to science. 

So, what are these living crusts and how did they reach their stony homes? All lichens are  strange composite organisms - part fungus and part plant. They are able to grow in such places because the single-celled microscopic plants, algae, that live inside them contain chlorophyll, and that chlorophyll does the same as the chlorophyll in higher plants. It makes sugar. Most of the lichen, however, is composed of mold-like fungus threads. The mold uses the majority of the sugars to built its own tissues, and in return gives the alga a secure and safe home. It farms the microscopic plant in a way that is similar to the way we farm things. Most of the milk produced by cows feeds people not calves. We protect our cows and the fungus protects its alga.

Most of these crustose lichens are gray, roughly circular stains on their stony substrates, and they are very difficult to tell from each other. The most common of them is probably Porpidia. Although it is basically rather nondescript, it possesses three features which stand out. You have probably never seen these features, but once you do see them you will probably not be able to un-notice them when you next see them. It is a curious thing that many common things go un-noticed unless our attention is drawn to them. In many cases these phenomena, which we unconsciously experienced many times, cannot fail to be seen once the requisite pathways have been activated in the brain. Porpidia usually has many round, raised black spots all over its surface. These are the fruiting bodies, apothecia, where the spores are produced. They are especially noticeable when they grow in concentric circular patterns. The second feature appears when two Porpidias touch one another. A black line forms between them. Several different  fungi and lichens make such lines. It is not known why this is so, but the most commonly accepted theory is territoriality. This is a fence between two different individuals. Something like Robert Frost’s poem that “good fences make good neighbors”. The third feature can only be seen after the lichen is damaged or dies, and the underlying rock becomes visible. Rock lichens produce acids that dissolve and react with minerals in the rock. In this case there is a reaction with iron compounds, and rusty patches are made. And you thought that gray, nondescript lichens are boring!

Probably the most striking of the crust lichens are the firedots (Caloplaca sp.). Most of these are bright orange in color, and are most common at two extremes of elevation - at the seashore and on high alpine mountains. At both these extremes there is bright sunlight, but there is something else the firedots like, and that is lots of fertilizer. Most lichens live under low nutrient conditions, but these orange lichens are exceptions. They like to grow where the birds like to perch. It has been suggested that the high nitrogen or calcium levels in bird droppings are the critical factors. Whatever the reason next time you are at the  beach look at the boulders where the seagulls sit and see if there are any orange splotches. 

Lichens growing on rocks above the timberline.
Photo by Kelly Sekhon

Bare, exposed rock surfaces are some of the most extreme environments on the planet. Yet, even here life thrives. On casual inspection it may not look alive, but these organisms are growing, photosynthesizing, reproducing and making chemicals that only living things can make. It has even been suggested that some of these rock lichens could thrive on Mars! They are that adapted to harsh conditions. In fact some of them cannot survive in more benign situations. They are the pioneers. As the minerals are dissolved by the lichens the rock is pitted, and a thin layer of soil is produced. The rough surface is then able to support more complex organisms, such as mosses and larger lichens. Next time you pass some bare rocks take at look at the simple ecosystem that lives here. These are the pioneers.   

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