Diversity of Microbial Communities: The Case of Solar Salterns

Abstract

1. Questions The diversity of microbial communities is a very elusive reality (Pedrós-Alió 1993). As microbial ecologists we assume there is a certain number of species in an ecosystem and that each one of those species is represented by a certain number of individuals. If we could only identify all those little balls and rods under the microscope and assign each one to the appropriate species then, we think, we would know the diversity of that microbial community. The number of species would tell us the richness. An index such as Shannon's would tell as the diversity, and another index such as the equitability index would provide an estimate of evenness. These indexes could be calculated for different communities and their diversities compared. Unfortunately, there are several problems when attempting to do such a calculation. The first problem, of course, is the difficulty in delineating what a microbial species is. However, this is not easy for any living being. There are many definitions of species for large animals and plants for example (Rosselló-Mora and Amann, 2001) and it is very difficult to delineate some species even with the well-studied birds and mammals. Fortunately, diversity can be calculated with other units besides species. All that is needed is to group individuals into non-overlapping classes according to a consistent classification criterion (Kolasa and Biesiadka, 1984; Margalef, 1974). Several descriptors that fulfill these requirements (and are appropriate for microbes) are the number of distinct populations in a flow cytometry graph, the number of DNA bands in a DGGE gel, or the number of clones in a DNA clone library. These units are operational taxonomic units (OTU). A second problem concerns the sample size. In a forest, for example, the number of tree species found increases as the area of forest sampled increases. Eventually, the number of species stabilizes, when the area sampled is enough to accommodate all the tree species in that ecosystem. In fact, the shape of the species vs. sample size curve is very informative about the structure of the community. If we were to increase the sample size much further we would find a size where the number of new species would increase again. This is the result of having a sampling area larger than an ecosystem. If we were sampling a Mediterranean oak forest, and we were to keep increasing the sample area, we would eventually include a portion of a pine forest growing next to the oak forest. Obviously the number of new species would suddenly start to increase. What is the size