Eukaryotic molecular biodiversity: systematic approaches for the assessment of symbiotic associations

'Biodiversity' addresses the wealth of species that constitute the biosphere. Notwithstanding that they have been regarded as mental constructs in the past, species are really existing entities that form and disappear in the course of evolution. Molecular techniques allow to trace the dynamics of speciation and to determine the relatedness of species and the genetic diversity within populations. These techniques also permit to recognize the incredible diversity of protists: their importance for the global conversion of biomass and energy had been greatly underestimated until recently. Because it is not possible to 'count' all species living on earth, a variety of approaches have been used to estimate global biodiversity. Such estimations are extrapolations of historical trends or of punctual assessments of the biodiversity of selected ecosystems. Therefore, new concepts are required to calculate global biodiversity. Systematic approaches that evaluate small, complex biotopes exhaustively, or that calculate the number of symbionts or parasites on the basis of their potential hosts have already led to a substantial revision of earlier estimations. Here, an evaluation of potential animal hosts for methanogenic archaea and intestinal protists is described that reveals the importance of host taxonomy for the assessments. If molecular techniques can confirm the presumed specificity of symbiotic and parasitic associations a substantial revision of the current assumptions about the biodiversity of such organisms will be necessary.