Biodiversity and Ecosystem Functioning in Transitional Waters; the point of view of a microbial ecologist

Abstract

1 - The study of biodiversity of prokaryotes in transitional waters is hampered by the fact that we do not posses any comprehensive inventories of prokaryotic species. Microbial biodiversity is thus often estimated indirectly by extraction of nucleic acids from the natural environment and by using 16S ribosomal RNA gene sequences as phylogenetic and taxonomic markers. Experimental microbial ecology has contributed over the years to developing niche-assembly theories of community assemblage and I review some of their major findings. Finally, the aim of establishing a link between biodiversity and ecosystem functioning is quite recent and has prompted a novel experimental approach that can be applied in microbial ecology. The following observations have been forwarded. 2 - The species richness for prokaryotes is difficult to assess in transitional waters, because of an unclearly defined species concept and because the Bacteriological Code requires living cultures as type strains for taxonomic descriptions; therefore, the number of validly described prokaryote species is still very limited (i.e. currently less than 10,000). 3 - The major emphasis on niche assembly theories in microbial ecology is a consequence of the paradigm "everything is everywhere, but the environment selects" quoted by Baas Becking in 1934. Hence, insights on how competition mechanisms can contribute to sustaining microbial biodiversity in relatively homogeneous model systems have tremendously increased during the last decade. Nevertheless, I argue that coastal lagoons and other transitional waters are environments characterised by strong exchanges with adjacent local microbial communities, where community assemblage may be strongly impacted by dispersal processes. 4 - The study of biodiversity and ecosystem functioning is based on quantifying a process rate or a yield (the ecosystem function) as a function of a manipulated biodiversity in an artificial community. A positive relation between biodiversity and ecosystem function has been explained by 1) sampling effect (i.e. a higher biodiversity correlates with a higher chance of providing a home for the most productive species), 2) more efficient resource exploitation for species showing complementary niche differentiation. I argue that synergistic phenomena due to positive interactions among species may also play an important role.