Taxonomic diversity and metabolic activity of microbial communities in the water column of the central Baltic Sea

Abstract

The water column of the Gotland Deep, an anoxic basin in the central Baltic Sea, was investigated as a representative and well-characterized part of a coastal marine ecosystem. We analyzed the overall structure of the bacterioplankton community by high resolution electrophoresis of the low-molecular-weight RNA (LMW RNA = 5S rRNA and tRNA). The results gave us a direct overview of the taxonomic diversity of the bacterial community (i.e. the number of taxa and their relative amounts in a sample). For further information about the identity of single taxa, we used partial sequencing of the 5S rRNA. Throughout the water column, we observed a rather limited number of abundant bacterial taxa (i.e. 3–7) with an increase of bacterial diversity in layers of increased turnover of organic matter. Comparison of samples taken from the euphotic zone during two consecutive summers indicated high similarity of taxonomic structure. One of the 5S rRNA bands occurring at the oxic—anoxic interface showed a partial sequence identical to Thiobacillus denitrificans, a species whose presence was hypothesized earlier from biogeochemical investigations. By using this novel approach, we could combine a direct overview of bacterial diversity with the ability to further analyze single species and thus provide means to elucidate major bacterial catalysts of biogeochemical cycling.