Phylogeny and Abundance of Novel Denitrifying Bacteria Isolated from the Water Column of the Central Baltic Sea

The Baltic Sea is an estuarine ecosystem where denitrification in the low oxic and anoxic parts of the deep water contributes significantly to the nitrogen budget. Seventy-six heterotrophic, denitrifying, strains have been isolated by four cultivation procedures from the water column of the Gotland Deep, the main anoxic basin of the Central Baltic. Phylogenetic positions of representative strains of 10 different genotypes, grouped beforehand by low molecular weight (LMW) RNA profiling, were estimated by 16S rRNA sequence analysis. The 10 genotypes consisted of two members of the alpha subclass of the Proteobacteria and eight members of the gamma subclass. The major fraction of the genotypes was considered to be novel species or even genera. The gamma-Proteobacteria were the most abundant of the denitrifying isolates (96% of the total isolates) with a predominance of Shewanella baltica (77%), whereas the alpha-Proteobacteria were represented by single isolates. The diversity spectrum of Baltic sea denitrifying isolates was rather distinct from that previously described for marine and freshwater environments. Denitrifying bacteria could be isolated from all depths of the water column with the highest diversity and abundance of genotypes detected in samples of the oxic-anoxic interface, the layer of high in situ denitrification. For success of isolation of phylogenetically divers denitrifiers, both sample origin and cultivation procedure were observed to have an impact.