Free-living and aggregate-associated Planctomycetes in the Black Sea.

We examined the distribution of uncultured Planctomycetes phylotypes along depth profiles spanning the redox gradient of the Black Sea suboxic zone to gain insight into their respective ecological niches. Planctomycetes phylogeny correlated with depth and chemical profiles, implying similar metabolisms within phylogenetic groups. A suboxic zone sample was split into > 30 and < 30 μm fractions to examine putative aggregate-attached and free-living Planctomycetes. All identified Planctomycetes were present in the > 30 μm fraction except for members of the Scalindua genus, which were apparently free-living. Sequences from Candidatus Scalindua, known to carry out the anammox process, formed two distinct clusters with nonoverlapping depth ranges. One cluster, only 97.1% similar to the named species, was present at high nitrite/nitrate and low ammonium concentrations in the upper suboxic zone. We propose this sequence type be named 'Candidatus Scalindua richardsii'. A second cluster, containing sequences more similar to 'Candidatus Scalindua sorokinii', was present at high ammonium and low nitrite conditions in the lower suboxic zone. Sequences obtained from the sulfidic zone (1000 m depth) yielded Planctomycetes from two uncharacterized Planctomycetacia clusters and three potentially new genera as well as sequences from the uncultured OP3 phylum.