Distribution, ecology and molecular identification of Thioploca from Danish brackish water sediments.

The distribution of Thioploca populations was investigated in Danish fjords, brackish lakes and coastal waters. Thioploca was found in three geographically distinct populations, where biomasses reached 33.8+/-14.3 g wet weight m(-2) (mean+/-SD). Mats or lawns were not formed at the sediment surfaces and Thioploca biomasses peaked 4-7 cm into the sediment and extended down to 18 cm depth. Morphology and 16S rRNA gene sequences classified all populations as Thioploca ingrica. A sequence divergence of 1.7-2.2% indicated that T. ingrica comprise at least two genotypes. Physiological analysis showed that T. ingrica accumulate nitrate in concentrations of approximately 3 mM and that bicarbonate and acetate are used as a carbon source. The presence of oxygen promoted carbon incorporation, but T. ingrica could survive up to 3 months without an external supply of nitrate or oxygen. Thioploca ingrica populations were exclusively found close to river outlets in a bioturbated sediment with separate sulphidic spots and worm burrow walls containing nitrate and oxygen. It is hypothesized that the subsurface T. ingrica have a special advantage in this heterogeneous environment using their sheath surrounding the bacterial trichomes when navigating between electron donor and acceptor.