Vertical distribution of sulfate-reducing bacteria at the oxic-anoxic interface in sediments of the oligotrophic Lake Stechlin

Vertical distribution of sulfate-reducing bacteria (SRB) and sulfate reduction rates were investigated in littoral and profundal sediments of the oligotrophic Lake Stechlin. SRB counted by the most probable number (MPN) technique showed two peaks, one at the oxic-anoxic interface, and the second deeper in the reduced sediment layer. Sulfate reduction rates determined as potential rates in anoxic sediment slurries showed a similar vertical distribution with the highest values in the anoxic zones. The highest cell numbers observed were 1.3 x 10(5) cells cm(-3) with lactate as substrate. SRB were also found in oxic sediment layers in numbers similar to those detected in anoxic zones. From the highest positive MPN tubes 34 pure cultures were isolated. Physiological characterization including nutritional behaviour, tests for desulfoviridin and spore formation turned out that nearly all isolates belonged to the genera Desulfovibrio, Desulfomicrobium, and Desulfotomaculum. SRB from the oxic sediment layers revealed a higher oxygen tolerance and capacity of oxygen respiration than isolates from the anoxic sediment layers, However, no sulfate reduction was observed in the presence of oxygen, since oxygen was preferentially reduced.