VERTICAL-DISTRIBUTION OF SULFATE-REDUCING BACTERIA AT THE OXIC-ANOXICINTERFACE IN SEDIMENTS OF THE OLIGOTROPHIC LAKE STECHLIN

Vertical distribution of sulfate-reducing bacteria (SRB) and sulfate r eduction rates were investigated in littoral and profundal sediments o f the oligotrophic Lake Stechlin. SRB counted by the most probable num ber (MPN) technique showed two peaks, one at the oxic-anoxic interface , and the second deeper in the reduced sediment layer. Sulfate reducti on rates determined as potential rates in anoxic sediment slurries sho wed a similar vertical distribution with the highest values in the ano xic 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 l ayers in numbers similar to those detected in anoxic zones. From the h ighest positive MPN tubes 34 pure cultures were isolated. Physiologica l characterization including nutritional behaviour, tests for desulfov iridin and spore formation turned out that nearly all isolates belonge d 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 sedim ent layers, However, no sulfate reduction was observed in the presence of oxygen, since oxygen was preferentially reduced.