Ecology of Thioploca spp.: Nitrate and Sulfur Storage in Relation to Chemical Microgradients and Influence of Thioploca spp. on the Sedimentary Nitrogen Cycle

Microsensors, including a recently developed NO(3)(−) biosensor, were applied to measure O(2) and NO(3)(−) profiles in marine sediments from the upwelling area off central Chile and to investigate the influence of Thioploca spp. on the sedimentary nitrogen metabolism. The studies were performed in undisturbed sediment cores incubated in a small laboratory flume to simulate the environmental conditions of low O(2), high NO(3)(−), and bottom water current. On addition of NO(3)(−) and NO(2)(−), Thioploca spp. exhibited positive chemotaxis and stretched out of the sediment into the flume water. In a core densely populated with Thioploca, the penetration depth of NO(3)(−) was only 0.5 mm and a sharp maximum of NO(3)(−) uptake was observed 0.5 mm above the sediment surface. In sediments with only few Thioploca spp., NO(3)(−) was detectable down to a depth of 2 mm and the maximum consumption rates were observed within the sediment. No chemotaxis toward nitrous oxide (N(2)O) was observed, which is consistent with the observation that Thioploca does not denitrify but reduces intracellular NO(3)(−) to NH(4)(+). Measurements of the intracellular NO(3)(−) and S(0) pools in Thioploca filaments from various depths in the sediment gave insights into possible differences in the migration behavior between the different species. Living filaments containing significant amounts of intracellular NO(3)(−) were found to a depth of at least 13 cm, providing final proof for the vertical shuttling of Thioploca spp. and nitrate transport into the sediment.