Oxygen- and Nitrogen-Dependent Sulfur Metabolism in the Thiotrophic Clam Solemya reidi.

We investigated aerobic and anaerobic thiotrophic metabolism by the gutless clam Solemya reidi and its intracellular symbiotic bacteria. Mean environmental sulfide concentrations in porewater next to animals varied from a high of 888 μM to a low of 17 μM in different sediment samples, while mean thiosulfate concentrations were very low (1-13 μM). The blood of freshly collected clams contained up to 300 μM thiosulfate but little sulfide (≤12 μM). In experimental incubations, clams were able to take up thiosulfate, yet under no conditions could the animals concentrate thiosulfate above external concentrations. Thiosulfate accumulation in the blood during incubations was the result of aerobic but not anaerobic sulfide oxidation by the animals. This finding and previous observations of the presence of high concentrations of thiosulfate in the blood of field-caught clams indicate that the animal portion of the symbiosis normally functions aerobically. The intact symbiosis exhibited nitrate and nitrite respiration under anoxic conditions. Nitrate respiration was stimulated by sulfide, as well as thiosulfate, while nitrite respiration was only stimulated by sulfide. Nitrate respiration also occurred when whole animals were under oxic conditions. Respiration measurements showed that the bacterial symbionts were capable of direct sulfide oxidation. Sulfide-stimulated oxygen consumption by bacterial preparations from the gills of mud-maintained clams reached a maximum rate at 25 μM sulfide and showed no apparent inhibition at sulfide concentrations up to 1 mM sulfide.