JOYNER, J.L.*; LEE, R.W.: The role of sulfur-containing amino acids in sulfide detoxification by chemoautotrophic bacteria-mollusc symbioses

Molluscs living at hydrothermal vents and in reducing sediments are exposed to high levels of the metabolic toxin, hydrogen sulfide. Many of these molluscs harbor intracellular chemoautotrophic bacteria. The symbiotic bacteria oxidize hydrogen sulfide to power the fixation of CO₂ into carbohydrates. How the molluscs transport toxic hydrogen sulfide to the symbionts is not fully known. Incorporation of sulfur into amino acids such as taurine and thiotaurine may prevent sulfide-induced damage to host cells and maintain free amino acid pools in host tissues. High levels of taurine and thiotaurine have been documented in tissues of several vent and reducing sediment molluscs. We examined the possible role of taurine and thiotaurine as intermediates or endproducts in sulfide detoxification using Solemya velum as a model organism. The protobranch bivalve S. velum lives in sulfidic sediments and harbors chemoautotrophic symbionts in its gill tissue. Clams were incubated in a flow-through respirometry system under controlled sulfide and oxygen conditions. Sulfide exposure resulted in increased taurine and thiotaurine levels in S. velum gill tissue. Immunocytochemical studies showed localization of taurine to the host cytosol or to the symbionts, depending upon the experimental treatment. In comparative investigations involving non-symbiotic sulfide-tolerant bivalves (Geukensia demissa and Yoldia sp.), we did not find a relationship between gill taurine or thiotaurine levels and sulfide exposure. These results indicate that production of taurine in response to sulfide exposure may be a capability of symbiotic but not non-symbiotic molluscs and an important mechanism of sulfide tolerance. This work was supported by NSF IBN 0076604.