Possible roles of sulfur-containing amino acids in a chemoautotrophic bacterium-mollusc symbiosis

Invertebrate hosts of chemoautotrophic symbionts face the unique challenge of supplying their symbionts with hydrogen sulfide while avoiding its toxic effects. The sulfur-containing free amino acids taurine and thiotaurine may function in sulfide detoxification by serving as sulfur storage compounds or as transport compounds between symbiont and host. After sulfide exposure, both taurine and thiotaurine levels increased in the gill tissues of the symbiotic coastal bivalve Solemya velum. Inhibition of prokaryotic metabolism with chloramphenicol, inhibition of eukaryotic metabolism with cycloheximide, and inhibition of ammonia assimilation with methionine sulfoximine reduced levels of sulfur-containing amino acids. Chloramphenicol treatment inhibited the removal of sulfide from the medium. In the absence of metabolic inhibitors, estimated rates of sulfide incorporation into taurine and thiotaurine accounted for nearly half of the sulfide removed from the medium. In contrast, amino acid levels in the nonsymbiotic, sulfide-tolerant molluscs Geukensia demissa and Yoldia limatula did not change after sulfide exposure. These findings suggest that sulfur-containing amino acids function in sulfide detoxification in symbiotic invertebrates, and that this process depends upon ammonia assimilation and symbiont metabolic capabilities.