THE GILL SYMBIONT OF THE HYDROTHERMAL VENT MUSSEL BATHYMODIOLUS-THERMOPHILUS IS A PSYCHROPHILIC, CHEMOAUTOTROPHIC, SULFUR BACTERIUM

Certain hydrothermal vent invertebrates, e.g. Riftia pachyptila and Ca lyptogena magnifica, are clearly established as harboring dense popula tions of chemoautotrophic sulfur bacteria in specialized tissues. By c ontrast, the physiological characteristics of the abundant intracellul ar gill symbiont of the vent mussel Bathymodiolus thermophilus have be en questioned. The low activities of enzymes diagnostic for CO2 fixati on (Calvin cycle) and for sulfur-driven energy generation, as measured by other investigators, have been attributed to bacterial contaminati on of the gill surface. Based on research at the Galapagos Rift hydrot hermal vents in 1988 and subsequent laboratory experiments, the curren t study confirms that the B. thermophilus symbiont is a psychrophile f or which thiosulfate and sulfide stimulate CO2 fixation. It strongly i ndicates that the symbiont is a chemoautotroph by establishing the fol lowing: (1) Sulfide and thiosulfate can stimulate CO2 fixation by part ially purified symbionts by up to 43-fold and 120-fold, respectively; (2) the ribulose-1,5-bisphosphate carboxylase/oxygenase activity of th e symbiont is sufficient to account for its sulfide- or thiosulfate-st imulated CO, incorporation; (3) the symbiont's molar growth yield on t hiosulfate, as judged by CO2 incorporation, is indistinguishable from that of free-living chemoautotrophs. Due to the high protein-degrading activity of B. thermophilus gill lysate, it is also suggested that ho st lysis of symbionts plays a more important role in the nutrition of the vent mussel than in R. pachyptila or C. magnifica, for which no co mparable protein-degrading activity was found.