Abstract
ABSTRACT The large tropical lucinid clam Codakia orbicularis has a symbiotic relationship with intracellular, sulfide-oxidizing chemoautotrophic bacteria. The respiration strategies utilized by the symbiont were explored using integrative techniques on mechanically purified symbionts and intact clam-symbiont associations along with habitat analysis. Previous work on a related symbiont species found in the host lucinid Lucinoma aequizonata showed that the symbionts obligately used nitrate as an electron acceptor, even under oxygenated conditions. In contrast, the symbionts of C. orbicularis use oxygen as the primary electron acceptor while evidence for nitrate respiration was lacking. Direct measurements obtained by using microelectrodes in purified symbiont suspensions showed that the symbionts consumed oxygen; this intracellular respiration was confirmed by using the redox dye CTC (5-cyano-2,3-ditolyl tetrazolium chloride). In the few intact chemosymbioses tested in previous studies, hydrogen sulfide production was shown to occur when the animal-symbiont association was exposed to anoxia and elemental sulfur stored in the thioautotrophic symbionts was proposed to serve as an electron sink in the absence of oxygen and nitrate. However, this is the first study to show by direct measurements using sulfide microelectrodes in enriched symbiont suspensions that the symbionts are the actual source of sulfide under anoxic conditions.