Gill Hemoglobin May Deliver Sulfide to Bacterial Symbionts ofSolemya velum(Bivalvia, Mollusca)

Two different hemoglobins occur in nearly equal concentrations in the gill of the bivalve mollusc, Solemya velum (total hemoglobin concentration is 200 µM/kg wet weight gill). A spectrophotometric study of intact gill filaments demonstrates that in the absence of sulfide, the gill hemoglobin may be oxygenated and deoxygenated, with part (5-20%) in the aquoferric form. In the presence of sulfide, about half of the gill hemoglobin is rapidly and reversibly converted to ferric hemoglobin, which then binds sulfide to form ferric hemoglobin sulfide (ferric hemoglobin with sulfide ligated to the heme iron in the distal ligand position); the balance continues to bind oxygen as oxyhemoglobin. S. velum inhabits reduced marine sediments where oxygen and hydrogen sulfide meet, and houses a dense population of intracellular chemoautotrophic sulfur-oxidizing symbiotic bacteria in its gill. We suggest that gill hemoglobins may mediate sulfide and oxygen delivery to the bacterial symbiont. Because sulfide is the dominant electron donor to fuel the Solemya/bacteria symbiosis, a cytoplasmic sulfide-binding protein that prevents the spontaneous reaction of sulfide with oxygen may be of utility in the nutrition of the animal.