HEME-PROTEINS IN SULFIDE-OXIDIZING BACTERIA MOLLUSK SYMBIOSES

A number of bivalve mollusc species in the families Vesicomyidae, Luci nidae and Solemyidae from sulfide-rich sediments harbor sulfide-oxidiz ing bacteria as intracellular symbionts in gills. Cytoplasmic hemoglob in, relatively uncommon in symbiont-free gills, is a nearly constant f eature in these symbiont-harboring bivalve gills and may function in t he delivery of oxygen and sulfide to ensure symbiont autotrophy and ho st cell respiration. However, biochemical characteristics of isolated hemoglobins from these species differ vastly. For example, within clam s of the same genus, gill hemoglobin concentrations vary from micromol ar to millimolar. Ligand reaction rate constants of multiple gill hemo globins from a single species differ by three orders of magnitude. Gil l hemoglobins from congeners form different derivatives in the presenc e of sulfide. Some hemoglobins react avidly with hydrogen sulfide whil e others appear refractory. This assortment of characteristics suggest s that the role of hemoglobin in each symbiotic association cannot be generalized.