Hydrogen Sulfide Reduction of Symbiont Cytochrome c552 in Gills of Solemya reidi (Mollusca).

The gill of the protobranch clam Solemya reidi houses a dense population of intracellular symbiotic chemoautotrophic sulfur-oxidizing bacteria that fix carbon dioxide into sugars and supply the carbon nutrition of the host. The gill is divided into a bacteriocyte (cells with intracellular symbionts) domain and a domain of mitochondria-rich, symbiont-free ciliated cells. Optical spectra, recorded separately from each domain, are dominated by hemoglobin. Only oxygenated and deoxygenated hemoglobin were detected in the gill. In sharp contrast to the gill of the congener Solemya velum, ferric hemoglobin sulfide was not detected, suggesting that this species, if formed, is short lived. The spectral contribution of hemoglobin may be cancelled or subtracted in difference spectra. Difference spectra of each gill domain in nitrogen minus the same tissue in air show a complement of reduced cytochromes, demonstrating that both symbiont and mitochondrial cytochromes are reduced by endogenous substrate. Difference spectra of the bacteriocyte domain exposed to hydrogen sulfide (air containing 1.4 torr hydrogen sulfide minus air) show only the contribution of reduced symbiont cytochrome c552. The extent of reduction increases monotonically with ambient pH2S, suggesting that, by analogy with some free-living sulfur-oxidizing bacteria, cytochrome c552 is near the point of entry of electrons into the symbiont electron transport chain. Difference spectra of muscle or of the ciliated domain under these same conditions show reduced cytochrome c550, cytochrome b and cytochrome oxidase, suggesting that host mitochondria may accept electrons from hydrogen sulfide.