Chemolithoheterotrophy in a metazoan tissue: Sulfide supports cellular work in ciliated mussel gills

Hydrogen sulfide, a common constituent of marine intertidal sediments, is b oth a potent toxin of aerobic cellular respiration and an electron-rich mol ecule used by some prokaryotic organisms as a source of energy, In ciliated gills from Geukensia demissa, a marine mussel from sulfide-rich sediments, sulfide oxidation supports cellular work, Evidence for this comes from mea surements of ciliary beat frequency (f(CB)) as a measure of ATP turnover ra te, the rate of gill oxygen consumption ((M) over dot (O2)) as a measure of ATP production rate, and mitochondrial cytochrome redox state as an indica tor of the path of electron flow, Results from experiments performed in the presence and absence of the mitochondrial complex III inhibitor antimycin A to limit endogenous carbon substrate oxidation showed that exposure to su lfide stimulated oxygen consumption and ciliary beating, with cytochrome c being the dominant reduced species. These results, along with the resultant f(CB)/(M) over dot (O2), ratio, are qualitatively and quantitatively consi stent with the hypothesis that electrons from sulfide oxidation support mit ochondrial ATP production, We propose that Geukensia demissa gills use sulf ide as a respiratory substrate when given the choice and thus function meta bolically as facultative chemolithoheterotrophs. Similar conclusions could not be drawn for the ciliated gills from Mytilus edulis, a marine mussel fr om aerated habitats, or for the ciliated lungs from the phylogenetically di stinct leopard frog Rana pipiens.