Animals that harbor symbiotic sulfide-oxidizing chemoautotrophic bacteria (thiotrophic symbioses) must expose themselves to both oxygen and sulfide to provide these substrates to their symbionts. The production of free radicals during sulfide oxidation has previously been proposed, but not proven, and if true, this could impose an oxidative stress on these symbioses. Here, I show that that both oxygen- and sulfur-centered free radicals are produced during sulfide autoxidation, and that this does impose an oxidative stress on organisms. Activities of antioxidant enzymes in animal tissues of the symbiotic bivalve Solemya velum and the related but non-symbiotic protobranch Yoldia limatula increase following simultaneous exposure to oxygen and sulfide. Furthermore, I present evidence that low-molecular-weight compounds, particularly glutathione, are important antioxidants in these animals.

In addition to these radicals, sulfide autoxidation produces substantial chemiluminescence. This sulfide-dependent chemiluminescence may explain light emission by the deep-sea hydrothermal vents.

Most superoxide dismutase activity in S. velum derives from the mangano- form of the enzyme, which in eukaryotes is usually restricted to the mitochondria. The unusual proportion of Mn-SOD in this clam may be owing to the need to maintain activity in the face of chronic sulfide exposure, since the usual cytosolic cupro-zinc SOD is inhibited by sulfide, whereas Mn-SOD is probably not.

Tissue homogenates from these clams and from three phylogenetically diverse species of hydrothermal vent invertebrates are able to suppress the formation of free radicals during sulfide autoxidation to differing degrees. In general, tissues from species which are from the highest sulfide environments (S. velum, Riftia pachyptila, and Bythograea thermydron) suppress radical formation in the presence of 1 mM sulfide, whereas those from species (Y. limatula and Bathymodiolus thermophilus) which inhabit environments having lower levels of sulfide cannot. This does not mean that defenses of the latter species are insufficient to protect against sulfide-dependent oxidative stress, because these species probably never experience sulfide concentrations as high as 1 mM. Rather, the interspecific difference indicates that these defenses are particularly robust in those species which obligately experience high sulfide concentrations.