Oxidation of reduced sulfur compounds requires the presence of oxidized reagents such as oxygen and nitrate. Hydrothermal vents are characterized by conditions of high hypoxia. In hypoxic conditions, sulfur-storing organisms start producing hydrogen sulfide. Therefore, the production of in H2S in anaerobic conditions is common among thiotrophic symbiosis. H2S can be damaging for some physiological processes as it inhibits the activity of cytochrome c oxidase, consequentially impairing oxidative phosphorilation. In R. pachyptila the production of hydrogen sulfide starts after 24h of hypoxia. In order to avoid physiological damage some animals, including Riftia pachyptila are able to bind H2S to haemoglobin in the blood to eventually expel it in the surrounding environment.
