Anaerobic sulfur metabolism in thiotrophic symbioses

Hydrogen sulfide is generally accepted to be the energy source for the esta blishment of sulfur-oxidizing symbiotic communities. Here, we show that sul fur-storing symbioses not only consume but also produce large amounts of hy drogen sulfide. The prerequisite for this process appears to be the absence of oxygen, Anaerobic sulfide production is widespread among different thio trophic symbioses from vent and non-vent sites (Riftia pachyptila, Calyptog ena magnifica, Bathymodiolus thermophilus, Lucinoma aequizonata and Calypto gena elongata), The extent of H2S generation correlates positively with the amount of elemental sulfur stored in the symbiont-bearing tissues of the h osts. Sulfide production starts a few hours after anoxia sets in, with H2S initially accumulating in the circulatory system before it is excreted into the surrounding environment. We propose that not sulfate but the elemental sulfur deposited in the symbionts serves as a terminal electron acceptor d uring anoxia and is reduced to sulfide. In anoxia-tolerant symbioses such a s L. aequizonata, anaerobic sulfur respiration may be important for produci ng maintenance energy to help the species survive several months without ox ygen, The increased levels of cysteine in the gills of L. aequizonata may b e caused by a lack of reoxidation due to the absence of oxygen.