SULFUR CYCLING IN CATENOCOCCUS-THIOCYCLUS

Since its isolation from marine volcanic areas, Catenococcus thiocyclu s has been known to be able to oxidize thiosulfate to tetrathionate, b ut the benefits gained from the reaction were unknown, The energy to b e gained from such a reaction is so small (1 electron per mol of thios ulfate, compared with 8 electrons if the thiosulfate is oxidized to su lfate) that it seemed unlikely to be a useful metabolic reaction. Howe ver, continuous culture experiments have now revealed that C. thiocycl us is able to gain metabolically useful energy from this oxidation (bi omass yields increased by approximately 20% after the addition of 7.75 mM thiosulfate to medium containing 20 mM acetate) by combining it wi th the chemical reduction of the tetrathionate by sulfide. The enzymes for thiosulfate oxidation appear to be constitutive. Moreover, with a suitable primary energy source (e.g. glucose), C. thiocyclus can redu ce sulfur (S degrees) to sulfide and Fe3+ to Fe2+. A chemical reaction then generates FeS. Such reactions may have important implications fo r the sulfur cycle at oxic:anoxic interfaces in marine and freshwater systems.