Bacterial sulfate reduction within reduced microniches of oxidized marine sediments

Bacterial sulfate reduction was demonstrated in the oxidized surface layers of a coastal marine sediment using a radiotracer technique. The obligate anaerobic process takes place within reduced sediment pellets of 50 to 200 m diameter. The H₂S produced diffuses out into the interstitial solution and is oxidized before any detectable accumulation takes place. This microniche structure explains the presence of sulfate-reducing (Desulfovibrio spp.) and sulfide oxidizing (Beggiatoa spp.) bacteria and of ferrous sulfide and pyrite in the oxidized sediment. Sulfate reduction was also demonstrated within detrital particles experimentally decomposed in oxic seawater or sediment. The limiting conditions for the maintenance of a reduced microniche within an oxic environment is discussed in terms of a theoretical model.