PRESENCE OF SULPHATE-REDUCING BACTERIA IN OXIDISED HABITATS
During the second half of the nineteenth century the formation of sulphide from sulphate was recognised as a biogenic process (Meyer, 1864). While it was initially suggested that algae were the catalysing organisms (Cohn, 1867), Hoppe-Seyler demonstrated in 1886 that the process required anoxic conditions and was chemotrophic, requiring external electron donors. In 1895, Beijerinck proved that sulphate reduction is catalysed by bacteria and described the first pure culture, Spirillum desulfuricans. This organism was described as strictly anaerobic and was irreversibly inhibited by oxygen.
The view that sulphate-reducing bacteria (SRB) are extremely sensitive to oxygen started to change in the late 1970s when sulphate reduction was demonstrated to occur also in oxidised sediment layers which showed no traces of FeS and were considered oxic (Jørgensen, 1977). Similarly, cultivation-based studies revealed the presence of viable sulphate reducers within these layers (Laanbroek and Pfennig, 1981; Battersby et al., 1985; Jørgensen and Bak, 1991). However, it was found that oxygen did not penetrate as deep into sediments as previously assumed and that large parts of the oxidised, hence FeS-free layers, were in fact anoxic. In sediment layers that contain oxidised manganese or iron species, sulphide can be chemically reoxidised to elemental sulphur (Aller and Rude, 1988), or, in the case of manganese oxide, even to thiosulphate (Schippers and Jørgensen, 2001).