INTRODUCTION

Sulphate-reducing bacteria (SRB) are anaerobes, which derive energy for growth from anaerobic metabolism, coupling the oxidation of organic substrates with the dissimilatory reduction of sulphate to hydrogen sulphide (sulphate respiration). Although generally considered as strict anaerobes, more and more data indicate a higher abundance and metabolic activity in oxic zones of biotopes, such as marine and freshwater sediments, than in neighbouring anoxic zones (Ravenschlag et al., 2000; Sass et al., 1997; 1998). A well-documented example of sulphate reduction under oxic conditions is also provided by cyanobacterial mats. Here a zone of photosynthetic oxygen synthesis overlaps with a zone of sulphide production by SRB and a zone of oxygen-dependent microbial sulphide oxidation, creating steep, opposing gradients of oxygen and sulphide, which fluctuate with the rhythm of day and night (Canfield and des Marais, 1991; Teske et al., 1998; Caumette et al., 1994). In cyanobacterial mats from the saline evaporation pond in Baja California, Desulfobacter and Desulfobacterium are restricted to greater depths while the Desulfococcus and Desulfovibrio groups are predominant in the upper part of the photo-oxic zone (Risatti et al., 1994). The high numbers of SRB found in these oxic environments indicate that these organisms are able to deal with temporal exposures to oxygen concentrations as high as 1.5 mM (Sigalevich and Cohen, 2000).

In these oxygen-exposed systems SRB of the genus Desulfovibrio are among the most oxygen-tolerant, e.g. Desulfovibrio oxyclinae was isolated from Solar Lake microbial mats (Krekeler et al., 1997) and Desulfovibrio desulfuricans strain DvO1 was isolated from activated sludge aerated to atmospheric oxygen saturation (Kjeldsen et al., 2005).