Ecophysiology of sulphate-reducing bacteria in environmental biofilms

Abstract

INTRODUCTION Sulphate-reducing bacteria (SRB) are a phylogenetically and physiologically diverse group of bacteria, characterized by their versatile metabolic capability to use various electron acceptors and donors (Widdel, 1988). SRB are therefore universally distributed in diverse environments and play significant ecophysiological roles in anaerobic biomineralization pathways. The degradation of organic matter by a complex microbial community is governed to a large extent by available electron acceptors. The terminal stages of the anaerobic mineralization of organic matter is catalyzed by SRB and methanogens, and their competitive and cooperative interactions have been described previously (Oude Elferink et al ., 1994). Typical domestic wastewaters contain sulphate concentrations of 100–1000 μM and relatively low dissolved oxygen due to the lower solubility and rapid depletion of this gas by biological activity. Thus, sulphate reduction can be the dominant terminal electron accepting process and account for up to 50% of mineralization of organic matter in wastewater biofilms (Kuhl and Jorgensen, 1992; Okabe et al ., 2003a). Multiple electron donors and electron acceptors are present in the wastewaters. As a result, wastewater biofilms are very complex multispecies biofilms, displaying considerable heterogeneity, with regard to both the microorganisms present and their physicochemical microenvironments. Sulphate reduction is anticipated to take place in the deeper anoxic biofilm strata even though the bulk liquid is oxygenated. It is, therefore, thought that a successive vertical zonation of respiratory processes can be found in aerobic wastewater biofilms with a typical thickness of only a few millimeters (Ito et al ., 2002b; Kuhl and Jorgensen, 1992; Okabe et al ., 1999a; 2003a; Ramsing et al. , 1993).