Energy metabolism and phylogenetic diversity of sulphate-reducing bacteria

Sulphate-reducing bacteria (SRB) are those prokaryotic microorganisms, both bacteria and archaea, that can use sulphate as the terminal electron acceptor in their energy metabolism, i.e. that are capable of dissimilatory sulphate reduction. Most of the SRB described to date belong to one of the four following phylogenetic lineages (with some examples of genera): (i) the mesophilic d-proteobacteria with the genera Desulfovibrio, Desulfobacterium, Desulfobacter, and Desulfobulbus; (ii) the thermophilic Gram-negative bacteria with the genus Thermodesulfovibrio; (iii) the Grampositive bacteria with the genusDesulfotomaculum; and (iv) the Euryarchaeota with the genus Archaeoglobus (Castro et al., 2000). A fifth lineage, the Thermodesulfobiaceae, has been described recently (Mori et al., 2003). Many SRB are versatile in that they can use electron acceptors other than sulphate for anaerobic respiration. These include elemental sulphur (Bottcher et al., 2005; Finster et al., 1998), fumarate (Tomei et al., 1995), nitrate (Krekeler and Cypionka, 1995), dimethylsulfoxide (Jonkers et al., 1996), Mn(IV) (Myers and Nealson, 1988) and Fe(III) (Lovley et al., 1993; 2004). Some SRB are even capable of aerobic respiration (Dannenberg et al., 1992; Lemos et al., 2001) although this process appears not to sustain growth, and probably provides these organisms only with energy for maintenance. Since dissimilatory sulphate reduction is inhibited under oxic conditions, SRB can grow at the expense of sulphate reduction only in the complete absence of molecular oxygen. SRB are thus considered to be strictly anaerobic microorganisms and are mainly found in sulphate-rich anoxic habitats (Cypionka, 2000; Fareleira et al., 2003; Sass et al., 1992). These conditions apply in marine sediments since ocean water is rich in sulphate, 1