Published online by Cambridge University Press: 22 August 2009
INTRODUCTION
An early focus on the use of molecular techniques to characterize natural populations of sulphate-reducing microorganisms (SRM) derived from the close relationship between their phylogenetic affiliation and their capability to anaerobically respire with sulphate. In other words, all so-far characterized SRM associate with lineages in the tree of life that predominantly consist of sulphate reducers. Known SRM are affiliated with two divisions (phyla) within the Archaea (the euryarchaeotal genus Archaeoglobus species and the crenarchaeotal genera Caldivirga and Thermocladium, affiliated with the Thermoproteales) and five divisions within the Bacteria (the Deltaproteobacteria, endospore-forming Desulfotomaculum, Desulfosporosinus, and Desulfosporomusa species within the Firmicutes division, Thermodesulfovibrio species within the Nitrospira division, and two divisions represented by Thermodesulfobacterium species and the recently isolated Thermodesulfobium narugense, the exact phylogenetic position of the latter is still ambiguous). Most described SRM are either Gram-positive bacteria with a low G+C content or Gram-negative Deltaproteobacteria. However, it is important to note that almost all major physiological properties of cultured and uncultured SRM, such as substrate usage patterns, the ability to completely oxidize a substrate to CO2, and alternative ways of anaerobic energy generation cannot be unambiguously determined from comparative analysis of their 16S rRNA genes.
The generally tight association between phylogenetic affiliation and sulphate-reducing physiology offered a foundation to directly associate the population structure determined by 16S rRNA sequence type and process. These studies have now been complemented by the use of highly conserved genes in the pathway for sulphate respiration.
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