New insights on old bacteria: diversity and function of morphologically conspicuous sulfur bacteria in aquatic systems

Colourless sulfur bacteria were among the first groups of biogeochemically important bacteria to be studied by microbial ecologists. This was in part due to the fact that several species are large and often produce macroscopi cally visible structures such as mats. Although some of these sulfur bacter ia have been studied for over a century, few have been obtained in axenic c ulture and thus, until recently, relatively little was known regarding thei r relationship to other bacteria or their role in the environment. The intr oduction of molecular biological methods to microbial ecology has allowed m any of these distinctive uncultured bacteria to be characterized phylogenet ically and in situ measurements have revealed aspects of their behaviour an d their involvement in specific biogeochemical processes. Culture based stu dies of some morphologically distinctive sulfur bacteria, principally isola tes of different Beggiatoa species, have revealed that they exhibit a wide range of metabolic activities that span heterotrophy to obligate chemolitha utotrophy. This knowledge has informed studies of uncultured sulfur bacteri a such as Thioploca and Achromatium. The combination of molecular biologica l analyses, chemical and process measurements, and culture based studies th at have allowed the ecology of these bacteria to be interrogated are discus sed.