Yet-to-be Cultured Microorganisms Relevant to Methane Fermentation Processes

Abstract

Anaerobic wastewater treatment sludge represents part of the natural microbial population found in various anaerobic ecosystems. The sludge drives the degradation of organic pollutants in wastewater, generally converting complex organic compounds into carbon dioxide and methane. This anaerobic conversion (methanogenesis) relies greatly on close interaction among a variety of different trophic groups of anaerobes within the sludge. Recent culture-independent molecular inventories of the microbial community of anaerobic (methanogenic) sludges revealed a vast diversity of microbes, the majority of which have not yet been cultivated and hence whose ecophysiological functions remain largely unknown. Two ongoing challenges in this field are to explain the in situ functions of these uncultured organisms and to establish how they interact with other organisms to make the methanogenic degradation of complex organic matter possible. In this review, recent progress in the study of the composition of microbial communities and the functions of individual populations in anaerobic sludge is summarized, with a special emphasis placed on the ecology and function of yet-to-be cultured microbes. In addition, it is also emphasized that the granular sludge in upflow anaerobic sludge blanket (UASB) reactors is an ideal model ecosystem with which to explore the functions of uncultured lineages of anaerobes and to elucidate the interaction of such organisms with other trophic groups of microbes in situ.