DIRECT LINKING OF MICROBIAL-POPULATIONS TO SPECIFIC BIOGEOCHEMICAL PROCESSES BY C-13-LABELING OF BIOMARKERS

Recent advances in the application of molecular genetic approaches hav e emphasized our potentially huge underestimate of microbial diversity in a range of natural environments(1). These approaches, however, giv e no direct information about the biogeochemical processes in which mi croorganisms are active(2). Here we describe an approach to directly l ink specific environmental microbial processes with the organisms invo lved, based on the stable-carbon-isotope labelling of individual lipid biomarkers. We demonstrate this approach in aquatic sediments and pro vide evidence for the identity of the bacteria involved in two importa nt biogeochemical processes: sulphate reduction coupled to acetate oxi dation in estuarine and brackish sediments(3,4), and methane oxidation in a freshwater sediment(5). Our results suggest that acetate added i n a C-13-labelled form was predominantly consumed by sulphate-reducing bacteria similar to the Gram-positive Desulfotomaculum acetoxidans an d not by a population of the more widely studied Gram-negative Desulfo bacter spp. Furthermore, C-13-methane labelling experiments suggest th at type I methanotrophic bacteria dominate methane oxidation at the fr eshwater site.