Hydrocarbon-degrading sulfate-reducing bacteria in marine hydrocarbon seep sediments

Abstract

Microorganisms are key players in our biosphere because of their ability to degrade various organic compounds including a wide range of hydrocarbons. At marine hydrocarbon seeps, more than 90% of sulfate reduction (SR) is potentially coupled to non-methane hydrocarbon oxidation. Several hydrocarbon-degrading sulfate-reducing bacteria (SRB) were enriched or isolated from marine sediments. However, in situ active SRB remained largely unknown. In the present thesis, the global distribution and abundance of SRB at diverse gas and hydrocarbon seeps was investigated by catalyzed-reporter deposition fluorescence in situ hybridization (CARD-FISH). The majority of Deltaproteobacteria was assigned to specific SRB groups, for instance on average 83% and 61% at gas and hydrocarbon seeps. Members of the Desulfosarcina/Desulfococcus (DSS) clade significantly dominated all sites, suggesting their important role in hydrocarbon degradation processes. Furthermore, butane- and dodecane-degrading SRB were identified from two contrasting marine hydrocarbon seeps using 13C-stable-isotope probing techniques. The identified key players affiliated with four distinct groups, of which three belonged to the DSS clade. Specific groups were, according to their ability to oxidize short-chain alkanes (SCA) or long-chain alkanes (LCA), denoted as SCA-SRB1 and SCA-SRB2 as well as LCA-SRB1 and LCA-SRB2 . Based on the obtained data it is assumed that diverse and highly specialized DSS organisms are involved in hydrocarbon degradation at marine seeps rather than generalists of one dominant subgroup. At marine hydrocarbon seeps, groups SCA-SRB1 and SCA-SRB2 constituted up to 31 and 9% of all Deltaproteobacteria, respectively. In addition, LCA-SRB2 comprised up to 6% of all detected Deltaproteobacteria. Furthermore, activities for these groups were analyzed on the cellular level by Nanometer-scale Secondary Ion Mass Spectrometry (NanoSIMS). Alkane oxidation rates for specific groups were determined to be on average between 45 and 58 amol butane and 1 amol dodecane per cell and per day. Extrapolated data indicate that specific alkane-degrading SRB groups have the potential to contribute up to 100% of the total SR rates at seeps from the Gulf of Mexico. Therefore, alkane-degrading SRB groups may significantly impact sulfur and carbon cycles at marine hydrocarbon seeps. In addition, based on the obtained data, members of the uncultured group SEEP-SRB2 are hypothesized to be involved in hydrocarbon degradation. SEEP-SRB2 were visualized for the first time using CARD-FISH and were detected either in association with methanotrophic archaea (ANME 2/SEEP2 and ANME-1/SEEP2 consortia) or as single cells. Furthermore, the high abundance of SEEP-SRB2 indicates their important ecological role at marine hydrocarbon seeps.