Reduced TCA cycle rates at high hydrostatic pressure hinder hydrocarbon degradation and obligate oil degraders in natural, deep-sea microbial communities

Petroleum hydrocarbons reach the deep-sea following natural and anthropogenic factors. The process by which they enter deep-sea microbial food webs and impact the biogeochemical cycling of carbon and other elements is unclear. Hydrostatic pressure (HP) is a distinctive parameter of the deep sea, although rarely investigated. Whether HP alone affects the assembly and activity of oil-degrading communities remains to be resolved. Here we have demonstrated that hydrocarbon degradation in deep-sea microbial communities is lower at native HP (10 MPa, about 1000 m below sea surface level) than at ambient pressure. In long-term enrichments, increased HP selectively inhibited obligate hydrocarbon-degraders and downregulated the expression of beta-oxidation-related proteins (i.e., the main hydrocarbon-degradation pathway) resulting in low cell growth and CO2 production. Short-term experiments with HP-adapted synthetic communities confirmed this data, revealing a HP-dependent accumulation of citrate and dihydroxyacetone. Citrate accumulation suggests rates of aerobic oxidation of fatty acids in the TCA cycle were reduced. Dihydroxyacetone is connected to citrate through glycerol metabolism and glycolysis, both upregulated with increased HP. High degradation rates by obligate hydrocarbon-degraders may thus be unfavourable at increased HP, explaining their selective suppression. Through lab-scale cultivation, the present study is the first to highlight a link between impaired cell metabolism and microbial community assembly in hydrocarbon degradation at high HP. Overall, this data indicate that hydrocarbons fate differs substantially in surface waters as compared to deep-sea environments, with in situ low temperature and limited nutrients availability expected to further prolong hydrocarbons persistence at deep sea

Monitoring of microbial hydrocarbon remediation in the soil

Bioremediation of hydrocarbon pollutants is advantageous owing to the cost-effectiveness of the technology and the ubiquity of hydrocarbon-degrading microorganisms in the soil. Soil microbial diversity is affected by hydrocarbon perturbation, thus selective enrichment of hydrocarbon utilizers occurs. Hydrocarbons interact with the soil matrix and soil microorganisms determining the fate of the contaminants relative to their chemical nature and microbial degradative capabilities, respectively. Provided the polluted soil has requisite values for environmental factors that influence microbial activities and there are no inhibitors of microbial metabolism, there is a good chance that there will be a viable and active population of hydrocarbon-utilizing microorganisms in the soil. Microbial methods for monitoring bioremediation of hydrocarbons include chemical, biochemical and microbiological molecular indices that measure rates of microbial activities to show that in the end the target goal of pollutant reduction to a safe and permissible level has been achieved. Enumeration and characterization of hydrocarbon degraders, use of micro titer plate-based most probable number technique, community level physiological profiling, phospholipid fatty acid analysis, 16S rRNA- and other nucleic acid-based molecular fingerprinting techniques, metagenomics, microarray analysis, respirometry and gas chromatography are some of the methods employed in bio-monitoring of hydrocarbon remediation as presented in this review

Direct linking of microbial populations to specific biogeochemical processes by 13C-labelling of biomarkers

Recent advances in the application of molecular genetic approaches have emphasized our potentially huge underestimate of microbial diversity in a range of natural environments(1). These approaches, however, give no direct information about the biogeochemical processes in which microorganisms are active(2). Here we describe an approach to directly link specific environmental microbial processes with the organisms involved, based on the stable-carbon-isotope labelling of individual lipid biomarkers. We demonstrate this approach in aquatic sediments and provide evidence for the identity of the bacteria involved in two important biogeochemical processes: sulphate reduction coupled to acetate oxidation in estuarine and brackish sediments(3,4), and methane oxidation in a freshwater sediment(5). Our results suggest that acetate added in a C-13- labelled form was predominantly consumed by sulphate-reducing bacteria similar to the Gram-positive Desulfotomaculum acetoxidans and not by a population of the more widely studied Gram-negative Desulfobacter spp. Furthermore, C-13-methane labelling experiments suggest that type I methanotrophic bacteria dominate methane oxidation at the freshwater site. [KEYWORDS: Sulfate-reducing bacteria; gradient gel-electrophoresis; fatty-acid profiles; ribosomal-rna; methanotrophic bacteria; estuarine sediments; marine-sediments; sp-nov; acetate; desulfovibrio]