Description: Gulf of Mexico cold seeps characterized by variable compositions and magnitudes of hydrocarbon seepage were sampled in order to investigate the effects of natural oils, methane, and non-methane hydrocarbons on microbial activity, diversity, and distribution in seafloor sediments. Though some sediments were characterized by relatively high quantities of oil, which may be toxic to some microorganisms, high rates of sulfate reduction (SR, 27.9714.7 mmol m2 d1), anaerobic oxidation of methane (AOM, 16.276.7 mmol m2 d1), and acetate oxidation (2.7470.76 mmol m2 d1) were observed in radiotracer measurements. In many instances, the SR rate was higher than the AOM rate, indicating that non-methane hydrocarbons fueled SR. Analysis of 16S rRNA gene clone libraries revealed phylogenetically diverse communities that were dominated by phylotypes of sulfate-reducing bacteria (SRB) and anaerobic methanotrophs of the ANME-1 and ANME-2 varieties. Another group of archaea form a Gulf of Mexico-specific clade (GOM ARC2) that may be important in brine-influenced, oil-impacted sediments from deeper water. Additionally, species grouping within the uncultivated Deltaproteobacteria clades SEEP-SRB3 and -SRB4, as well as relatives of Desulfobacterium anilini, were observed in relatively higher abundance in the oil-impacted sediments, suggesting that these groups of SRB may be involved in or influenced by degradation of higher hydrocarbons or petroleum byproducts.

Description: Phytoplankton blooms in surface waters of the oceans are known to influence the food web and impact microbial as well as zooplankton communities. Numerous studies have investigated the fate of phytoplankton-derived organic matter in surface waters and shelf sediments, however, little is known about the effect of sinking algal biomass on microbial communities in deep-sea sediments. Here, we analyzed sediments of four regions in the Southern Atlantic Ocean along the Antarctic Polar Front that had different exposures to phytoplankton bloom derived organic matter. We investigated the microbial communities in these sediments using high-throughput sequencing of 16S rRNA molecules to determine microorganisms that were active and catalyzed reporter deposition fluorescence in situ hybridization to infer their abundance and distribution. The sediments along the Antarctic Polar Front harbored microbial communities that were highly diverse and contained microbial clades that seem to preferably occur in regions of high primary productivity. We showed that organisms affiliated with the gammaproteobacterial clade NOR5/OM60, which is known from surface waters and coastal sediments, thrive in the deep-sea. Benthic deep-sea NOR5 were abundant, diverse, distinct from pelagic NOR5 and likely specialized on the degradation of phytoplankton-derived organic matter, occupying a similar niche as their pelagic relatives. Algal detritus seemed to not only fuel the benthic microbial communities of large areas in the deep-sea, but also to influence communities locally, as we found a peak in Flavobacteriaceae-related clades that also include degraders of algal biomass. The results strongly suggest that phytoplankton-derived organic matter was rapidly exported to the deep-sea, nourished distinct benthic microbial communities and seemed to be the main energy source for microbial life in the seafloor of vast abyssal regions along the Antarctic Polar Front.