Abstract
Marine subsurface environments such as deep-sea sediments, house abundant and diverse microbial communities that are believed to influence large-scale geochemical processes. These processes include the biotransformation and mineralization of numerous petroleum constituents. Thus, microbial communities in the Gulf of Mexico are thought to be responsible for the intrinsic bioremediation of crude oil released by the Deepwater Horizon (DWH) oil spill. While hydrocarbon contamination is known to enrich for aerobic, oil-degrading bacteria in deep-seawater habitats, relatively little is known about the response of communities in deep-sea sediments, where low oxygen levels may hinder such a response. Here, we examined the hypothesis that increased hydrocarbon exposure results in an altered sediment microbial community structure that reflects the prospects for oil biodegradation under the prevailing conditions. We explore this hypothesis using metagenomic analysis and metabolite profiling of deep-sea sediment samples following the DWH oil spill. The presence of aerobic microbial communities and associated functional genes was consistent among all samples, whereas, a greater number of Deltaproteobacteria and anaerobic functional genes were found in sediments closest to the DWH blowout site. Metabolite profiling also revealed a greater number of putative metabolites in sediments surrounding the blowout zone relative to a background site located 127 km away. The mass spectral analysis of the putative metabolites revealed that alkylsuccinates remained below detection levels, but a homologous series of benzylsuccinates (with carbon chain lengths from 5 to 10) could be detected. Our findings suggest that increased exposure to hydrocarbons enriches for Deltaproteobacteria, which are known to be capable of anaerobic hydrocarbon metabolism. We also provide evidence for an active microbial community metabolizing aromatic hydrocarbons in deep-sea sediments of the Gulf of Mexico.
Highlights
The Deepwater Horizon (DWH) blowout resulted in the largest marine US oil spill to date, in which 4.1 million barrels of crude oil flowed into the depths (∼1500 m) of the Gulf of Mexico (Operational Science Advisory Team, 2010)
Two previous deep-sea metagenomic studies resulted in the suggestion that there is a core metagenomic structure for deepsea sediments, composed of four main microbial groups (Euryarchaeota, Proteobacteria, Firmicutes, and Chloroflexi) that can vary depending on specific parameters, such as depth, organic carbon content, and geography (Biddle et al, 2008; Biddle et al, 2011)
These four microbial taxa were detected in GoM sediment samples in the present work; they do not constitute the four major groups detected in this report (Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria)
Summary
The Deepwater Horizon (DWH) blowout resulted in the largest marine US oil spill to date, in which 4.1 million barrels of crude oil flowed into the depths (∼1500 m) of the Gulf of Mexico (Operational Science Advisory Team, 2010). An estimated 78% of the oil was depleted through either human intervention or natural means by August 2010 (Ramseur, 2010), the fate of the remaining 22% was uncertain. Evidence subsequently showed that both oil (Hazen et al, 2010; Mason et al, 2012) and gas (Kessler et al, 2011) persisted in the Gulf of Mexico water column, affecting deep-sea (>1000 m) microbial communities that potentially facilitate the biodegradation of residual hydrocarbons. Much of the hydrocarbons from sub-sea oil spills and natural seeps may rise to the surface, there are water-soluble components in oil as well as hydrocarbons adhering to solid particulates that can settle in deep-sea sediments (Ramseur, 2010). After the 1979 Ixtoc I oil spill, for example, in which over three million barrels of oil flowed into the Gulf of Mexico, it is estimated that 25% of the oil was transported to the sea floor (Jernelov and Linden, 1981)
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