Microbial response to oil enrichment in Gulf of Mexico sediment measured using a novel long-term benthic lander system

Beth N Orcutt,Peter R Girguis,Neha Sarode,Laura L Lapham,Kathleen S Marshall,Kelly J Whaley-Martin,C Geoff Wheat,Jennifer Delaney,Greg Slater,Jody W Deming,Laurenz Thomsen

doi:10.1525/elementa.129

Abstract

Weathered crude oil sank to the seafloor following the Deepwater Horizon disaster in 2010, removing this oil from further physical and photo-chemical degradation processes and leaving benthic processes as the mechanisms for altering and remediating this hydrocarbon source. To quantify potential microbial oil degradation rates at the seafloor, and associated changes in sediment microbial community structure and pore fluid composition, we used a benthic lander system to deploy novel sediment flow-through chambers at a natural hydrocarbon seep in the Gulf of Mexico (at a depth of 1226 m in lease block GC600) roughly 265 km southwest of the Deepwater Horizon wellhead (at 1500 m depth). Sediment amended with 20% unweathered crude oil had elevated rates of sulfate reduction over the course of the 5-month-long experiment as compared to an unamended control, yielding potential rates of sulfate reduction (600–800 mmol m–2 d–1) among the highest measured in hydrocarbon-influenced seafloor sediment. Oil amendment also stimulated methane production towards the end of the experiment, and led to slightly higher cell densities without significant changes in microbial community structure, based on 16S rRNA gene sequence libraries and fatty acid profiles. Assuming a link between sulfate reduction and hydrocarbon degradation, these results suggest that electron acceptor availability may become limiting in heavily oiled deep-sea environments, resulting in minimal degradation of deposited oil. This study provides unique data on seafloor sediment responses to oil deposition, and reveals the value of using observatories to fill the gap in understanding deep-sea microbial processes, especially for ephemeral and stochastic events such as oil spills.

Highlights

A massive amount of oil was released into the Gulf of Mexico after the Deepwater Horizon blowout from April to July 2010 (Crone and Tolstoy, 2010; MacDonald, 2010; Joye et al, 2011; McNutt et al, 2012), with roughly 5–15% of that oil sinking to the seafloor (Valentine et al, 2014; Chanton et al, 2015)
As the Gulf of Mexico has many natural oil seeps (MacDonald et al, 1996, 2002), it has been suggested that some Gulf of Mexico sediment microorganisms are “primed” to begin oil degradation quickly following an oil-loading event (Valentine et al, 2010; Kessler et al, 2011); a lag in microbial response to hydrocarbon inputs might be expected based on slow growth rates of microorganisms involved in anaerobic hydrocarbon consumption (Nauhaus et al, 2007)
Microbial Methane Observatory for Seafloor Analysis (MIMOSA) concept, configuration, and sampling strategy The goal of MIMOSA was to examine the dynamics of hydrocarbon cycling and microbial community structure in deep-sea sediment to better understand the response of microorganisms to oil release, as happened following the Deepwater Horizon event

Summary

Introduction

A massive amount of oil was released into the Gulf of Mexico after the Deepwater Horizon blowout from April to July 2010 (Crone and Tolstoy, 2010; MacDonald, 2010; Joye et al, 2011; McNutt et al, 2012), with roughly 5–15% of that oil sinking to the seafloor (Valentine et al, 2014; Chanton et al, 2015). As the Gulf of Mexico has many natural oil seeps (MacDonald et al, 1996, 2002), it has been suggested that some Gulf of Mexico sediment microorganisms are “primed” to begin oil degradation quickly following an oil-loading event (Valentine et al, 2010; Kessler et al, 2011); a lag in microbial response to hydrocarbon inputs might be expected based on slow growth rates of microorganisms involved in anaerobic hydrocarbon consumption (Nauhaus et al, 2007) It is unclear if microbial communities degrading sedimented oil would exhaust the available pool of electron donors (i.e., oxygen, nitrate, sulfate) needed for complete hydrocarbon oxidation. Since the Deepwater Horizon event, studies have aimed to determine rates of oil degradation and the oil-degrading microorganisms in bottle experiments using coastal sands and muds (Mortazavi et al, 2012; Singh et al, 2014)

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Conclusion