Sulfate and methane sediment profile changes over time with the addition of surface oil, Laboratory Experiments, September 2013 to April 2014

On April 20, 2010, The Deepwater Horizon oil spill released an unprecedented amount of oil into the Gulf of Mexico deep water, some of which was deposited to the seafloor. While studies reported that endemic microbial communities were able to breakdown this oil, it is still unclear how microbial communities in the sediments responded to this influx of oil. In conjunction with in situ experiments in the Gulf of Mexico, we carried out laboratory experiments to explore how anaerobic processes such as sulfate reduction and methanogenesis degrade oil and at what rates. We hypothesized that the addition of oil will stimulate sulfate reducers due to the influx of a new carbon source. This, in turn, will shift zones of microbial activity so that methanogens will begin to produce methane at a shallower depth than the controls. For the experiment, sediment cores (~25 cm long) were collected from a shallow-water estuary (St Mary's College, MD) where salinity is 13 psu. The cores were permanently fitted with Rhizone samplers that filter porewater (0.2μm) every 3-5 cm. For the experiment, there were two treatments, one with surface sediment and original carbon content, and one treatment where the surface sediment was added to oil in a 5:1 ratio by volume. Five milliliters of either treatment material was then added to the surface sediment of the core and left at room temperature for the duration of the 3 month experiment. Pore-water sub-samples were then collected every week. Pore-water samples were measured for sulfate concentrations, and a methane profile was determined at the end of the experimental period. Overall, the sulfate levels in the oiled core decreased to lower concentrations than in the control core, and methane levels increased to a much greater amount, when sulfate was exhausted. A small increase in methane was seen in the deepest part of the control core. This supports our hypothesis that methane will be present at shallower depths in sediments when oil is introduced. This may have long-term consequences on methane levels that may potentially rise to the surface through physical processes and function as a greenhouse gas. While these in-lab results shed light on oil degradation dynamics with natural microbial populations, they may not truly reflect activity in the Gulf — in-situ experiments remain the most accurate way to study true environmental conditions.