Water Accommodated Fraction of Macondo Oil Has Limited Effects on Nitrate Reduction in Northern Gulf of Mexico Salt Marsh Sediments Regardless of Prior Oiling History

Coastal marshes provide valuable ecosystem services including the removal of excess nitrogen (N) prior to reaching coastal waters. Crude oil contamination can disrupt N cycling processes, and while the impacts of crude oil on marsh structure and function are well studied, less is known about the effects of different oil components. The objective of this study was to determine how water accommodated fraction (WAF) of oil impacts marsh sediment N cycling capacity from three marshes with differing characteristics. One site was previously oiled following the Deepwater Horizon (DWH) oil spill while the other two sites had no known history of oil spills. We measured 16S rRNA gene composition from sediments collected from each marsh then conducted a laboratory incubation experiment on sediments treated with different concentrations (0%, 25%, 100%) of WAF. The DWH impacted site had a lower number of observed microbial taxa and lower Chao1 diversity, but a higher relative abundance of putative hydrocarbon degraders compared to the other sites. While there was no treatment effect of WAF on sediment denitrification, denitrification potential rates were 2.4 × higher in the DWH impacted sediment compared to the other sites. There were no differences in dissimilatory nitrate reduction to ammonium (DNRA) potential rates across sites, but 100% WAF treatments increased rates nearly twofold at one of the unoiled sites. These results suggest oil contamination alters the microbial community structure and impacts N cycling processes in salt marsh sediments.