Microbial Community Analysis of Recently Deposited Marine Oil Snow in the Gulf of Mexico Between 2010 to 2018

During the Deepwater Horizon oil spill substantial amounts of the spilled oil was transported to the seafloor through the widely reported and now recognized marine oil snow sedimentation and flocculent accumulation (MOSSFA) event. MOSSFA occurs when natural organic rich material aggregates in ways like marine snow to entrain and transport oil droplets from the water column to the benthos. An estimated 111 million liters of oil was transferred to the seafloor during the Deepwater Horizon oil spill through MOSSFA processes. Numerous microbes are able to utilize the carbon sources in marine oil snow as a source of structural carbon and energy, yet the spatial and temporal variability of marine oil snow microbial communities is undocumented. Here we examine a long-term time series of microbial community change in recently deposited sediments from a deepwater site in the Gulf of Mexico (OC-26). This site was highly impacted from Deepwater Horizon MOSSFA-related oil input. We characterized the biogeochemistry of this layer, determined rates of microbial activity and assessed the microbial community composition in both the deep sediment and the associated MOSSFA layer. The MOSSFA layers were organic rich, yet they remained oxic. Rates of sulfate reduction in these layers were muted but rates of nitrate reduction were elevated. The microbial community in MOSSFA layers was originally very similar to the community observed in surface oil slicks. Over time, the community changed but even after several years, compositional differences between MOSSFA layers and the deeper sediments layers remained. MOSSFA layers were also distinct from microbial populations in control (no MOSSFA layer) surface sediments. Surprisingly long time periods were required for benthic microbial populations to respond effectively to such pulses of MOSSFA-derived material deposition.