Sediment-oil agglomerates in situ incubation in Pensacola Beach sand

Replicate homogenized Sediment-oil agglomerates (SOAs) were collected from the coastline immediately after the DWH disaster. SOAs were buried from 5 to 55cm depth in Pensacola Beach sand and sampled from October 2010 to December 2013. The abundance and composition of microbial communities was determined using next generation sequencing and qPCR of SSU rRNA genes, respectively, for SOAs sampled in a time series. Taxonomic diversity of microorganisms decreased in SOAs in comparison to surrounding or pristine sands. Hydrocarbon-degrading bacteria were enriched and a succession of microbial populations was observed that paralleled the chemical evolution of petroleum hydrocarbons as determined in our companion study. Bacterial abundance was two to four orders of magnitude higher in SOAs in comparison to surrounding sands, indicating that these large oil residues are hotspots of microbial growth. Quantification of nitrogenase genes (nifH) in SOAs revealed a bloom in nitrogen-fixing prokaryotes or diazotrophs late in the time series. Thus, the coupling of nitrogen fixation to hydrocarbon degradation appears to be an important control over the fate of oil in macroscopic oil aggregates. Our observations are corroborated by predictions from inferred metagenomic analysis and predictions aid in linking specific taxa to the degradation of specific hydrocarbon compounds and diazotrophy.