Physical and chemical dispersion of oil released from the Deepwater Horizon wellhead between April 20 and July 15, 2010 resulted in fine droplets and dissolved hydrocarbons within the deep water column. Alkanes and polycyclic aromatic hydrocarbons were rapidly biodegraded in the water column as evidenced by detailed chemistry measurements using GC and GC-MS analyses of over 10,000 water samples. Hydrocarbon biodegradation was rapid even at 5C. Microbial biodegradation removed many of the toxic components and reduced the overall impact of the oil released from the well. Larger droplets of oil rose to the surface forming oil slicks and some of the released oil impacted portions of the coastline with some marsh areas being heavily oiled.  DNA was extracted from impacted and reference marsh sites and genomic analyses (PhlyoChip, Geochip, and whole metagenome sequencing analyses) were performed. Microbial community composition varied with level of oiling, depth in the sediment profile, and extent of biodegradation. Specific populations of oil degrading bacteria and methanogenic archaea were elevated above reference sites in the presence of oil; as the oil weathered and concentrations of oil decreased the microbial community structure returned toward baseline composition. Anaerobic oil-degrading taxa predominated in the anaerobic surface layers at oil-contaminated sites. At one site, Desulfococcus oleovorans represented 51% of total sequences identified in the near-surface layer in fall when hydrocarbon concentrations were high, but decreased to 34% of total sequences identified in spring when hydrocarbon concentrations were lower. At this site the archaean Methanoplanus petrolearius comprised up to 63% of the total microbial community at 8-12 cm in fall but declined to 10% of the community in spring. Microbial community diversity was lower in oil impacted marsh sediments than in reference sediments but increased toward diversities characteristic of reference sites as the oil weathered.