21Response actions to the Deepwater Horizon oil spill included the injection of ~771,000 gallons 22 (2,900,000 L) of chemical dispersant into the flow of oil near the seafloor. Prior to this incident, 23 no deepwater applications of dispersant had been conducted and thus no data exists on the 24 environmental fate of dispersants in deepwater. We used ultrahigh resolution mass 25 spectrometry and liquid chromatography with tandem mass spectrometry (LC/MS/MS) to 26 identify and quantify one key ingredient of the dispersant, the anionic surfactant DOSS (dioctyl 27 sodium sulfosuccinate), in the Gulf of Mexico deepwater during active flow and again after 28 flow had ceased. Here we show that DOSS was sequestered in deepwater hydrocarbon plumes 29 at 1000-1200m water depth and did not intermingle with surface dispersant applications. 30Further, its concentration distribution was consistent with conservative transport and dilution 31 at depth and it persisted up to 300 km from the well, 64 days after deepwater dispersant 32 applications ceased. We conclude that DOSS was selectively associated with the oil and gas 33 phases in the deepwater plume, yet underwent negligible, or slow, rates of biodegradation in 34 the affected waters. These results provide important constraints on accurate modeling of the 35 deepwater plume and critical geochemical contexts for future toxicological studies. 36 37 38

Methane was the most abundant hydrocarbon released during the 2010 Deepwater Horizon oil spill in the Gulf of Mexico. Beyond relevancy to this anthropogenic event, this methane release simulates a rapid and relatively short-term natural release from hydrates into deep water. Based on methane and oxygen distributions measured at 207 stations throughout the affected region, we find that within ~120 days from the onset of release ~3.0 × 10(10) to 3.9 × 10(10) moles of oxygen were respired, primarily by methanotrophs, and left behind a residual microbial community containing methanotrophic bacteria. We suggest that a vigorous deepwater bacterial bloom respired nearly all the released methane within this time, and that by analogy, large-scale releases of methane from hydrate in the deep ocean are likely to be met by a similarly rapid methanotrophic response.

The Deepwater Horizon event resulted in suspension of oil in the Gulf of Mexico water column because the leakage occurred at great depth. The distribution and fate of other abundant hydrocarbon constituents, such as natural gases, are also important in determining the impact of the leakage but are not yet well understood. From 11 to 21 June 2010, we investigated dissolved hydrocarbon gases at depth using chemical and isotopic surveys and on-site biodegradation studies. Propane and ethane were the primary drivers of microbial respiration, accounting for up to 70% of the observed oxygen depletion in fresh plumes. Propane and ethane trapped in the deep water may therefore promote rapid hydrocarbon respiration by low-diversity bacterial blooms, priming bacterial populations for degradation of other hydrocarbons in the aging plume.

Microbial communities present in the Gulf of Mexico rapidly responded to the Deepwater Horizon oil spill. In deep water plumes, these communities were initially dominated by members of Oceanospirillales, Colwellia, and Cycloclasticus. None of these groups were abundant in surface oil slick samples, and Colwellia was much more abundant in oil-degrading enrichment cultures incubated at 4°C than at room temperature, suggesting that the colder temperatures at plume depth favored the development of these communities. These groups decreased in abundance after the well was capped in July, but the addition of hydrocarbons in laboratory incubations of deep waters from the Gulf of Mexico stimulated Colwellia's growth. Colwellia was the primary organism that incorporated 13 C from ethane and propane in stable isotope probing experiments, and given its abundance in environmental samples at the time that ethane and propane oxidation rates were high, it is likely that Colwellia was active in ethane and propane oxidation in situ. Colwellia also incorporated 13 C benzene, and Colwellia's abundance in crude oil enrichments without natural gas suggests that it has the ability to consume a wide range of hydrocarbon compounds or their degradation products. However, the fact that ethane and propane alone were capable of stimulating the growth of Colwellia, and to a lesser extent, Oceanospirillales, suggests that high natural gas content of this spill may have provided an advantage to these organisms. methane | biodegradation | marine bacteria | archaea | alkane

Following the Deepwater Horizon disaster, the effect of weathering on surface slicks, oil-soaked sands, and oil-covered rocks and boulders was studied for 18 months. With time, oxygen content increased in the hydrocarbon residues. Furthermore, a weathering-dependent increase of an operationally defined oxygenated fraction relative to the saturated and aromatic fractions was observed. This oxygenated fraction made up >50% of the mass of weathered samples, had an average carbon oxidation state of −1.0, and an average molecular formula of (C 5 H 7 O) n . These oxygenated hydrocarbon residues were devoid of natural radiocarbon, confirming a fossil source and excluding contributions from recent photosynthate. The incorporation of oxygen into the oil's hydrocarbons, which we refer to as oxyhydrocarbons, was confirmed from the detection of hydroxyl and carbonyl functional groups and the identification of long chain (C 10 −C 32 ) carboxylic acids as well as alcohols. On the basis of the diagnostic ratios of alkanes and polycyclic aromatic hydrocarbons, and the context within which these samples were collected, we hypothesize that biodegradation and photooxidation share responsibility for the accumulation of oxygen in the oil residues. These results reveal that molecular-level transformations of petroleum hydrocarbons lead to increasing amounts of, apparently recalcitrant, oxyhydrocarbons that dominate the solvent-extractable material from oiled samples.