Award: OCE-1045115

1) Microbial community succession in the Gulf of Mexico after the oil spill. We have shown a distinct community succession of hydrocarbon-degrading bacteria in the water column of the Gulf of Mexico, near the Macondo wellhead. The oil spill completely reorganized microbial carbon processing and microbial community structure; as the most conspicuous oil-spill-related bacteria, members of the genus Cycloclasticus and the Oceanospirillales dominated water samples during the early oil spill stages before disappearing in the months after the oil spill. (A) Mixtures of fresh crude oil and sea-water (such as those occurring at the sea surface) are colonized, already at an early stage (May 2010), by aromatic-degrading members of the genus Cycloclasticus. These bacteria are indigenous to the Gulf of Mexico, and in many other marine habitats with hydrocarbons present. (B) Members of the Oceanospirillales dominate the deep-water hydrocarbon plume at ca, 1100 to 1300 m depth near the wellhead almost entirely, at least until the end of May 2010 (sampled May 31 2010). These bacteria are not available in culture, and their detailed metabolism and carbon source is therefore uncertain; their closest cultured relatives specialize in oxidation of unbranched alkanes, a major component of petroleum hydrocarbons. They are no longer detected after approx. mid-June 2010, possibly reflecting the microbial consumption of the deepwater hydrocarbon plume, consisting of highly volatile alkanes. (C) In June, the Oceanospirillales community yields to other genera of hydrocarbon-degrading bacteria and common marine heterotrophs (Cycloclasticus and Colwellia); this marks the transition to a microbial community that also degrades the biomass produced by the oil spill, no longer only the petroleum compounds itself. (D) In September, the water column near the wellhead harbors a more diverse bacterial community, with Cycloclasticus and the methylotroph Methylophilus present as minor components. Analysis of the microbial community of water column samples after September 2010 is in progress. We will differentiate the oil spill impact from the natural background in the Gulf of Mexico, at a distance and near natural hydrocarbon seeps. 2) Microbial transformation of petroleum hydrocarbons in the water column. The Deepwater Horizon oil spill triggered a complex cascade of microbial responses that reshaped the dynamics of heterotrophic carbon degradation and the turnover of dissolved organic carbon (DOC), as well as the transport behavior of crude oil in the water column. A 21-day laboratory incubation of oil-contaminated water in rotating glass bottles demonstrated that oil degrading bacteria associated with an oil slick from the spill site rapidly catalyzed the formation of macroscopic aggregates with incorporated oil droplets (oil aggregates). Oil aggregates were densely colonized by heterotrophic bacteria that showed elevated rates of enzymatic activity (lipase hydrolysis) indicative for oil degradation. We also found in bottle waters enhanced microbial growth and activities (β-glucosidase; leucine aminopeptidase) not directly associated with primary oil-degradation, as well as a twofold increase in DOC. Concurrent changes in fluorescence properties of colored dissolved organic matter (CDOM) indicated an increase in oil-derived, alkylated polycyclic aromatic hydrocarbons (PAHs) in the DOC pool. Our data show that microbial activities, together with physical mixing, enhance the formation of oil aggregates in oil-contaminated surface seawater. These aggregates likely mediate, by two distinct mechanisms, the transfer of hydrocarbons to the deep sea: a microbially-derived flux of PAHs from sinking oil aggregates into the ambient water column, and rapid sedimentation of the oil aggregates themselves, leading to massive accumulation of oily particulate matter, as observed on the seafloor around the spill site several months post-oil spill. 3) Microbial Necromas...