The biodegradation of oil and the dispersant Corexit 9500 in Arctic seawater

Abstract

As oil and gas production continues in the Arctic, oil exploration and shipping traffic have increased due to the decline of Arctic sea ice. This increased activity in the Arctic Ocean poses a risk to the environment through the potential release of oil from cargo ships, oil tankers, pipelines, and future oil exploration. Understanding the fate of oil is crucial to understanding the impacts of a spill on the marine ecosystem. Previous oil biodegradation studies have demonstrated the ability of Arctic and sub-Arctic microorganisms to biodegrade oil; however, the rate at which oil degrades and the identity of indigenous oil-degrading microorganisms and functional genes in Arctic seawater remain unknown. In addition to oil, it is also important to understand the fate and effects of chemicals potentially used in oil spill response. Corexit 9500 is a chemical dispersant that is pre-approved for use in sub-Arctic seawater and is likely the dispersant of choice for spill responders in Arctic offshore environments. Currently no literature exists concerning the biodegradation of Corexit 9500 in Arctic seawater. Here we investigate the fate of oil, chemically dispersed oil, and the chemical dispersant, Corexit 9500, in laboratory mesocosms containing freshly collected Arctic surface seawater. The objectives of these experiments were to calculate the extent and rate of biodegradation (based on GC/MS & LC/MS/MS analysis) and to identify bacteria (determined using 16S rRNA gene sequencing) and genes (based on GeoChip 5.0 microarray) potentially involved in the biodegradation process. Indigenous microorganisms degraded both fresh and weathered oil, in both the presence and absence of Corexit 9500, with oil losses ranging from 36-41% within 28 days and 46-61% within 60 days. The biodegradation of the active components of Corexit 9500, which are dioctyl sodium sulfosuccinate (DOSS) and non-ionic surfactants, was also measured after 28 days. Biodegradation of DOSS was 77% in offshore seawater and 33% in nearshore seawater. Non-ionic surfactants were non-detectable after 28 days. Taxa known to include oil-degrading bacteria (e.g. Oleispira, Polaribacter, and Colwellia) and oilbiodegradation genes (e.g. alkB) increased in relative abundance in response to both oil and Corexit 9500. These results increase our understanding of oil and dispersant biodegradation in the Arctic and suggest that some bacteria may be capable of biodegrading both oil and Corexit 9500. We also sought to understand baseline abundances of taxa known to include oildegrading bacteria and functional genes involved in oil biodegradation in an offshore oil lease area. Aerobic oil-degradation genes (based on GeoChip 5.0 microarray) and taxa (determined using 16S rRNA gene sequencing) known to include oil-degrading bacteria were identified in seawater from the surface, middle, and bottom of the water column. Bacterial community structure differed significantly by depth (surface water vs. bottom water), while the relative abundance of major functional gene categories did not differ with depth. These findings support previous observations that two different water masses contribute to a stratified water column in the summer open-water season of the oil lease area, but indicate that potential function is fairly similar with depth. These results will contribute to understanding the potential for oil biodegradation throughout the Arctic water column and the fundamental microbial ecology of an offshore oil lease area. Together, these mesocosm experiments and in situ studies address important data gaps concerning the fate of spilled oil and Corexit in Arctic seawater. These results provide novel insight into the ability of Arctic bacteria to biodegrade crude oil and Corexit 9500, and suggest similarities between Arctic and temperate deep-sea environments in regards to taxa and functional genes that respond to oil and Corexit.