The fate and biodegradation of petroleum hydrocarbons and dispersant in sediments of coastal ecosystems can be influenced significantly by the presence of organisms inhabiting the sediment. This is especially so for benthic animals that are active bioturbators. Effectively serving as ecosystem engineers, they greatly modify physical and chemical conditions in the sediment. Burrowing, feeding and other activities by these organisms move sediment between surface and subsurface zones, increase sediment surface area, increase pore-water movement within the sediment, and increase the rate of water and chemical exchange between the water column and sedimentary pore-waters. These activities consequently change sediment characteristics such as redox potential, organic content, and particle size. These changes in turn affect microbial activity and abundance. In the case of oiled sediment, bioturbators can have a direct affect on the distribution of the oil within the sediment as well as exchange between the water column and sediment. Indirectly, their effect on sediment characteristics, microbial abundance and the amount of sediment/water interface is likely to affect the rate of microbial degradation of oil.
This project adopts experimental approaches for assessing the influence of bioturbators on biodegradation of petroleum. The research will be conducted with two abundant and widely distributed groups of bioturbators; ghost shrimp and razor clams. These groups differ in burrowing depth and various other bioturbation characteristics, though both are ecosystem engineers and common inhabitants of inshore Gulf of Mexico intertidal to shallow subtidal zones. Experiments will be conducted in greenhouse mesocosms containing natural sediment with oil introduced alternatively at the surface, in subsurface sediments, or as a water-soluble fraction (with and without dispersant) in the overlying water column. Experiments will also be conducted with a single aromatic hydrocarbon. Variables to be measured in the mesocosms include hydrocarbon distribution between sediment and water column, hydrocarbon distribution within the sediment, hydrocarbon concentration over time, microbial community composition, and hydrocarbon-degradation capacity of the microbial community. The project will also assess whether the microbial population that develops in the greenhouse mesocosms is comparable to that at Gulf of Mexico sites where sediment and animals were collected.
This project will use an interdisciplinary approach (combining the expertise of an environmental toxicologist, a benthic ecologist, a marine microbiologist, and a chemist) to address research theme 2 “Chemical evolution and biological degradation of the petroleum/dispersant systems and subsequent interaction with coastal, open-ocean, and deepwater ecosystems”. Specifically, the project investigates whether animals inhabiting shallow subtidal sediments and shorelines will have an effect on where the oil is found and on the rate at which the oil will be broken down by microbes. It will shed light on such questions as: “Will oil that has seeped into sediment be redistributed to the surface by the activity of animals burrowing in the sediment? And: “Will the presence of active burrowing animals impact the rate at which microbes break down the oil?” Determining the role of bioturbators in the fate and biodegradation of oil in sediment will improve the ability for assessing long-term impacts of oil spills. The scientific impact of this study will be an increased understanding of processes that affect oil distribution between sediment and the water column, the rate of microbial degradation in bioturbated environments, and the potential role of bioturbators. Societally, the project potentially educates a concerned public on the term of risks from oil that has washed onto shores or soaked into nearshore sediment, while also providing insight into potential roles played by healthy infaunal communities in the course of oil degradation. The project will also contribute to the training of young scientists.