Microbial Ecology of Coastal Ecosystems: Investigations of the Genetic Potential for Anaerobic Hydrocarbon Transformation and the Response to Hydrocarbon Exposure

Microbial-mediated hydrocarbon transformation plays a vital role in the attenuation of natural and anthropogenic-sourced petroleum contamination in the environment, particularly in marine systems. Indigenous microbial communities in marine habitats are resilient to influxes of petroleum, and it is well documented that many taxa are capable of responding and utilizing these compounds. Coastal ecosystems are often either subjected to or at risk for oil contamination and are of particular concern due to their significant environmental and economic value. The research projects presented here focused on coastal ecosystems and investigated microbial community compositions via next-generation sequencing of 16S rRNA genes, the genetic potential for anaerobic hydrocarbon biodegradation within these communities via molecular surveys of marker genes, and the response of anaerobic populations to exposure of a hydrocarbon via microcosm studies or to products of hydrocarbon transformation processes (i.e. photolysis) via sulfate reduction assays (SRAs). Chesapeake Bay is the largest estuary in the United States, and experiences high nutrient loading and water column hypoxia due to watershed runoff, as well as petroleum contamination from urban runoff, atmospheric deposition, and spills directly into the water column. Past studies have demonstrated that aerobic hydrocarbon-degrading bacteria can be enriched from the water column and from the sediment. However, evidence for anaerobic biodegradation of hydrocarbons had not been demonstrated at the time of our study. Given the recurring seasonal water column hypoxia and the transient exposure to hydrocarbons, we hypothesized that the potential for degradation under anaerobic conditions may exist in Chesapeake Bay sediments. Here, molecular surveys and microcosms were utilized to investigate microbial community composition and the potential for anaerobic hydrocarbon degradation among sites along a transect of the Bay. Sampling locations were chosen both within and outside areas of recurring hypoxia. Distinct geochemical gradients along the transect were revealed. Low oxygen, low sulfate, and high methane concentrations were observed in the upper Bay, as were significantly higher levels of taxa associated with anaerobic processes (e.g., sulfate reducers and methanogens). In contrast, higher oxygen, higher sulfate, and very low methane were measured in the lower Bay. Sulfate-reducers and methanogens decreased in abundance in lower Bay sediments as well. Similarly, molecular surveys showed more frequent detection of marker genes associated with the anaerobic activation of hydrocarbons via the ‘fumarate addition’ pathway (e.g., assA, bssA) in the upper Bay, and microcosms established under sulfate-reducing and/or methanogenic conditions suggested that the model hydrocarbon, hexadecane, was being converted to methane by indigenous sediment communities obtained from the upper Bay sites. These findings illustrate the variability of microbial communities between different locations in…