Microbial Community Response to Oil-Impacted Marsh Soils Treated With Nitrogen and Rhamnolipid Bio-Surfactant.

Understanding microbial community dynamics is important for guiding the remediation of contaminated marsh soils as it is closely linked to mechanisms that control degradation processes. Rhamnolipid has been used to enhance the bioremediation oil-contaminated upland soils but its role in wetland soils has not been adequately evaluated. In this study, petroleum hydrocarbon degradation in a Louisiana marsh soil was investigated using an integrated approach incorporating chemical monitoring of hydrocarbon degradation and total carbon respiration with microbial community characterization. The collected saline marsh soil was treated with crude oil, rhamnolipid biosurfactant and nitrogen, and incubated in glass jars. Methane, CO₂ and N₂O concentrations in the jar head spaces were measured using a GC equipped with FID and ECD detectors. One portion of incubated samples were periodically sampled and characterized for the change in individual hydrocarbon compounds using GC-MS following extraction using dichloromethane (DCM). The other portion was used for DNA extraction followed by 16s-rRNA pyrosequencing for characterizing microbial community change over time. The results showed that the combined nitrogen and rhamnolipid biosurfactant treatment enhanced the degradation of total petroleum hydrocarbons. Alkane compounds were generally degraded at a faster rate than aromatic compounds.  However, dibenzo(a,h)anthracene showed a different trend than the rest of aromatic compounds with a slower degradation under biosurfactant and nitrogen treatment.  Overall, rhamnolipid biosurfactant had a generally positive effect on petroleum hydrocarbon degradation in marsh soils.