Microbial Dynamics during Bioremediation of a Crude Oil-Contaminated Coastal Wetland

In 1996, a controlled crude oil application was conducted at a Texas intertidal, coastal wetland to determine the effectiveness of two biostimulation treatments in these sensitive areas. An inorganic nutrient treatment and inorganic nutrient plus a potential electron acceptor (nitrate) treatment were examined. As part of this research, polycyclic aromatic hydrocarbon (PAH)-degrading, aliphatic-degrading, and total heterotrophic microbial numbers were monitored. Using a randomized, complete block design consisting of 21 plots, microbial data from biostimulation treatment plots were statistically compared to oiled control plots to assess treatment differences. Sediment samples from all plots receiving oil showed exponential increases in the numbers of aliphatic (n-alkane) and PAH-degrading microorganisms. This increase was observed at both 0 to 5 cm and 5 to 10 cm sample depths. Statistical analysis, however, revealed no significant differences in the numbers of aliphatic-degrading or PAH-degrading microorganisms between treatment plots and oiled control plots or between treatments on any sample day. The numbers of PAH- and aliphatic-degrading microorganisms returned to near pre-application levels by the end of the monitoring period. Ratios of hydrocarbon-degrading microbes to total heterotrophs also increased as a result of the oil application and returned to pre-application levels by the end of the monitoring period. Overall, the populations of hydrocarbon-degrading microorganisms illustrated a well-documented response to crude oil. However, the addition of the biostimulation treatments did not significantly increase the numbers of aliphatic-degrading, PAH-degrading, or total heterotrophic microorganisms over populations on control plots.