The effect of bioremediation on microbial populations in an oil-contaminated coastal wetland

Abstract

A series of controlled, crude oil applications was carried out in a Texas coastal wetland near the Houston Ship Channel to determine the effectiveness of bioremediation in these sensitive areas. The first application, conducted in 1996, was performed to assess the use of diammonium phosphate and diammonium phosphate plus nitrate (potential electron acceptor) as treatments to stimulate microbial growth and hydrocarbon degradation. The second application, conducted in 1997, was performed to determine the potential of two commercial bioaugmentation products to supplement native microbial populations and enhance hydrocarbon biodegradation. Diammonium phosphate was also re-evaluated during this phase as a biostimulation treatment. For each application, 2 1 test plots were used and data from bioremediation treatment plots were compared to oiled control plots to assess treatment differences. Sediment samples from each phase of research (each application) were analyzed for petroleum hydrocarbon concentrations, microbial numbers, nutrient levels, and acute toxicity. Only data from the microbial enumerations are presented here. For both applications, sediment samples from all oiled test plots showed exponential increases in the numbers of aliphatic hydrocarbon and polycyclic aromatic hydrocarbon (PAH)-degrading microorganisms. These increases were observed at 0-5 cm and 5- 10 cm depths. For the first application, the average numbers of aliphatic-degrading and PAH-degrading microorganisms on nutrient amended plots were slightly higher, though not significantly higher, than populations on oiled control plots on most sample days. Total heterotroph numbers increased slightly following the oil application and were slightly higher on nutrient-amended plots. The numbers of hydrocarbon degraders and total heterotrophs returned to near pre-application levels by the end of the monitoring period. The bin/augmentation products examined during the second application did not significantly increase the numbers of hydrocarbon-degrading microorganisms (aliphatic-degrading or PAH- degrading) on any sample day. The microbial dynamics from both phases of research illustrated a well-documented response to a petroleum hydrocarbon input. Overall, the bioremediation treatments examined in both phases of research did not appear to have a significant impact on the numbers of hydrocarbon degraders or total heterotrophs.