Comparative effects of fossil fuel hydrocarbons on microbial activity in freshwater sediments

Studies of freshwater sediment microbial activity were conducted using adenosine triphosphate (ATP) and dehydrogenase enzyme activities as measures of community viability and metabolic activity. A field study of two sites on Melton Hill Lake and a small pond in east Tennessee was conducted on a seasonal basis to provide background information on possible seasonal fluctuations in microbial activity in three different aquatic ecosystems. A microcosm experiment simulating conditions at one of the field sites was conducted to determine possible effects of hydrocarbons from chronic low-to moderate-level inputs of a diesel fuel marine (DFM) oil or a synthetic coal derived crude (ASO) oil on the microbial community. Adenosine triphosphate was extracted with 0.6 N cold sulfuric acid and ATP quantified via bioluminescence. A rapid dehydrogenase analysis procedure was developed by modifying several standard dehydrogenase analysis procedures. The correlation coefficients of the results of the field dehydrogenase analyses and microcosm dehydrogenase analyses using 10 and 60 minute incubations were correlated (∝ =0.0001) at r=0.907 for the field, and r=0.686 for the microcosm equilibration period and r=0.892 for the microcosm treatment period. The results of the field study showed that ATP content was significantly different (∝ =0.05) at the three sites, but not on a seasonal basis. Field dehydrogenase data showed site and seasonal effects on microbial activity.

Both oil treatments may have had significant effects on ATP content after the first oil treatment. The microcosm sediments were not significantly different in ATP content at the end of the experiment. When the treatment period as a whole is considered, comparing treatment means, the hydrocarbon treatments appear to lower ATP content significantly. Dehydrogenase activity, when considered for individual microcosms on separate sampling days, was the same for all the microcosms immediately after the first treatment. After the second treatment, the ASO-treated microcosms were significantly lower in activity than the controls and DFM-treated microcosms. After the fourth hydrocarbon treatment, the DFM-treated microcosm sediments tended to have greater dehydrogenase activity than the controls, which tended to have greater activity than the ASO-treated microcosms. At the end of the experiment, after the fifth oil treatment, the controls had significantly greater dehydrogenase activity than the ASO-treated microcosms, while the DFM-treated microcosms tended to have activity midway between the controls and ASO-treated microcosms. When mean overall treatment statistics are compared for the treatment period, dehydrogenase activity in the ASO-treated microcosms was significantly lower than the controls and DFM-treated systems.

The results of this general examination of the effects of allochthonous hydrocarbons on microbial activity reveals a need for more detailed study.