Degradation of a petroleum hydrocarbon in coastal sediments

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Abstract
The bacterial degradation of naphthalene in surface sediments from an oil5polluted Danish marine inlet has been investigated by two methods, i.e. traditional shake-flask incubation and intact sediment cores brought to the laboratory. The degradation was estimated on the basis of released radioactive CO2 after addition of 14C-naphthalene. The shake-flask experiment showed a significant increase in the number of oil-degrading bacteria presumably caused by the physical disturbance, while the number in the sediment cores remained relatively constant. The degradation rate and the turnover time of naphthalene were of the same order of magnitude in the two experiments. However, the shake-flasks revealed a high degradation rate in the beginning of the experiment, whereafter the degradation decreased, possibly due to lack of oxygen and/or nutrients. The sediment cores showed the fastest degradation after three days but suffered from oxygen deficiency after five days. After addition of labelled substrate to the sediment cores, this system needed more than 24 hours to reach maximum degradation rate. The shake-flask experiment could only be retained for some hours and underestimated the turnover time compared with the sediment core microcosms. One of the requirements for shake-flask experiments (transport kinetics) is not met, i.e. constant bacterial population size. Better results will be obtained using sediment core microcosms to estimate natural degradation rates of organic substances. The experiments indicated a turnover time of naphthalene of two years at the sampling point.