OIL POLLUTION IN THE NORTH-SEA - A MICROBIOLOGICAL POINT-OF-VIEW

In this study we determined oil degradation rates in the North Sea und er most natural conditions. We used the heavy fuel oil, Bunker C, the major oil pollutant of the North Sea, as the model oil. Experiments we re conducted in closed systems with water sampled during winter and re peated under identical conditions with water collected during summer. No nitrogen or phosphorous was added and conditions were chosen such t hat neither oxygen nor nutrients, present in the water, would become l imiting during the experiments. We detected a fourfold increased degra dation rate for water samples taken in summer (18 degrees C water temp erature) as compared to water sampled in winter (4 degrees C water tem perature). Under the assumption that biodegradation of oil can be rega rded as a Michaelis-Menten type kinetic reaction, the kinectic constan ts V-max and K-M were determined for oil biodegradation at 4 degrees C and 18 degrees C. At both temperatures K-M was about 40 ppm, whereas V-max was 3-4 times higher at 18 degrees C. From both V-max and the re sults of fermentation studies, we determined the maximum rates of Bunk er C oil degradation in the North Sea as similar to 20 g m(-3)a(-1) at 4 degrees C in winter and 60-80 g m(-3)a(-1) at 18 degrees C in summe r. Furthermore, while over 25% of the oil was degraded within 6 weeks in summer, only 6.6% of the oil was degraded in winter. A higher incub ation temperature in winter (18 degrees C) increased both the rate and the percentage of oil degraded, but degradation did not reach the lev el obtained during the summer. While these data reflect the oxidation only of the hydrocarbons, we conducted experiments directly in the ope n sea to determine the contribution of abiotic factors to oil removal. Approximately 42 % of the oil was lost within 6 weeks under these con ditions in summer and 65 % in winter. However, CC-MS analysis of the r ecovered oil showed no significant change in the alkane pattern that w ould indicate enhanced degradation. Thus, mainly abiotic factors such as erosion and dispersion rather than degradation were responsible for enhanced oil removal. Especially the high loss during winter can be a ttributed to frequent storms resulting in greater dispersion. In concl usion, the higher oil degrading potential of the microbial population in the North Sea was represented by a four times faster oil degradatio n during the summer. In-situ experiments showed that abiotic factors c an have an equal (summer) or even higher (winter) impact on oil remova l.