Influence of daylight on potential biodegradation of diesel and crude oil in Antarctic seawater

The effects of daylight exposure on Antarctic coastal bacterial communities contaminated by diesel fuel and ‘Arabian light’ crude oil addition were studied in artificial mesocosms during the austral summers of 1991–1992, 1992–1993 and 1993–1994 in the Terre Adelie land area. In order to study the possible influence of photo-oxidation, two sets of experiments were conducted in covered and noncovered batches. Daily sampling allowed regular surveys of the changes of total bacterial abundance, mean cell volumes, saprophytic and hydrocarbon-utilizing bacterial communities. The results clearly revealed a significant response of the Antarctic bacterial community to crude oil addition. Two orders of magnitude increases were observed after contamination. Concomitant enrichments of saprophytic and hydrocarbon-utilizing bacteria occurred during these periods. Hydrocarbon-utilizing bacteria ranged from 0.001% of the total community before contamination to more than 80% after 2 weeks of contamination with crude oil. Chemical analysis of the residual hydrocarbon fraction after 5 weeks of contamination confirmed this potential biodegradation. Solar radiation had no measurable effect upon crude oil-contaminated seawater. In contrast, there was a clear toxic effect upon bacterial communities contaminated with diesel. Data suggest that the initial state of the bacterial communities can play a major role in the potential biodegradation. Some surface bacterial assemblages seem to demonstrate a better resistance to solar radiation than deeper ones