RESUMEN
In November 2002, the oil tanker Prestige sank off the northwestern coast of Spain, spilling more than 50,000 tons of petroleum with disastrous ecological and economical consequences. In order to analyse the harmful consequences of the oil spill on marine microalgae, short- and long-term effects of oil samples from the Prestige spill were studied using laboratory cultures of Dunaliella tertiolecta (strain Dt1Lwt). Significant inhibition of photosynthesis (assessed by F(v)/F(m), ETR(max) and alpha estimations) was observed after only 1h of oil exposure with clear concentration dependency. Three days later, photosynthetic activity remained inhibited although cell survival was only slightly effected. In cultures exposed to the lowest oil concentration, mitotic rates and percentage of motile cells were 17-33% and 12-42% of the controls, respectively. After 1 month, neither dividing nor motile cells were observed at the highest oil concentrations. However, after further incubation, occasionally the growth of rare cells resistant to oil was found. A fluctuation analysis was carried out to distinguish between resistant cells arising from rare spontaneous mutations and resistant cells arising from physiological or other mechanisms of adaptation. The existence of rapid evolution as result of preselective mutations from petroleum sensitivity to petroleum resistance was observed. Resistant cells arose by rare spontaneous mutations prior to the addition of oil, with a mutation rate of 2.76x10(-5) oil-resistant mutants per cell division. Apparently, rare spontaneous preselective mutations are able to assure the survival of microalgae in oil-polluted environments.