Fate of oil from two major oil spills: Role of microbial degradation in removing oil from the Amoco Cadiz and IXTOC I spills

The fate of oil was examined in intertidal sediments impacted by the Amoco Cadiz spill in Brittany, France, and in surface waters contaminated by the IXTOC I well blowout in the Bay of Campeche, Mexico. The role of microbial degradation in removing the contaminating hydrocarbons was assessed by laboratory determinations of numbers of hydrocarbon utilizing microorganisms, measurement of microbial hydrocarbon biodegradation potentials, by chemical analyses of samples incubated under controlled conditions and by examination of the evolution of the hydrocarbon mixture in field samples collected after the spill. Analyses included the use of ¹⁴C radiolabelled hydrocarbon tracers, glass capillary-gas chromatography and mass spectrometry. Numbers of hydrocarbon utilizing microorganisms were found to be elevated by several orders of magnitude in oil contaminated samples. Numbers of hydrocarbon utilizing microorganisms appear to provide a useful index of environmental hydrocarbon contamination. However, environmental constraints on microbial processes appear to be most important in determining actual rates of biodegradation rather than numbers of available hydrocarbon utilizing microorganisms. Very high rates of degradation were found in intertidal sediments impacted by the Amoco Cadiz spill. In contrast, rates of degradation in the Gulf of Mexico were found to be much lower, partly due to nutrient limitations and partly due to the formation of large masses of oil-water emulsion or mousse.