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Estuarine sediment microcosms were treated with combinations of diesel, copper (at two levels), and a mixture of heavy metals (mercury, cadmium, lead, and chromium; at two levels) mimicking the contaminant loadings found in harbor sediments. The effects on the microbial community were monitored by polar lipid fatty acid analysis. Diesel addition increased microbial biomass, caused shifts in some fatty acid structural groups, and decreased starvation biomarkers. Incorporation of diesel hydrocarbons into lipids was expressed as an increase in the proportion of odd-carbon-number fatty acids. No treatment with the metals mixture (mercury, cadmium, lead, and chromium) alone significantly changed any parameter derived from the polar lipid fatty acids, but the increase in microbial biomass from diesel addition was higher with the metals mixture, possibly because of indirect effects caused by reductions in grazing resulting from metal-induced toxicity to bacteriovorous nematodes. Copper also modified the effects of diesel addition, preventing biomass increase but not diesel degradation, suggesting that some of the energy gained from diesel oxidation was expended combating copper toxicity. In the present study, observations indicate that metals in general, and copper in particular, can modify the response of sedimentary microorganisms to petroleum-hydrocarbon contaminants. [ABSTRACT FROM AUTHOR]