Effects of oil and gas well-drilling fluids on the biomass and community structure of microbiota that colonize sands in running seawater

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Abstract
Well-drilling fluid and a number of the known components (barite, clay, Aldacide®, Surflo®, and Dowicide®, were tested for effects on the biomass and community structure of the microbiota that colonize marine sands exposed for eight weeks to running ambient seawater. Shading the microbiota from light depressed the microflora without a significant effect on the biomass, while well-drilling fluids layered on the surface or mixed with the sand significantly increased a component of the bacteria and the microfauna as reflected in changes in the fatty acid composition. There were some shading effects from the surface layering of well-drilling fluids as reflected in the fatty acids from the microflora when compared to the sands mixed with well-drilling fluids. Barite had essentially no effect on the biomass or community structure while clays increased nearly all of the biomass indicators for the bacteria as well as the microfauna; the clay overlay mirrors the effect of the drilling fluids. Aldacide shifted the bacterial composition, depressing the proportions of microbes containing the cyclopropane fatty acids and the anaerobic pathways of desaturation. Concentrations of 1 and 15μg/L increased the bacterial biomass as reflected in the total lipid (16∶0) and extractable lipid phosphate coupled with a decrease in the total microeukaryotes. Surflo increased the biomass and shifted the bacterial community structure at concentrations between 4 and 800μg/L. The lowest level also stimulated the microfauna. Dowicide at 100μg/L increased the bacteria forming cisvaccenic acid and the microfauna similar to low concentrations of Surflo.