Environmental legislation has driven the reduction of sulfur levels in automotive fuels worldwide (<= 10 ppm). We evaluate the behavior of microbial biomass in terms of community composition, metabolite production, and degradation of the Brazilian blend B10, made with ultra-low-sulfur diesel (ULSD, 6.3 ppm), high-sulfur diesel (HSD, 327 ppm), and ultra-high sulfur diesel (UHSD, 861 ppm) during simulated storage. The microcosm was assembled in glass flasks containing an aqueous phase (mineral medium) and an oil phase (fuels) at two conditions of microbial contamination: natural;(similar to 10(3) colony-forming units (CFU) per liter) and inoculated(similar to 10(6) bacterial cells and fungal spores per milliliter), evaluated each 10 days for 40 days. The results showed that biomass production was more pronounced in inoculated treatment and could be described, at T-40 as UHSD < ULSD < HSD B10. The higher degradation of terminal methyl ester fraction (50 1%), and aromatic compounds (46 2%) was in HSD B10, and ULSD B10 suffered the lowest degradation (23 +/- 3%; 26 +/- 7%, respectively) (p < 0.05). Pseudomonas,(Proteobacteria) was the predominant bacterial genus at the interface (similar to 91%), but in the water phase, changes in relative abundance and development of Pandoraea (Proteobacteria) and Propionispora (Firmicutes) were observed (p < 0.05). Ascomycota and Basidiomycota were the most abundant fungal phyla (similar to,78%). Putative fatty acids myristic, palmitic, stearic, oleic, linoleic, and alpha-linolenic acid were detected in the water phase with high relative abundance at all sulfur levels compared to controls (p < 0.05), indicating the degradation products of the fatty acid methyl esters present in soybean biodiesel. The data set suggests that the reduction of sulfur content may have favored microbial growth; however, the addition of fatty acid methyl ester (FAIVLE), additives, and the origin of petroleum-based fuels may be a more -relevant factor in the B10-blend, aerobic biodegradation.