Biosurfactant production by hydrocarbon-degrading bacteria is strongly influenced by the nature and concentration of available carbon substrates, which determine hydrocarbon bioavailability and microbial metabolic responses. In this study, five Bacillus subtilis strains isolated from weathered oil-contaminated sites in Qatar were evaluated for growth and biosurfactant activity using diesel fuel, fresh corn oil, and five-times-overheated corn oil as sole carbon sources. Cultures were grown in mineral salts medium under controlled conditions, and biosurfactant production was assessed through emulsification activity (EA), solubilization activity (SA), and colony-forming unit counts. All strains grew on the tested substrates but exhibited distinct strain-specific responses. Fresh corn oil supported the highest biomass production with values up to 3.3 × 107 CFU mL−1, whereas the strongest emulsification activity yield was observed in diesel cultures at low carbon loading (59 ± 2.3 EU g−1 carbon. Five-times-overheated corn oil maintained more stable emulsification activity across a broader concentration range, indicating tolerance to oxidized hydrocarbons and adaptation to chemically altered substrates. Increasing hydrocarbon concentrations led to progressive declines in EA and SA, indicating inhibitory effects at high substrate loads. Overall, biosurfactant production did not correlate directly with biomass, highlighting the importance of substrate properties in regulating functional output. These findings demonstrate that substrate composition and concentration are key determinants of biosurfactant performance in B. subtilis isolates and support the potential use of waste oils as low-cost feedstocks for biotechnological and bioremediation applications.
Alsayegh et al. (Thu,) studied this question.