Exogenous surfactants enhance biodegradation of heavy used engine oil by increasing substrate surface area for thermophilic biosurfactants-producing bacteria

This study explored the identification of thermophilic biosurfactants producing bacteria and their potential for heavy used engine oil (HUEO) degradation with and without supplementation of exogenous surfactant. Among the various isolated bacteria Bacillus stercoris S1M and Brevibacillus agri ST2 were selected as potent biosurfactants producing and hydrocarbon (i.e., HUEO) degrading bacteria under thermophilic (50 °C) condition. Both strains viz. S1M and ST2 were found to be efficient biosurfactants producers with significant oil displacement activity (ODA) and emulsification indices (EI24%), with 29.5 ± 2.12 to 18.5 ± 4.94 mm ODA, and 65.25 ± 8.72 to 33.33 ± 9.42% EI to Kerosene oil, respectively). HUEO degradation under thermophilic conditions was evaluated for both strains and defined co-culture. The addition of exogenous surfactant further enhanced biodegradation efficiency for Bacillus stercoris S1M (increased from 46.40 ± 7.8 to 64.87 ± 14.00%), confirming their role in improving hydrocarbon bioavailability. Brevibacillus agri ST2 degradation increased modestly (70.77 ± 13.13 to 74.77 ± 1.46%) and its co-culture (66.39 ± 10.51 to 76.36 ± 7.22%). However, exogenous surfactant exerted an inhibitory effect on the growth of Brevibacillus agri ST2, and in the co-culture. This study demonstrates a novel dual role of surfactant in thermophilic hydrocarbon degradation. It acts not only as a bioavailability enhancer which significantly enhanced HUEO degradation by S1M and its co-culture, but also a potent disruptor of microbial synergy, possessing a growth-inhibitory effect on a key degrading partner i.e., ST2 strain. This reveals a critical strain-specific tradeoff between the metabolic enhancement and ecological interactions in designed bioremediation consortia.