Evaluation of the effect of nutrient ratios on biosurfactant production by Serratia marcescens using a Box-Behnken design

► Biosurfactant activity produced by S. marcescens is influenced by the C/N/Fe/Mg ratio. ► The best treatment using C/N = 5, C/Fe = 26,000 and C/Mg = 30 reduced surface tension (ST) to 30 mN m−1. ► The crude biosurfactant yield was 21.66 g L−1 and the purified yield was 4.1 g L−1. ► The biosurfactant reduced interfacial tension up to 1.5 mN m−1 and E24 to 71.4%. ► The biosurfactant increased the mobility of heavy oil (15° API) from 9.3 to 18 cm.