Enhanced aerobic denitrification performance with Bacillus licheniformis via secreting lipopeptide biosurfactant lichenysin

Aerobic denitrification efficiency is not always satisfactory and high nitrite accumulation and N2O emission occur frequently. Though NOx reduction during aerobic denitrification is influenced by neighboring microbes, the underlying mechanism has been seldomly studied. Here, we showed markedly improved denitrification performance of a common aerobic denitrifier in its response to co-culturing with an abundant co-existing bacterium Bacillus licheniformis, and NOx removal by B. licheniformis was marginal. Compared with that in solo culture, the growth rate of denitrifiers in co-culture varied slightly, but the cytoplasmic nitrate reductase (NAR) exhibited obvious reduction capability and its encoding genes narGHI increased to 95.5 ∼ 122.2 times. It suggested that B. licheniformis improved nitrate removal by enhancing NAR activity instead of denitrifiers density. Transwell assay demonstrated non-contacting B. licheniformis could also promote nitrate and nitrite removal, and following LC-MS showed noticeable difference in the concentration of lichenysin, a lipopeptides biosurfactant metabolite from B. licheniformis, in its solo-culture and co-culture with denitrifiers. Following observation that exogenous lichenysin, but lichenysin producing mutant strain, enhanced aerobic denitrification performance verified the critical role of lichenysin here. Further investigation illustrated that lichenysin promoted the membrane permeability of aerobic denitrifiers, which facilitated the transmembrane transport of nitrate and glucose, and subsequently made more nitrate accessible to NAR and more glucose participate into oxidation for NADH generation respectively. By these means, increased nitrate reduction with less nitrite accumulation and N2O emission was realized in the presence of B. licheniformis. Finally, enhancement of B. licheniformis on performance of aerobic denitrifying community from wastewater treatment plant was also observed in laboratory, which suggested the feasibility of its practical application in the future.