Expression of Vitreoscilla hemoglobin in Bacillus thuringiensis BMB171 can promote manganese(II) oxidation under oxygen-restricted conditions

Manganese (Mn) is one of the most abundant elements on the Earth's crust. Nowadays, soluble Mn(II) contamination in groundwater and surface water as well as industrial wastewater is a critical environmental issue in some regions of the world. The application of Mn(II)-oxidizing microorganisms to treat Mn(II)-contaminated water is one of the most useful approaches to solve this problem. However, dissolved oxygen is a primary limitation factor for both microbial growth and Mn(II) oxidation in bioreactor systems utilizing microbial biofilms. In the study reported here we demonstrated that heterologous expression of Vitreoscilla hemoglobin (VHb) in Bacillus thuringiensis BMB171 not only enhanced the bacterial cell density but also improved the total Mn(II) oxidation activity of the recombinant strain BMB-VHb(+) under oxygen-restricted conditions. Previous experiments have verified that VHb, an oxygen carrier, can facilitate the transport of oxygen into bacterial cells. Importantly, we revealed for the first time that the expression of VHb promoted the Mn(II) oxidation rate of each recombinant bacterial cell. There are two possible explanations: (1) VHb improves the concentration of substrate intracellular oxygen, and (2) VHb itself may have oxidase activity. Based on these results, we suggest that this strategy has potential applications for the treatment of the Mn(II)-contaminated water.