High-Rate, High-Yield Production of Methanol by Ammonia-Oxidizing Bacteria

Abstract

The overall goal of this study was to develop an appropriate biological process for achieving autotrophic conversion of methane (CH₄) to methanol (CH₃OH). In this study, we employed ammonia-oxidizing bacteria (AOB) to selectively and partially oxidize CH₄ to CH₃OH. In fed-batch reactors using mixed nitrifying enrichment cultures from a continuous bioreactor, up to 59.89 ± 1.12 mg COD/L of CH₃OH was produced within an incubation time of 7 h, which is approximately ten times the yield obtained previously using pure cultures of <i>Nitrosomonas europaea</i>. The maximum specific rate of CH₄ to CH₃OH conversion obtained during this study was 0.82 mg CH₃OH COD/mg AOB biomass COD-d, which is 1.5 times the highest value reported with pure cultures. Notwithstanding these positive results, CH₄ oxidation to CH₃OH by AOB was inhibited by NH₃ (the primary substrate for the oxidative enzyme, ammonia monooxygenase, AMO) as well as the product, CH₃OH, itself. Further, oxidation of CH₄ to CH₃OH by AOB was also limited by reducing equivalents supply, which could be overcome by externally supplying hydroxylamine (NH₂OH) as an electron donor. Therefore, a potential optimum design for promoting CH₄ to CH₃OH oxidation by AOB could involve supplying NH₃ (needed to maintain AMO activity) uncoupled from the supply of NH₂OH and CH₄. Partial oxidation of CH₄-containing gases to CH₃OH by AOB represents an attractive platform for the conversion of a <i>gaseous</i> mixture to an <i>aqueous</i> compound, which could be used as a commodity chemical. Alternately, the nitrate and CH₃ OH thus produced could be channeled to a downstream anoxic zone in a biological nitrogen removal process to effect nitrate reduction to N₂, using an <i>internally</i> produced organic electron donor