초록
<P>The overall goal of this study was to develop an appropriate biological process for achieving autotrophic conversion of methane (CH<SUB>4</SUB>) to methanol (CH<SUB>3</SUB>OH). In this study, we employed ammonia-oxidizing bacteria (AOB) to selectively and partially oxidize CH<SUB>4</SUB> to CH<SUB>3</SUB>OH. In fed-batch reactors using mixed nitrifying enrichment cultures from a continuous bioreactor, up to 59.89 ± 1.12 mg COD/L of CH<SUB>3</SUB>OH 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 CH<SUB>4</SUB> to CH<SUB>3</SUB>OH conversion obtained during this study was 0.82 mg CH<SUB>3</SUB>OH COD/mg AOB biomass COD-d, which is 1.5 times the highest value reported with pure cultures. Notwithstanding these positive results, CH<SUB>4</SUB> oxidation to CH<SUB>3</SUB>OH by AOB was inhibited by NH<SUB>3</SUB> (the primary substrate for the oxidative enzyme, ammonia monooxygenase, AMO) as well as the product, CH<SUB>3</SUB>OH, itself. Further, oxidation of CH<SUB>4</SUB> to CH<SUB>3</SUB>OH by AOB was also limited by reducing equivalents supply, which could be overcome by externally supplying hydroxylamine (NH<SUB>2</SUB>OH) as an electron donor. Therefore, a potential optimum design for promoting CH<SUB>4</SUB> to CH<SUB>3</SUB>OH oxidation by AOB could involve supplying NH<SUB>3</SUB> (needed to maintain AMO activity) uncoupled from the supply of NH<SUB>2</SUB>OH and CH<SUB>4</SUB>. Partial oxidation of CH<SUB>4</SUB>-containing gases to CH<SUB>3</SUB>OH 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 CH<SUB>3</SUB> OH thus produced could be channeled to a downstream anoxic zone in a biological nitrogen removal process to effect nitrate reduction to N<SUB>2</SUB>, using an <I>internally</I> produced organic electron donor.</P><P><B>Graphic Abstract</B><BR><IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2013/esthag.2013.47.issue-7/es3042912/production/images/medium/es-2012-042912_0002.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/es3042912'>ACS Electronic Supporting Info</A></P>