|Cartoon of the process. Click to enlarge.|
Researchers at Columbia University have developed a biological process utilizing autotrophic ammonia-oxidizing bacteria (AOB) for the conversion of methane (CH4) to methanol (CH3OH). A paper on their work is published in the ACS journal Environmental Science & Technology.
In fed-batch reactors using mixed nitrifying enrichment cultures from a continuous bioreactor, up to 59.89 ± 1.12 mg COD/L (COD = chemical oxygen demand, an indirect measurement of organic compounds in water) of CH3OH was produced within an incubation time of 7 h—approximately 10x the yield obtained previously using pure cultures of Nitrosomonas europaea. The maximum specific rate of CH4 to CH3OH conversion obtained during this study was 0.82 mg CH3OH COD/mg AOB biomass COD-d—1.5x times the highest value reported with pure cultures.
There are intense efforts globally to develop biobased fuels, chemicals, and energy. While ethanol has been of primary focus in the past few years, it should be noted that other chemicals and biofuels such as methanol can be also attractive. In addition to being used in gasoline blends, methanol can be used in fuel cells, combined with long-chain fatty acids and lipids to form biodiesel, or chemically dimerized to dimethyl ether (DME, also a fuel). Methanol is also one of the most widely used chemicals for enhancing denitrification in wastewater treatment. Methanol is commonly produced from natural gas, by chemical catalysis. The chemical pathway first involves the oxidation of CH4 to CO2 and H2 and subsequent reduction of CO2 to CH3OH, and is quite economically and energy intensive and redundant.
Given that natural gas reserves are finite, it might be more sustainable to look toward alternate sources of CH4 to produce CH3OH, such as anaerobic digester gas, biogas, or landfill gas, which in addition contain moisture and CO2. However, the primary limitation to the more widespread use of such gas mixtures is the cost and energy required to purify the CH4 present and the challenges of handling a gaseous stream.
On the other hand, ammonia-oxidizing bacteria (AOB) can oxidize CH4 to CH3OH via the nonspecific action of the enzyme ammonia monooxygenase (AMO). The other benefit of using bacterial conversion of CH4 to CH3OH is that the contaminants such as moisture and CO2, which need to be removed from anaerobic digestion gas or biogas for chemical conversion to CH3OH, do not pose a limitation for biological conversion. In fact autotrophic AOB can also utilize the CO2 contained in gas mixtures for cell synthesis.—Taher and Chandran
The rationale behind using AOB to oxide methane instead of using methane-oxidizing bacteria (MOB) is rather straightforward, the authors note in their paper: MOB oxidize methane completely to CO2, which cannot be used readily as a fuel. In other words, if MOB were to be used for methanol production, there would need to be some likely non-trivial engineering to selectively inhibit the metabolic pathways that further process CH3OH.
AOB only oxidize CH4 partially to CH3OH—and possibly to trace amounts of formaldehyde (HCHO), which is highly toxic to AOB. Feedback inhibits any further oxidation of CH3OH to HCHO.
AOB do not derive any energy or reducing equivalents from this process, and since AMO requires reducing power to function, continued CH4 oxidation can likely be limited unless reducing power is supplied externally. NH3 is not an ideal or direct source of reducing power, since it can competitively inhibit methane oxidation. The researchers posited that the use of an alternate reducing power source such as NH2OH could promote AOB-mediated CH4 oxidation to CH3OH. They also hypothesized that uncoupling NH3 and CH4 feeding strategies could promote CH4 oxidation to CH3OH by avoiding competition between these two substrates for AMO.
The results obtained highlight the metabolic versatility of AOB to convert CH4 to CH3OH and point to the possibility of developing engineered processes to promote the production and utilization of CH4 as a chemical needed for enhanced denitrification. Once optimized, the successful implementation of this process could potentially allow wastewater treatment plants to offset some of their CH3OH costs.
Consequently, the overall greenhouse footprint of wastewater treatment plants (by lowering CH4 release as well as recovering CH3OH) could be reduced. At the same time, through this microbially mediated approach, redundancies in currently followed chemical conversion of CH4 to CH3OH can be avoided. Further mechanistic and modeling studies are needed to understand the substrate and product fluxes during AOB mediated oxidation of CH4 to CH3OH oxidation and to maximize the kinetics and yield of CH3OH.—Taher and Chandran
Edris Taher and Kartik Chandran (2013) High-Rate, High-Yield Production of Methanol by Ammonia-Oxidizing Bacteria. Environmental Science & Technology doi: 10.1021/es3042912