Metabolic manipulation of methanogens for methane machinations

The U.S.A is moving toward energy independence; a distant memory are the long lines for gasoline of the 1970s and fading is the promise of competitive biofuels from Escherichia coli (Liu and Khosla, 2010; Steen et al., 2010). The reason is cheap methane. Global amounts of shale gas total 7300 trillion cubic feet (U.S. Energy Information Administration, 2013) and its major constituent is methane. This remarkable availability of methane is now driving synthetic biology, and an exciting prediction is that methane will be harnessed for biotechnological applications using not the traditional workhorse E. coli or aerobic methanotrophs, but instead, using archaeal strains, specifically methanogens, in anaerobic fermentations based on biosynthetic pathways such as that recently shown to convert methane to the biotechnological building block acetate (Soo et al., 2016). As opposed to chemical plants which employ Fischer– Tropsch processes to convert methane into liquid fuels and require complex technology that demands largescale investment up to ~$20 billion, biological conversion of methane is a more economically and environmentally sustainable, as it requires a smaller footprint and is less technologically complex (Haynes and Gonzalez, 2014). Hence, harnessing methane has been recognized as one of the most important near-term goals for biochemical engineering (Lee and Kim, 2015). Notably, from the recent realization that anaerobic processes confer higher energy and carbon yield efficiencies with lower CO2 emissions than aerobic ones for converting methane into products (Haynes and Gonzalez, 2014), there is interest in using anaerobes rather than the traditional, better-studied aerobic methanotrophs (Lawton and Rosenzweig, 2016). The first process used to capture methane anaerobically for biotechnology applications (Soo et al., 2016) is based on the natural process of anaerobic methane oxidation (AOM), which efficiently captures up to 300 Tg of methane per year to limit global methane emissions (Knittel and Boetius, 2009). AOM occurs in natural consortia consisting of anaerobic methanotrophic archaeal populations and syntrophic bacteria. Methane is activated by reversing methanogenesis and was hypothesized to be catalysed by methyl-coenzyme M reductase (Mcr) based on the prevalence of mcr genes in ANME populations (Hallam et al., 2004) and the trace AOM seen in the anaerobic methanogens Methanothermobacter marburgensis (Scheller et al., 2010) and Methanosarcina acetivorans (Moran et al., 2005, 2007). This hypothesis had been difficult to prove as these natural consortia are enigmatic due to their long lag phase (~60 years) (Dale et al., 2008) and doubling time (~7 months) (Nauhaus et al., 2007). Critically, no one has been able to culture these organisms independently (Scheller et al., 2010). Of course the way to circumvent the problem of not being able to culture anaerobic methanotrophic archaeal populations is to utilize the metagenome of these organisms from a microbial mat in the Black Sea (Meyerdierks et al., 2010; Shima et al., 2012). From this metagenome, Soo et al. (2016) cloned the genes encoding the Mcr (3.9 kb) and expressed this 280 kDa heterohexameric (abc)2 protein complex in the methanogenic host M. acetivorans. This host was chosen as it has the largest archaeal genome (Galagan et al., 2002), is genetically tractable (Kohler and Metcalf, 2012) and encodes a native Mcr for producing methane during methanogenesis; hence, it was reasoned that this host may be able to provide the methylthio-F430 cofactor (or suitable substitute) to produce active Mcr from the anaerobic methanotrophic archaeal population. The M. acetivorans host also contains all the enzymes required to convert Received 18 September, 2016; accepted 18 September, 2016. *For correspondence. E-mail tuw14@psu.edu; Tel. (+)1 814-8634811; Fax (1) 814-865-7846. Microbial Biotechnology (2017) 10(1), 9–10 doi:10.1111/1751-7915.12425 Funding Information This work was supported by the Army Research Office (W911NF14-1-0279).