Scholarly article on topic 'Metabolic manipulation of methanogens for methane machinations'

Metabolic manipulation of methanogens for methane machinations Academic research paper on "Environmental biotechnology"

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Academic research paper on topic "Metabolic manipulation of methanogens for methane machinations"

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Crystal Ball

Metabolic manipulation of methanogens for methane machinations

Thomas K. Wood1'2 *

1 Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802-4400, USA. 2Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, PA 16802-4400, USA.

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 biotechnologi-cal 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 large-scale 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

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 (2016) 0(0), 000-000. doi:10.1111/1751-7915.12425 Funding Information

This work was supported by the Army Research Office (W911NF-14-1-0279).

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 anaerobi-cally 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 Methanothermobac-ter marburgensis (Scheller et al., 2010) and Methanosar-cina 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 (aPy)2 protein complex in the methanogenic host M. ace-tivorans. 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 methanogene-sis; 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 methan-otrophic archaeal population. The M. acetivorans host also contains all the enzymes required to convert

© 2016 The Authors. Microbial Biotechnology published by John Wiley & Sons Ltd and Society for Applied Microbiology.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and

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2 T. K. Wood

captured methane to acetate, by running methanogenesis in reverse (after methanogenic Mcr is replaced with the methanotrophic Mcr from the Black Sea). In effect, the first anaerobic organism that grows on methane as a pure culture was created. Remarkably, the engineered strain grows as a biofilm on solid ferric chloride which was reduced by the electrons generated by growth on methane (Soo et al., 2016). Carbonate in the medium and methane in the headspace were converted into the two carbons of acetate as shown by 13C labelling of both substrates (Soo et al., 2016). These results in effect put an end to the decade's old debate about whether methane can be fixed by running methanogenesis in reverse (Knittel and Boetius, 2009). They also will now enable Mcr to be studied biochemically as it is produced for the first time in active form.

Critically, by capturing methane anaerobically in a pure strain, a new field of biochemical engineering has been created, that of the anaerobic conversion of methane for biotechnological applications. Although not an end in itself, the acetate produced from methane by reversing methanogenesis (Soo et al., 2016) is a building block for many products. For example, via additional metabolic engineering and production of active methan-otrophic Mcr, we have now converted methane into the pure stereoisomer L-lactate which may be used in cosmetics, foods and pharmaceuticals. In addition, by combining the engineered archaeal strain that captures methane with suitable consortia, we have found that we can directly convert methane into electricity in fuel cells. This allows electricity to be generated at remote fracking sites and foregoes the expenditure of billions of dollars that are required for methane transport as well as may help limit methane emissions (methane is a potent greenhouse gas). Therefore, the future is bright for harnessing biologically the world's deposits of methane; i.e. one can envision anaerobic cell factories in which myriad products are produced from methane as well as a methane-driven, electricity generating industry.

Conflict of interest

The author has no conflict of interest to declare. References

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© 2016 The Authors. Microbial Biotechnology published by John Wiley & Sons Ltd and Society for Applied Microbiology.