Biofuels: Microbially Generated Methane and Hydrogen

Abstract

The production of methane (CH4) or hydrogen (H2) from renewable biomass by microorganisms growing anaerobically has the potential for contribution to independence from fossil fuels. Anaerobes function in Nature by converting biomass to CH4 through food chains comprised of fermentative and acetogenic species, which decompose the complex biomass to H2, formate and acetate that are further metabolised to CH4 by methanogens. Methanogens reduce the concentration of products to levels that permit the initial decomposition of biomass by fermentative and acetogenic species. Current H2 production relies extensively on energy‐intensive fossil fuel sources. Photosynthetic and fermentative species offer more efficient routes for H2 production. Although fermentatives have significantly higher production rates, they have lower yields of H2 but may be a source of other valuable compounds that are synthesised along with H2. Further research must be conducted on obtaining H2 from reductive pools of NAD(P)H to increase yields and increase economic competitiveness.

Key Concepts:

  • Methane production from biomass, an essential component of the global carbon cycle, requires a microbial food chain.

  • Methane‐producing species (methanogens) are terminal organisms of the food chain metabolising acetate, formate and H2 that are metabolic products of species initiating decomposition of the biomass.

  • The methyl group of acetate is reduced to methane with electrons derived from oxidation of the carbonyl group.

  • Carbon dioxide is reduced to methane with electrons derived from oxidation of H2 or formate.

  • Current hydrogen production relies on fossil fuel sources.

  • Photosynthetic hydrogen production yields are high, but rates are slow.

  • Dark hydrogen production rates are fast, but yields are low.

  • Higher yields through dark fermentation may be obtained using pathways that use NAD(P)H.

  • Additional products such as 1,3‐propanediol and ethanol may be obtained along with hydrogen in dark fermentations.

Keywords: food chain; anaerobic; aceticlastic; fermentation; NADPH; hydrogenase; nitrogenase

Figure 1.

The global carbon cycle. Aerobic O2‐requiring conversions are shown in solid red arrows and anaerobic conversions in solid blue arrows. Black dotted arrows symbolise diffusion of substrates and products across the interface of aerobic and anaerobic zones.

Figure 2.

Composite of CO2 reduction and aceticlastic methane‐producing pathways. The left arm leading to CH3‐H4M(S)PT shows reactions (1–4) unique to the aceticlastic pathway and the right arm leading to CH3‐H4M(S)PT shows reactions (5–9) unique to the CO2 reduction pathway. Both pathways have in common reactions (10, 11 and 12) leading to the formation of CH4 from the methyl groups of CH3‐H4M(S)PT. Abbreviations: ATP, adenosine triphosphate; H4SPT, tetrahydrosarcinapterin; H4MPT, tetrahydromethanopterin; Fd, ferredoxin; CoA, coenzyme A; CoM, coenzyme M; CoB, coenzyme B; MF, methanofuran; F420, coenzyme F420.

Figure 3.

Comparison of electron transport pathways in acetotrophic methanogens. (a) H2‐dependent. (b) H2‐independent. Abbreviations: Ech, Ech hydrogenase; Fdr, ferredoxin reduced; Fdo, ferredoxin oxidised; Vho, Vho hydrogenase; MP, methanophenazine; HdrDE, heterodisulfide reductase; CoM‐SH, coenzyme M; CoB‐SH, coenzyme B; Atp, ATP synthase; Cyt c, cytochrome c; MaRnf, Rnf complex from Methanosarcina acetivorans; Mrp, putative sodium/proton antiporter.

Figure 4.

Fermentative pathways leading to production of H2 and other important products. Metabolites shown in blue indicate carbon substrates, whereas those in red indicate final fermentation products. Abbreviations: Fdred, reduced ferredoxin; Fdox, oxidised ferredoxin; Pi, orthophosphate.

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McAnulty, Michael J, Vepachedu, Venkata R, Wood, Thomas K, and Ferry, James G(Mar 2013) Biofuels: Microbially Generated Methane and Hydrogen. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020375]