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Biogeochemical processes of methane emission and uptake

Biogeochemical processes of methane emission and uptake. Edward Hornibrook Bristol Biogeochemistry Research Centre Department of Earth Sciences University of Bristol. Outline. 1. Methanogenesis & methanotrophy. 2. Anaerobic C mineralisation in wetlands - uncertainties?.

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Biogeochemical processes of methane emission and uptake

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  1. Biogeochemical processes of methane emission and uptake Edward Hornibrook Bristol Biogeochemistry Research Centre Department of Earth Sciences University of Bristol

  2. Outline 1. Methanogenesis & methanotrophy 2. Anaerobic C mineralisation in wetlands - uncertainties? 3. Stable isotopes & methane 4. Current BBRC research

  3. Alessandro Volta (1776) "Combustible Air" Wolfe (1993)

  4. methanogens methanotrophs Universal Phylogenetic Tree of Life (16S & 18S RNA) Madigan et al (2003)

  5. C6H12O6 + 6 O2 6 CO2 + 6 H2O DG0 = -2870 kJ/mol C6H12O6 3 CO2 + 3 CH4 DG0 = -418 kJ/mol

  6. I. CO2-type substrates • Carbon dioxide, CO2 • Formate, HCOO- • Carbon monoxide, CO III. Acetotrophic substrates • Acetate, CH3COO- • Pyruvate, CH3COCOO- Methanogenic Substrates II. Methyl substrates • Methanol, CH3OH • Methylamine, CH3NH3+ • Dimethylamine, (CH3)2NH2+ • Trimethylamine, (CH3)3NH+ • Methylmercaptan, CH3SH • Dimethylsulphide, (CH3)2S

  7. Methanosarcinales 7 Genera & 19 species; Substrates: mainly methanol & methylamines; Methanosarcina & Methanosaeta + acetate; Methanohalophilus + methylsulphides; Methanosalsum + dimethylsulphide Diversity of methanogenic Archaea Methanobacteriales 5 Genera & 25 species; Substrates: mainly H2 + CO2, formate; Methanosphaera + methanol, Methanothermus+ reduction of S0 Methanococcales 5 Genera & 9 species; Substrates: mainly H2 + CO2, formate; Methanococcus + pyruvate Methanomicrobiales 8 Genera & 22 species; Substrates: mainly H2 + CO2, formate; Methanocorpusculum, Methanoculleus & Methanolacinia + alcohols Methanopyrales 1 Genera & 1 species:Methanopyrus; hyperthermophile (110°C) Substrates: H2 + CO2

  8. CH3CH2COO- CH3CH2CH2COO- H2 + CO2 + HCOO- fermentive bacteria acetogenic bacteria CH3COO- H2 + CO2 methanogenic Archaea Anaerobic Chain of Decay complex organics (cellulose, hemicellulose) homoacetogenic bacteria

  9. kJ/reaction DG DG0' -207 -319 C6H12O6 + 4 H2O  2 CH3COO- + 2 HCO3- + 4 H+ + 4 H2 -135 -284 C6H12O6 + 2 H2O  CH3(CH2)2COO- + 2 HCO3- + 3 H+ + 2 H2 +48 -18 CH3(CH2)2COO- + 2 H2O  2 CH3COO- + H+ + 2 H2 +76 -6 CH3CH2COO- + 3 H2O  CH3COO- + HCO3- + H+ + H2 +19 -37 2 CH3CH2OH + 2 H2O  2 CH3COO- + 2 H+ + 4 H2 +47 -18 C6H5COO- + 6 H2O  3 CH3COO- + CO2 + 2 H+ + 3 H2 4 H2 + HCO3- + H+ CH4 + 3 H2O -136 -3 -31 -25 2 CH3COO- + H2O  CH4 + HCO3- -7 DG0' standard conditions: solutes 1 M; gases 1 atm The importance of syntrophy 4 H2 + 2 HCO3- + H+ CH3COO- + 4 H2O -105 DG  typical in situ abundance of reactants & products: VFAs 1 mM; HCO3- 5 mM; glucose 10 mM; CH4 0.6 atm; H2 10-4 atm Madigan et al (2003)

  10. Methanotrophic Bacteria Aerobic methane oxidation (Proteobacteria) • Low affinity methanotrophs (culturable) • High affinity methanotrophs (no isolates to date) 2. Anaerobic methane oxidation • Marine environments • Methanogen/ sulphate-reducer consortia

  11. methane mono- oxygenase CH4 ===> CH3OH Substrates used by methylotrophs & methanotrophs • Methane, CH4 • Methanol, CH3OH • Methylamine, CH3NH3+ • Dimethylamine, (CH3)2NH2+ • Trimethylamine, (CH3)3NH+ • Tetramethylammonium, (CH3)4N+ • Trimethylamine N-oxide, (CH3)3NO • Trimethylsulphonium, (CH3)3S+ • Formate, HCOO- • Formamide, HCONH2 • Carbon monoxide, CO • Dimethyl ether, (CH3)2O • Dimethyl ether, (CH3)2O • Dimethyl carbonate, CH3OCOOCH3 • Dimethyl sulphoxide, (CH3)2SO • Dimethylsulphide, (CH3)2S

  12. Methanotrophic Bacteria Type I (Ribulose monophosphate C-assimilation pathway) Methylomonas,Methylomicrobium, Methylobacter, Methylococcus Type II (Serine C-assimilation pathway) Methylosinus,Methylocystis, Methylocella*, Methylocapsa* *acidophiles isolated from peat bogs (Dedysh et al. 2000; 2002)

  13. Anaerobic C Mineralisation in Wetlands Tenet 1: Methanogenesis is the terminal step in anaerobic decay of organic matter in freshwater wetlands. Tenet 2:In most freshwater systems, 2/3 of methanogenesis occurs via acetate fermentation and 1/3 by CO2 reduction (H2). Vile et al. (2003). Global Biogeochem. Cycles17(2), 1058. • anaerobic C mineralisation in freshwater wetlands along a natural sulphate gradient • 36 to 27% SO42- reduction vs. <<1% methanogenesis • ? fermentation of organic acids  CO2 Wieder & Lang (1988). Biogeochemistry5, 221-242. • anaerobic C mineralisation in West Virginian Sphagnum bog • 38 to 64% SO42- reduction vs. 2.8 to 11.7% methanogenesis Bridgham et al. (1998). Ecology79, 1545-1561. • anaerobic C mineralization via methanogenesis: 0.5% in bogs and <2% in fens

  14. Decoupling of Terminal Carbon Mineralisation Pathway Lansdown et al. (1992). Geochim. Cosmochim. Acta56(9), 3493-3503. • Kings Lake Bog, Washington State (ombrotrophic peatland) • CH4 derived mainly from CO2/H2; confirmed with 14C tracer experiments Hines et al. (2001). Geophys. Res. Lett.28(22), 4251-4254. • northern wetlands: CH4 derived mainly from CO2/H2 • Acetate accumulation to high levels; ultimately degraded aerobically to CO2 • ?contribution to high levels of DOC/ organic acids in ombrotrophic bogs

  15. spring- summer early spring winter 800 600 acetate (mM) 400 200 0 Buck Hollow Bog (Michigan, USA) -45 -50 d13C-CH4 (‰) -55 -60 CR AF CR -65 Jul Jun Apr Nov Jan Feb 20 15 soil (peat) temperature (°C) 10 5 0 Jul Jun Apr Nov Jan Feb Avery et al. (1999)

  16. 5 1000 0 800 600 -5 Acetate (mM) 400 Depth (cm) -10 200 -15 100 -20 25 -25 0 1999 Turnagain Bog (ombrotrophic peatland, Anchorage Alaska; pH 4.6 to 5.1) Duddleston et al. (2002). Geophys. Res. Lett.28(22), 4251-4254.

  17. Questions • How much C in acetate normally destined for CH4 is being converted to CO2? • How stable is the decoupling? 'Underachieving' northern wetlands? O2 CO2 SO42- H2S • Possible causes?: (i) temperature (ii) pH (iii) vegetation (iv) trophic level acetate  CH4 • What is the mechanism of acetate production? (i) heterotrophic or (ii) autotrophic VFAs • CH4 flux & VFAs? (Christensen et al. 2003) H2/CO2 CH4

  18. International Standard D, 13C, 15N, 18O or 34S enriched w.r.t. standard D, 13C, 15N, 18O or 34S depleted w.r.t. standard d-values - + 0 D, 13C, 15N, 18O, 34S (‰)

  19. atmospheric CH4 biological & abiological CH4 petroleum & coal eukaryotic algae VPDB C3 plants C4 plants freshwater carbonates marine carbonates atmospheric CO2 Stable Carbon Isotopes -90 -80 -70 -60 -50 -40 -30 -20 -10 0 +10 13C (‰) after Hoefs (1997)

  20. ~ -36±7‰ ~ -43±7‰ ~ -66±5‰ ~ -60‰ ~ -60‰ 13C of CH4 Sources 13Cwt. avg. ~ -54.4‰ 13Catmosphere ~ -47.3‰ Biomass Burning ~ -24±3‰ Coal Mining Natural Gas ~ -50±2‰ Landfills ~ -60±5‰ Ruminants Termites Rice Paddies ~ -63±5‰ Oceans Gas Hydrates -40 to -86‰ Freshwater ~ -70±5‰ -60±5‰ Natural Wetlands 0 10 15 20 25 5 Methane Flux (% of total) Tyler et al. (1988), Wahlen (1994), Quay et al. (1991, 1999), Breas et al (2002)

  21. CO + 1000 CH + 1000 CO -CH = DCO -CH = CO - CH 2 2 4 2 2 4 4 4 marine (CO2 reduction) DC ~ 86‰ DC ~ 54‰ DC ~ 40‰ freshwater (acetate fermentation) aC = 1.090 aC = 1.055 aC = 1.040 methanotrophy or thermogenesis 20 10 0 -10 13C-CO2 (‰) -20 -30 -40 -50 -120 -110 -100 -90 -80 -70 -60 -50 -40 -30 13C-CH4 (‰) Whiticar M. J., Faber E., and Schoell M. (1986) Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation - Isotope evidence. Geochimica et Cosmochimica Acta50, 693-709.

  22. d13C of CH4 with pathway confirmed with 14C tracers acetate d13C-CH4 CO2-reduction d13C-CH4 Study Environment coastal marine -62 ‰ -39 to -37 ‰ Alperin et al. (1992) peatland Lansdown et al. (1992) -73 ± 4 ‰ n/a rice paddy Sugimoto & Wada (1993) -43 to -30 ‰ -77 to -60 ‰ -43 ± 10 ‰ -62 to -58 ‰ coastal marine Blair et al. (1993) freshwater estuary n/a -72 ± 2.2 ‰ Avery (1996) peatland (May) Avery et al. (1999) -43.8 ± 12 ‰ -72 ± 1.3 ‰ peatland (June) Avery et al. (1999) -71 ± 1.3 ‰ -44.5 ± 5.4 ‰

  23. (r2 = 0.83; n = 29) Point Pelee Marsh: 180 cm = -21.3‰ surface = -42.3‰ (r2 = 0.64; n = 55) Sifton Bog: 20 DC = 54‰ DC = 86‰ 10 DC = 40‰ 0 13C-CO2 (‰) -10 -20 CR AF -30 -90 -80 -70 -60 -50 -40 -30 13C-CH4 (‰) Hornibrook et al. (2000)

  24. O = CH3 - C - O- C3 compost (soybean meal & rice straw): 13C = -26.5‰ dried rice plants: 13C (CH3COOH) = -32.1‰ kudzu (fresh green leaves): 13C (CH3COOH) = -32.9‰ 13C (CH3-) 13C (COOH) dried rice plants: -39.7‰ -24.4‰ kudzu: -42.9‰ -22.9‰ intersection: -42.3‰ (CH4) -21.3‰ (CO2) Sugimoto & Wada (1993)

  25. Bog 3850 Bog S4 Sugimoto & Wada (1993) = -23.9 ± 4.8‰ = -40.7 ± 6.1‰ Other Wetlands 20 CR AF 10 0 13C-CO2 (‰) -10 -20 -30 -40 -90 -80 -70 -60 -50 -40 -30 13C-CH4 (‰) Hornibrook et al. (2000)

  26. DC = 86‰ DC = 54‰ 500 cm 100 cm DC = 40‰ 170 cm 12 cm 0 cm 65 cm Rainy River Peatland (N. Ont.) Kings Lake Bog (WA, USA) Ellergower Moss (Scotland) Other Wetlands 20 CR AF 10 0 13C-CO2 (‰) -10 -20 -30 -40 -90 -80 -70 -60 -50 -40 -30 13C-CH4 (‰) Aravena et al. (1993), Lansdown et al. (1992), Waldron et al. (1999)

  27. -60±5‰ CH4 emissions from wetlands flux ? flux ? 20 CO2 reduction 10 acetate fermentation deep 13C-CO2 (‰) 0 -10 shallow shallow -20 -90 -80 -70 -60 -50 -40 -30 13C-CH4 (‰) Hornibrook et al. (2000)

  28. UK Sites • determine the prevalence of these d13C distributions in different classes of natural wetlands (SW England & Wales) • determine CH4 pathway predominance using 14C tracers • determine relationship between pore water distribution and d13C signature of CH4 emissions • Ms. Helen Bowes (NERC Ph.D. student)

  29. Field sites 1.Cors Caron 2.Tor Royal, Dartmoor 3.Llyn Mire 4.Blanket bog, Elan Valley 5.Gors Lywd, Elan Valley 6.Crymlyn Bog 7.Wicken Fen 7 5 4 1 3 6 2

  30. Summary • The relative proportions of anaerobic processes in freshwater wetlands needs to be better characterised. • How wide spread is decoupling of terminal stages of anaerobic C mineralisation in northern wetlands? • What controls decoupling? Can systems switch TCM processes? • Can stable isotope signatures of CH4 be used as an accurate proxy for biogeochemical and physical processes? Models • Better understanding of anaerobic C flow needed to represent microbial activity accurately in process-based models • Integrated models of gas abundance/ emission + accurate simulation of stable isotope signatures.

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