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? 3. Stable isotopes & methane 4. Current BBRC research

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

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

C 6 H 12 O O 2 6 CO H 2 O G 0 = kJ/mol C 6 H 12 O 6 3 CO CH 4 G 0 = -418 kJ/mol

Methanogenic Substrates I. CO 2 -type substrates Carbon dioxide, CO 2 Formate, HCOO - Carbon monoxide, CO II. Methyl substrates Methanol, CH 3 OH Methylamine, CH 3 NH 3 + Dimethylamine, (CH 3 ) 2 NH 2 + Trimethylamine, (CH 3 ) 3 NH + Methylmercaptan, CH 3 SH Dimethylsulphide, (CH 3 ) 2 S III. Acetotrophic substrates Acetate, CH 3 COO - Pyruvate, CH 3 COCOO -

Diversity of methanogenic Archaea Methanobacteriales 5 Genera & 25 species; Substrates: mainly H 2 + CO 2, formate; Methanosphaera + methanol, Methanothermus + reduction of S 0 Methanococcales 5 Genera & 9 species; Substrates: mainly H 2 + CO 2, formate; Methanococcus + pyruvate Methanomicrobiales 8 Genera & 22 species; Substrates: mainly H 2 + CO 2, formate; Methanocorpusculum, Methanoculleus & Methanolacinia + alcohols Methanosarcinales 7 Genera & 19 species; Substrates: mainly methanol & methylamines; Methanosarcina & Methanosaeta + acetate; Methanohalophilus + methylsulphides; Methanosalsum + dimethylsulphide Methanopyrales 1 Genera & 1 species: Methanopyrus; hyperthermophile (110°C) Substrates: H 2 + CO 2

Anaerobic Chain of Decay complex organics (cellulose, hemicellulose) complex organics (cellulose, hemicellulose) fermentive bacteria H 2 + CO 2 + HCOO - CH 3 CH 2 COO - CH 3 CH 2 CH 2 COO - CH 3 CH 2 COO - CH 3 CH 2 CH 2 COO - CH 3 COO - acetogenic bacteria H 2 + CO 2 methanogenic Archaea homoacetogenic bacteria

G 0' kJ/reaction G 0' standard conditions: solutes 1 M; gases 1 atm The importance of syntrophy C 6 H 12 O H 2 O 2 CH 3 COO HCO H H 2 C 6 H 12 O H 2 O CH 3 (CH 2 ) 2 COO HCO H H 2 CH 3 (CH 2 ) 2 COO H 2 O 2 CH 3 COO - + H H 2 CH 3 CH 2 COO H 2 O CH 3 COO - + HCO H + + H 2 2 CH 3 CH 2 OH + 2 H 2 O 2 CH 3 COO H H 2 C 6 H 5 COO H 2 O 3 CH 3 COO - + CO H H 2 4 H 2 + HCO H + CH H 2 O 2 CH 3 COO - + H 2 O CH 4 + HCO H HCO H + CH 3 COO H 2 O G G typical in situ abundance of reactants & products: VFAs 1 mM; HCO mM; glucose 10 M; CH atm; H atm Madigan et al (2003)

Methanotrophic Bacteria 1.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

Substrates used by methylotrophs & methanotrophs Methane, CH 4 Methanol, CH 3 OH Methylamine, CH 3 NH 3 + Dimethylamine, (CH 3 ) 2 NH 2 + Trimethylamine, (CH 3 ) 3 NH + Tetramethylammonium, (CH 3 ) 4 N + Trimethylamine N-oxide, (CH 3 ) 3 NO Trimethylsulphonium, (CH 3 ) 3 S + Formate, HCOO - Formamide, HCONH 2 Carbon monoxide, CO Dimethyl ether, (CH 3 ) 2 O Dimethyl carbonate, CH 3 OCOOCH 3 Dimethyl sulphoxide, (CH 3 ) 2 SO Dimethylsulphide, (CH 3 ) 2 S methane mono- oxygenase CH 4 ===> CH 3 OH

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)

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 CO 2 reduction (H 2 ). Vile et al. (2003). Global Biogeochem. Cycles 17(2), anaerobic C mineralisation in freshwater wetlands along a natural sulphate gradient 36 to 27% SO 4 2- reduction vs. <<1% methanogenesis ? fermentation of organic acids CO 2 Bridgham et al. (1998). Ecology 79, anaerobic C mineralization via methanogenesis: 0.5% in bogs and <2% in fens Wieder & Lang (1988). Biogeochemistry 5, anaerobic C mineralisation in West Virginian Sphagnum bog 38 to 64% SO 4 2- reduction vs. 2.8 to 11.7% methanogenesis

Decoupling of Terminal Carbon Mineralisation Pathway Hines et al. (2001). Geophys. Res. Lett. 28(22), northern wetlands: CH 4 derived mainly from CO 2 /H 2 Acetate accumulation to high levels; ultimately degraded aerobically to CO 2 ?contribution to high levels of DOC/ organic acids in ombrotrophic bogs Lansdown et al. (1992). Geochim. Cosmochim. Acta 56(9), Kings Lake Bog, Washington State (ombrotrophic peatland) CH 4 derived mainly from CO 2 /H 2 ; confirmed with 14 C tracer experiments

winter early spring spring- summer Avery et al. (1999) NovJan Feb Apr Jun Jul NovJan Feb Apr Jun Jul C-CH 4 () soil (peat) temperature (°C) Buck Hollow Bog (Michigan, USA) acetate ( M) CR AF

Duddleston et al. (2002). Geophys. Res. Lett. 28(22), Acetate ( M) Depth (cm) Turnagain Bog (ombrotrophic peatland, Anchorage Alaska; pH 4.6 to 5.1)

'Underachieving' northern wetlands? SO 4 2- H2SH2S O2O2 VFAs CO 2 acetate CH 4 H 2 /CO 2 CH 4 What is the mechanism of acetate production? (i) heterotrophic or (ii) autotrophic Possible causes?: (i) temperature (ii) pH (iii) vegetation (iv) trophic level Questions How much C in acetate normally destined for CH 4 is being converted to CO 2 ? How stable is the decoupling? CH 4 flux & VFAs? (Christensen et al. 2003)

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

VPDB C () atmospheric CH 4 biological & abiological CH 4 C 4 plants freshwater carbonates marine carbonates atmospheric CO 2 C 3 plants petroleum & coal eukaryotic algae Stable Carbon Isotopes after Hoefs (1997)

Methane Flux (% of total) Natural Wetlands Landfills Freshwater Gas Hydrates Oceans ~ -70±5 ~ -60 Ruminants Rice Paddies Termites ~ -63±5 ~ -50±2 ~ -60±5 ~ -66±5 Tyler et al. (1988), Wahlen (1994), Quay et al. (1991, 1999), Breas et al (2002) 13 C of CH 4 Sources Biomass Burning Coal Mining Natural Gas ~ -24±3 ~ -36±7 ~ -43±7 -60±5 -40 to C wt. avg. ~ C atmosphere ~ C wt. avg. ~ C atmosphere ~ -47.3

C-CH 4 () 13 C- CO 2 () marine (CO 2 reduction) Whiticar M. J., Faber E., and Schoell M. (1986) Biogenic methane formation in marine and freshwater environments: CO 2 reduction vs. acetate fermentation - Isotope evidence. Geochimica et Cosmochimica Acta 50, freshwater (acetate fermentation ) methanotrophy or thermogenesis C = C ~ 54 C = C ~ 40 C ~ 86 C = CO -CH = CO CH CO -CH = CO - CH

Environment CO 2 -reduction 13 C-CH 4 13 C of CH 4 with pathway confirmed with 14 C tracers acetate 13 C-CH 4 Study coastal marine peatland rice paddy coastal marine freshwater estuary peatland (May) peatland (June) Alperin et al. (1992) Lansdown et al. (1992) Sugimoto & Wada (1993) Blair et al. (1993) Avery (1996) Avery et al. (1999) ± to to ± ± ± to -37 n/a -43 to -30 n/a -43 ± ± ± 5.4

(r 2 = 0.64; n = 55) Sifton Bog: Hornibrook et al. (2000) C = C-CH 4 () 13 C- CO 2 () C = 54 C = 40 AF CR = = Point Pelee Marsh: (r 2 = 0.83; n = 29) 180 cm surface

intersection: (CH 4 ) ( CO 2 ) Sugimoto & Wada (1993) C 3 compost (soybean meal & rice straw): 13 C = dried rice plants: C (CH 3 - ) 13 C (COOH) dried rice plants: 13 C (CH 3 COOH) = kudzu (fresh green leaves): 13 C (CH 3 COOH) = kudzu: CH 3 - C - O - = O

C-CH 4 () 13 C- CO 2 () -40 Other Wetlands AF CR Bog 3850 Bog S4 = ± 6.1 = ± 4.8 Sugimoto & Wada (1993) Hornibrook et al. (2000)

C-CH 4 () 13 C- CO 2 () -40 Other Wetlands AF CR 12 cm 100 cm Aravena et al. (1993), Lansdown et al. (1992), Waldron et al. (1999) Kings Lake Bog (WA, USA) 0 cm 500 cm Ellergower Moss (Scotland) 65 cm 170 cm Rainy River Peatland (N. Ont.) C = 86 C = 54 C = 40

C-CH 4 () 13 C- CO 2 () shallow deep CH 4 emissions from wetlands CH 4 emissions from wetlands CO 2 reduction acetate fermentation -60±5 flux ? shallow Hornibrook et al. (2000)

UK Sites determine CH 4 pathway predominance using 14 C tracers determine the prevalence of these 13 C distributions in different classes of natural wetlands (SW England & Wales) determine the prevalence of these 13 C distributions in different classes of natural wetlands (SW England & Wales) determine relationship between pore water distribution and 13 C signature of CH 4 emissions determine relationship between pore water distribution and 13 C signature of CH 4 emissions Ms. Helen Bowes (NERC Ph.D. student)

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 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

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? 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. What controls decoupling? Can systems switch TCM processes? Can stable isotope signatures of CH 4 be used as an accurate proxy for biogeochemical and physical processes?