Biogeochemical processes of methane emission and uptake Edward Hornibrook Bristol Biogeochemistry...
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Transcript of Biogeochemical processes of methane emission and uptake Edward Hornibrook Bristol Biogeochemistry...
Biogeochemical processes of methaneemission and uptake
Edward HornibrookBristol Biogeochemistry Research Centre
Department of Earth SciencesUniversity 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)Universal Phylogenetic Tree of Life (16S & 18S RNA)
Madigan et al (2003)
methanogens
methanotrophs
C6H12O6 + 6 O2 6 CO2 + 6 H2O
G0 = -2870 kJ/mol
C6H12O6 3 CO2 + 3 CH4
G0 = -418 kJ/mol
Methanogenic SubstratesMethanogenic Substrates
I. CO2-type substrates • Carbon dioxide, CO2
• Formate, HCOO-
• Carbon monoxide, COII. Methyl substrates • Methanol, CH3OH • Methylamine, CH3NH3
+
• Dimethylamine, (CH3)2NH2+
• Trimethylamine, (CH3)3NH+
• Methylmercaptan, CH3SH • Dimethylsulphide, (CH3)2S
III. Acetotrophic substrates • Acetate, CH3COO-
• Pyruvate, CH3COCOO-
Diversity of methanogenic ArchaeaDiversity of methanogenic Archaea
Methanobacteriales5 Genera & 25 species; Substrates: mainly H2 + CO2, formate; Methanosphaera + methanol, Methanothermus + reduction of S0
Methanococcales5 Genera & 9 species; Substrates: mainly H2 + CO2, formate; Methanococcus + pyruvate
Methanomicrobiales8 Genera & 22 species; Substrates: mainly H2 + CO2, formate; Methanocorpusculum, Methanoculleus & Methanolacinia + alcohols
Methanosarcinales7 Genera & 19 species; Substrates: mainly methanol & methylamines;Methanosarcina & Methanosaeta + acetate; Methanohalophilus + methylsulphides; Methanosalsum + dimethylsulphide
Methanopyrales1 Genera & 1 species: Methanopyrus; hyperthermophile (110°C) Substrates: H2 + CO2
Anaerobic Chain of DecayAnaerobic Chain of Decay
complex organics(cellulose, hemicellulose)
complex organics(cellulose, hemicellulose)
fermentive bacteriafermentive bacteria H2 + CO2 + HCOO-H2 + CO2 + HCOO-CH3CH2COO-
CH3CH2CH2COO-
CH3CH2COO-
CH3CH2CH2COO-
CH3COO-CH3COO-acetogenic bacteriaacetogenic bacteria
H2 + CO2H2 + CO2
methanogenic Archaeamethanogenic Archaea
homoacetogenic bacteriahomoacetogenic bacteria
G0'
kJ/reaction
G0' standard conditions: solutes 1 M; gases 1 atm
The importance of syntrophyThe importance of syntrophy
C6H12O6 + 4 H2O 2 CH3COO- + 2 HCO3- + 4 H+ + 4 H2
C6H12O6 + 2 H2O CH3(CH2)2COO- + 2 HCO3- + 3 H+ + 2 H2
CH3(CH2)2COO- + 2 H2O 2 CH3COO- + H+ + 2 H2
CH3CH2COO- + 3 H2O CH3COO- + HCO3- + H+ + H2
2 CH3CH2OH + 2 H2O 2 CH3COO- + 2 H+ + 4 H2
C6H5COO- + 6 H2O 3 CH3COO- + CO2 + 2 H+ + 3 H2
4 H2 + HCO3- + H+ CH4 + 3 H2O
2 CH3COO- + H2O CH4 + HCO3-
4 H2 + 2 HCO3- + H+ CH3COO- + 4 H2O
-207
-135
+48
+76
+19
+47
-136
-31
-105
G-319
-284
-18
-6
-37
-18
-3
-25
-7
G typical in situ abundance of reactants & products: VFAs 1 mM; HCO3
- 5 mM; glucose 10 M; CH4 0.6 atm; H2 10-4 atm
Madigan et al (2003)
Methanotrophic BacteriaMethanotrophic 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 & methanotrophsSubstrates 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
methane mono-
oxygenase
CH4 ===> CH3OH
Methanotrophic BacteriaMethanotrophic 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 WetlandsAnaerobic 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. Cycles 17(2), 1058.• anaerobic C mineralisation in freshwater wetlands along a natural sulphate gradient• 36 to 27% SO4
2- reduction vs. <<1% methanogenesis• ? fermentation of organic acids CO2
Bridgham et al. (1998). Ecology 79, 1545-1561.
• anaerobic C mineralization via methanogenesis: 0.5% in bogs and <2% in fens
Wieder & Lang (1988). Biogeochemistry 5, 221-242. • anaerobic C mineralisation in West Virginian Sphagnum bog• 38 to 64% SO4
2- reduction vs. 2.8 to 11.7% methanogenesis
Decoupling of Terminal Carbon Mineralisation PathwayDecoupling of Terminal Carbon Mineralisation Pathway
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
Lansdown et al. (1992). Geochim. Cosmochim. Acta 56(9), 3493-3503.• Kings Lake Bog, Washington State (ombrotrophic peatland)• CH4 derived mainly from CO2/H2; confirmed with 14C tracer experiments
winter earlyspring
spring-summer
Avery et al. (1999)
Nov Jan Feb Apr Jun Jul
Nov Jan Feb Apr Jun Jul
-45
-50
-55
-60
-65
13C
-CH
4 (
‰)
soil
(pea
t)te
mpe
ratu
re (
°C)
20151050
Buck Hollow Bog (Michigan, USA)Buck Hollow Bog (Michigan, USA)
0
200
400
600
800
acet
ate
(M
)
CR CR AF
Duddleston et al. (2002). Geophys. Res. Lett. 28(22), 4251-4254.
1999
Ace
tate
(M
)
1000
800
600
400
200
100
25
0
5
0
-5
-10
-15
-20
-25
Dep
th (
cm)
Turnagain Bog (ombrotrophic peatland, Anchorage Alaska; pH 4.6 to 5.1)Turnagain Bog (ombrotrophic peatland, Anchorage Alaska; pH 4.6 to 5.1)
'Underachieving' northern wetlands?'Underachieving' northern wetlands?
SO42-
H2S
O2
VFAs
CO2
acetate CH4
H2/CO2 CH4
• 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 CH4
is being converted to CO2?• How stable is the decoupling?
• CH4 flux & VFAs? (Christensen et al. 2003)
-values-values
0 +
D, 13C, 15N, 18O, 34S (‰)
-
InternationalStandard
D, 13C, 15N, 18O or 34Sdepleted w.r.t. standard
D, 13C, 15N, 18O or 34Senriched w.r.t. standard
VPDBVPDB
-90 -80 -70 -60 -50 -40 -30 -20 -10 0 +10
13C (‰)
atmospheric CH4
biological & abiological CH4
C4 plants
freshwater carbonates
marine carbonates
atmospheric CO2
C3 plants
petroleum & coal
eukaryotic algae
Stable Carbon IsotopesStable Carbon Isotopes
after Hoefs (1997)
0 5 10 15 20 25Methane Flux (% of total)
Natural Wetlands
Landfills
FreshwaterGas Hydrates
Oceans
~ -70±5‰~ -60‰
~ -60‰
Ruminants
Rice PaddiesTermites
~ -63±5‰
~ -50±2‰~ -60±5‰
~ -66±5‰
Tyler et al. (1988), Wahlen (1994), Quay et al. (1991, 1999), Breas et al (2002)
13C of CH4 Sources13C of CH4 Sources
Biomass BurningCoal MiningNatural Gas
~ -24±3‰~ -36±7‰
~ -43±7‰
-60±5‰-40 to -86‰-40 to -86‰
13Cwt. avg. ~ -54.4‰13Catmosphere ~ -47.3‰
13Cwt. avg. ~ -54.4‰13Catmosphere ~ -47.3‰
-50
-40
-30
-20
-10
0
10
20
-120 -110 -100 -90 -80 -70 -60 -50 -40 -30
13C-CH4 (‰)
13C
-C
O2 (
‰)
marine(CO2 reduction)
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 Acta 50, 693-709.
freshwater(acetate fermentation)
methanotrophyor thermogenesis
C = 1.055
C ~ 54‰
C = 1.040
C ~ 40‰ C
~ 86‰
C = 1.090
CO -CH =CO + 1000
CH + 10002
42 4
CO -CH = CO - CH 2 4 2 4
EnvironmentCO2-reduction
13C-CH4
13C of CH4 with pathway confirmed with 14C tracers13C of CH4 with pathway confirmed with 14C tracers
acetate13C-CH4
Study
coastal marine
peatland
rice paddy
coastal marine
freshwaterestuary
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)
Avery et al. (1999)
-62 ‰
-73 ± 4 ‰
-77 to -60 ‰
-62 to -58 ‰
-72 ± 2.2 ‰
-72 ± 1.3 ‰
-71 ± 1.3 ‰
-39 to -37 ‰
n/a
-43 to -30 ‰
n/a
-43 ± 10 ‰
-43.8 ± 12 ‰
-44.5 ± 5.4 ‰
-10
-20
20
10
0
-30-30-40-50-60-70-80-90
13 13
2 4088 58 4C CCO CHΣ = − −. . (r2 = 0.64; n = 55)Sifton Bog:
Hornibrook et al. (2000)
C = 86‰
13C-CH4 (‰)
13 C
-C
O2
(‰
)
C = 54‰
C = 40‰
AFCR
= -21.3‰ 13
2CCO
13
4CCH = -42.3‰
Point Pelee Marsh: (r2 = 0.83; n = 29)
€
13CΣCO2= −0.45δ13CCH4
− 40.1
180 cm
surface
intersection: -42.3‰ (CH4) -21.3‰ (CO2)
Sugimoto & Wada (1993)
C3 compost (soybean meal & rice straw): 13C = -26.5‰C3 compost (soybean meal & rice straw): 13C = -26.5‰
dried rice plants: -39.7‰ -24.4‰
13C (CH3-)13C (COOH)
dried rice plants: 13C (CH3COOH) = -32.1‰dried rice plants: 13C (CH3COOH) = -32.1‰
kudzu (fresh green leaves): 13C (CH3COOH) = -32.9‰kudzu (fresh green leaves): 13C (CH3COOH) = -32.9‰
kudzu: -42.9‰ -22.9‰
CH3 - C - O-
=
O
-10
-20
20
10
0
-30
-30-40-50-60-70-80-90
13C-CH4 (‰)
13 C
-C
O2
(‰
)
-40
Other WetlandsOther Wetlands
AFCRBog 3850
Bog S4
13
4CCH = -40.7 ± 6.1‰
= -23.9 ± 4.8‰ 13
2CCO
Sugimoto & Wada (1993)
Hornibrook et al. (2000)
-10
-20
20
10
0
-30
-30-40-50-60-70-80-90
13C-CH4 (‰)
13 C
-C
O2
(‰
)
-40
Other WetlandsOther Wetlands
AFCR
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‰
-10
-20
20
10
0
-30-40-50-60-70-80-90
13C-CH4 (‰)
13 C
-C
O2
(‰
)
shallow
deep
CH4 emissionsfrom wetlands
CH4 emissionsfrom wetlands
CO2 reduction
acetate fermentation
-60±5‰flux ? flux ?
shallow
Hornibrook et al. (2000)
UK SitesUK Sites
• determine CH4 pathway predominance using 14C tracers• determine CH4 pathway predominance using 14C tracers
• determine the prevalence of these 13C distributions in different classes of natural wetlands (SW England & Wales)
• determine the prevalence of these 13C distributions in different classes of natural wetlands (SW England & Wales)
• determine relationship between pore water distribution and 13C signature of CH4 emissions
• determine relationship between pore water distribution and 13C signature of CH4 emissions
• Ms. Helen Bowes (NERC Ph.D. student)• Ms. Helen Bowes (NERC Ph.D. student)
Field sitesField sites
1.Cors Caron2.Tor Royal, Dartmoor3.Llyn Mire4.Blanket bog, Elan Valley5.Gors Lywd, Elan Valley6.Crymlyn Bog7.Wicken Fen
1.Cors Caron2.Tor Royal, Dartmoor3.Llyn Mire4.Blanket bog, Elan Valley5.Gors Lywd, Elan Valley6.Crymlyn Bog7.Wicken Fen
1
2
4
6
7
3
5
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 CH4 be used as an accurate proxy for biogeochemical and physical processes?