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)
methanogens
methanotrophs
Madigan et al (2003)
C6H12O6 + 6 O2  6 CO2 + 6 H2O
DG0 = -2870 kJ/mol
C6H12O6 3 CO2 + 3 CH4
DG0 = -418 kJ/mol
Methanogenic Substrates
I. CO2-type substrates
• Carbon dioxide, CO2
• Formate, HCOO• Carbon monoxide, CO
II. 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 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
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: H2 + CO2
Anaerobic Chain of Decay
complex organics
(cellulose, hemicellulose)
CH3CH2COOCH3CH2CH2COO-
acetogenic bacteria
fermentive bacteria
CH3COO-
H2 + CO2
methanogenic Archaea
H2 + CO2 + HCOO-
homoacetogenic bacteria
The importance of syntrophy
C6H12O6 + 4 H2O  2 CH3COO- + 2 HCO3- + 4 H+ + 4 H2
kJ/reaction
DG0' DG
-207
-319
C6H12O6 + 2 H2O  CH3(CH2)2COO- + 2 HCO3- + 3 H+ + 2 H2 -135
-284
CH3(CH2)2COO- + 2 H2O  2 CH3COO- + H+ + 2 H2
+48
-18
CH3CH2COO- + 3 H2O  CH3COO- + HCO3- + H+ + H2
+76
-6
2 CH3CH2OH + 2 H2O  2 CH3COO- + 2 H+ + 4 H2
+19
-37
C6H5COO- + 6 H2O  3 CH3COO- + CO2 + 2 H+ + 3 H2
+47
-18
4 H2 + HCO3- + H+  CH4 + 3 H2O
-136
-3
2 CH3COO- + H2O  CH4 + HCO3-
-31
-25
-105
-7
4 H2 + 2 HCO3- + H+  CH3COO- + 4 H2O
DG0'  standard conditions: solutes 1 M; gases 1 atm
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)
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, 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
monooxygenase
CH4 ===> CH3OH
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 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% SO42- reduction vs. <<1% methanogenesis
• ? fermentation of organic acids  CO2
Wieder & Lang (1988). Biogeochemistry 5, 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). Ecology 79, 1545-1561.
• anaerobic C mineralization via methanogenesis: 0.5% in bogs and <2% in fens
Decoupling of Terminal Carbon Mineralisation Pathway
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
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
Buck Hollow Bog (Michigan, USA)
winter
800
-50
600
-55
400
-60
200
CR
-65
soil (peat)
temperature (°C)
early springspring summer
CR
acetate (mM)
d13C-CH4 (‰)
-45
AF
0
Nov Jan Feb Apr Jun Jul
20
15
10
5
0
Nov Jan Feb Apr Jun Jul
Avery et al. (1999)
Turnagain Bog (ombrotrophic peatland, Anchorage Alaska; pH 4.6 to 5.1)
1000
0
800
-5
600
400
-10
200
-15
Acetate (mM)
Depth (cm)
5
100
-20
25
-25
0
1999
Duddleston et al. (2002). Geophys. Res. Lett. 28(22), 4251-4254.
'Underachieving' northern wetlands?
Questions
O2
CO2
SO4
2-
H 2S
acetate  CH4
• How much C in acetate normally destined for CH4
is being converted to CO2?
• How stable is the decoupling?
• Possible causes?: (i) temperature
(ii) pH
(iii) vegetation
(iv) trophic level
• What is the mechanism of acetate production?
VFAs
H2/CO2  CH4
(i) heterotrophic or (ii) autotrophic
• CH4 flux & VFAs? (Christensen et al. 2003)
d-values
International
Standard
D, 13C, 15N, 18O or 34S
depleted w.r.t. standard
-
D, 13C, 15N, 18O or 34S
enriched w.r.t. standard
0
dD, d13C, d15N, d18O, d34S (‰)
+
Stable Carbon Isotopes
atmospheric CH4
biological & abiological CH4
petroleum & coal
eukaryotic algae
VPDB
C3 plants
C4 plants
atmospheric CO2
freshwater carbonates
marine carbonates
-90 -80 -70 -60 -50 -40 -30 -20 -10
0
+10
d13C (‰)
after Hoefs (1997)
d13C of CH4 Sources
d13Cwt. avg. ~ -54.4‰
d13Catmosphere ~ -47.3‰
~ -24±3‰
Biomass Burning
~ -36±7‰
Coal Mining
~ -43±7‰
Natural Gas
~ -50±2‰
Landfills
~ -60±5‰
Ruminants
~ -66±5‰
Termites
Rice Paddies
~ -63±5‰
~ -60‰
Oceans
Gas Hydrates ~ -60‰
-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)
dCO2 + 1000
CO2 -CH 4 =
dCH 4 + 1000
DCO2 -CH4 = dCO2 - dCH 4
20
d13C-CO2 (‰)
10
0
-10
-20
-30
-40
-50
-120 -110 -100 -90
-80
-70
-60
-50
-40
-30
d13C-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 Acta 50, 693-709.
d13C of CH4 with pathway confirmed with 14C tracers
Environment
coastal marine
peatland
CO2-reduction
d13C-CH4
-62 ‰
-73 ± 4 ‰
acetate
d13C-CH4
Study
-39 to -37 ‰ Alperin et al. (1992)
n/a
Lansdown et al. (1992)
rice paddy
-77 to -60 ‰
-43 to -30 ‰ Sugimoto & Wada (1993)
coastal marine
-62 to -58 ‰
-43 ± 10 ‰
freshwater
estuary
-72 ± 2.2 ‰
n/a
peatland (May)
-72 ± 1.3 ‰
-43.8 ± 12 ‰ Avery et al. (1999)
peatland (June)
-71 ± 1.3 ‰
-44.5 ± 5.4 ‰ Avery et al. (1999)
Blair et al. (1993)
Avery (1996)
d13C-CO2 (‰)
Sifton Bog: d 13 CCO  0.88d 13 CCH  58.4 (r2 = 0.64; n = 55)
Point Pelee Marsh: d13CCO  0.45d13CCH  40.1 (r2 = 0.83; n = 29)
20
10
2
4
2
4

0
-10
-20
180 cm
d 13 CCO = -21.3‰
2
surface
CR
AF
d 13 CCH = -42.3‰
4
-30
-90
-80
-70
-60
-50
-40
-30
d13C-CH4 (‰)
Hornibrook et al. (2000)
C3 compost (soybean meal & rice straw): d13C = -26.5‰
dried rice plants: d13C (CH3COOH) = -32.1‰
kudzu (fresh green leaves): d13C (CH3COOH) = -32.9‰
=
O
CH3 - C - Od13C (CH3-)
dried rice plants:
kudzu:
intersection:
-39.7‰
-42.9‰
-42.3‰ (CH4)
d13C (COOH)
-24.4‰
-22.9‰
-21.3‰ (CO2)
Sugimoto & Wada (1993)
Other Wetlands
d13C-CO2 (‰)
20
Bog 3850
Bog S4
10
CR
AF
0
Sugimoto & Wada (1993)
-10
-20
-30
d 13 CCO = -23.9 ± 4.8‰
-40
-90
2
d 13 CCH = -40.7 ± 6.1‰
4
-80
-70
-60
-50
-40
-30
d13C-CH4 (‰)
Hornibrook et al. (2000)
Other Wetlands
d13C-CO2 (‰)
20
500 cm
10
CR
AF
100 cm
0
170 cm
-10
-20
-30
-40
-90
12 cm 0 cm
65 cm
Kings Lake Bog (WA, USA)
Ellergower Moss (Scotland)
Rainy River Peatland (N. Ont.)
-80
-70
-60
-50
-40
-30
d13C-CH4 (‰)
Aravena et al. (1993), Lansdown et al. (1992), Waldron et al. (1999)
d13C-CO2 (‰)
20
CO2 reduction
10
acetate
fermentation
deep
0
-10
-20
shallow
shallow
-90
-60±5‰
flux ?
-80
flux ?
-70
-60
CH4 emissions
from wetlands
-50
-40
-30
d13C-CH4 (‰)
Hornibrook et al. (2000)
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)
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
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.