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?