Regulation of Microbial Methane Cycling in Deep-Sea Sediments

Marshall Bowles, Vladimir Samarkin, Samantha JoyeDepartment of Marine Sciences, University of Georgia UGA

Investigations of Chemosynthetic

Communities on the Lower

Continental Slope of the Gulf of

Mexico

Contract No.: 1435-01-05-39187

DEEP

CHEMOSYNTHETIC

COMMUNITY

CHARACTERIZATION

CRUISE REPORT 7 May – 2 June 2006

October 2006

Site In Situ Concentration

Source

Gulf of Mexico (brine pool)

~25 mM Wankel et al., 2010

Gulf of Mexico (seep sediment) 14 mM

Lapham et al., 2008

Monterey Bay (bottom water

at seep)~20 mM

Pers. Comm. Wankel

Methane in the Deep-SeaMethaneFree gas

DissolvedHydrate

Site In Situ Concentration

Source

Gulf of Mexico (brine pool)

~25 mM Wankel et al., 2010

Gulf of Mexico (seep sediment) 14 mM

Lapham et al., 2008

Monterey Bay (bottom water

at seep)~20 mM

Pers. Comm. Wankel

Methane in the Deep-Sea

Measuring microbial processes

Radiotracers used for Methanotrophs:

14CH4 + Electron Acceptor →H14CO3-

Radiotracers used for Methanogens:

H14CO3- + 4H2 + H+→ 14CH4 + 3H2O

14CH3COO- + H2O → 14CH4 + HCO3-

Radiotracers used for Sulfate Reduction:

35SO42- + Electron Donor →HCO3- + H235S

SR:AOM

Median SR:AOM n

Oil/C2-C5 6 23

Cl- > 580 mM 5 12

High CH4 Flux 9 21

TOTAL 10 52

CH4 + SO42- + H+ → HCO3- + H2S + H2O SR:AOM = 1:1

10:1 is OK, SR can be fueled by other electron donors!

Bowles et al., 2010

AOM Rate=[CH4]× α/t ×(DPM-14CO2/DPM-14CH4)

?

[CH4], mM DPM-14CO2/DPM-14CH4

Rate (nmol cm-3 d-1)

Basis

0.5 0.01 5.3 measured ex situ

2 0.01 21.2 1 atm saturation

200 0.01 2120 in situ saturation

¡3 order of magnitude spread!

With higher [CH4], how much higher could AOM rates be?

How much higher could AOM rates be?

Km estimates range from few mM to 10 mM.

Rat

e (n

mol

cm

-3 d

-1)

[CH4]

The study sites

MCGB

MB Cold seep,gas, oil, mat samples, 5°C

Cold seep,gas, no oil, mat samples 5°C

Hydrothermal, gas, oil, mat samples,

5°C to >100°C

Putting the p back in the deep-sea

Add methane headspace using syringe

CH4

(gas)

Retractable butyl rubber

stopper

Inject isotope, then apply pressure

CH4

(dissolved)

DOES PRESSURE ALONE HAVE AN IMPACT AT CONSTANT METHANE CONCENTRATION?

CH4 = 10 mM

AOM RATES VERSUS [CH4]

AOM RATES VERSUS [CH4]

Site Km(mM)

Vmax(nmol cm-3 d-1)

R2

MC 9 5735 0.99

GB 18 5030 0.96

MB 4 280 0.97

SR VERSUS [CH4]

HCO3- MOG VERSUS [CH4]

PARTICULATE ORGANIC CARBON FORMATION VIA METHANE

SR:AOM VERSUS [CH4]at pressure

1:1

SR:AOM VERSUS [CH4]at pressure

1:1

AOM>>>SRThese findings call for an evaluation of our understanding of the anaerobic cycling of

methane.

What is/are the electron acceptor(s)?

What are the pathways?

What are the energetics?

WHAT MIGHT BE HAPPENING?We speculate that a mechanism involving hydrogen

release might exist: SR and MOG can be hydrogenotrophic processes, both

were stimulated by methane addition.

We can also speculate that there is formation of acetate via this pathway:

Preliminary experiments show the net production of a methane derived volatile organic (conceivably acetate).

Problems: Energetics are tricky and involve a massive hydrogen sink!

EARLY OCEAN IMPLICATIONS

CH4

(Kah et al., 2004)

EARLY OCEAN IMPLICATIONS

CH4

(Kah et al., 2004)

CH4

HCO3-

CH3COOH

CH3COOH

H2

H2

CONCLUSIONS

Accurate AOM rates require that we know in situ methane concentrations.

MOG rates at pressure were measured for the first time.

Pathways not including SR support a bulk of AOM in deep sea sediments measured at methane concentrations

approaching in situ values.

THE ENDTHE END

THANKS!Andreas TeskeScott WankelPeter GirguisKim Hunter

Christof Meile

This work was supported by the NOAA National Institute for Undersea Science and Technology and the National Science Foundation Biological Oceanography Program.