Methane production - Methanogenesis Substrates / pathways Isotopic studies Hydrogen cycling Methane consumption - Anaerobic methane oxidation Methane hydrates (Thermogenic methane) (Hydrothermal vent methane)

Methanogens (Zinder; Oremland) Archaea. Relatively few species (30-40), but highly diverse (3 orders, 6 families, 12 genera). Highly specialized in terms of food sources – Can only use simple compounds (1 or 2 carbon atoms), and many species can only use 1 or 2 of these simple compounds. Therefore, dependent on other organisms for their substrates; food web / consortium required to utilize sediment organic matter. Strict anaerobes.

Two main methanogenic pathways: CO 2 reduction Acetate fermentation Both pathways found in both marine and freshwater systems Many other substrates now recognized

CO 2 reduction Acetate fermentation Zinder, 1993

CO 2 reduction Acetate fermentation

Obligate syntrophy is common Both species (e.g., a methanogen and an acetogen) require the other: the acetogen provides the hydrogen; the methanogen prevents a build-up of hydrogen (which inhibits the acetogens) In marine sediments, methanogens are competitive only after sulfate is gone (< 0.2 mM sulfate). Sulfate reducers keep H 2 partial pressure too low for methanogens.

Syntrophy between an acetogen and a methanogen

Zinder, 1993

Whiticar et al. Dominant pathway for methanogenesis? Stable isotope approaches. Distinct  D (stable hydrogen isotope) values for CO 2 reduction and acetate fermentation, based on source of the hydrogen atoms. All H from water 3 of 4 H from acetate CH 3 COO - + H 2 O => CH 4 + HCO 3 - 4H 2 + HCO H + => CH 4 + 3H 2 O

CO 2 reduction - Slope near 1 Fermentation - Slope much lower Overlap in  13 C; separation in  D

CO 2 reduction - Slope near 1 Fermentation - Slope much lower Methanogenesis in freshwater systems dominated by acetate fermentation; in (sulfate-free) marine systems, by CO 2 reduction

What controls the  13 C of biogenic methane? (strongly depleted, with a wide range) N. Blair – link to organic C flux? Alperin et al., 1992: 120 day sediment incubations Measure concentrations and rates; Infer pathways and fractionations

Acetogenesis SO 4 -2 < 0.2 mM 120 day sediment incubations Methanogenesis Gas leak Alperin et al., 1992

Fraction from acetate Total CH 4 production  13 C DIC Increase due to CO 2 reduction  13 C CH 4 Shifting pathways, and source  13 C

 13 C of CH 4 from DIC,  –50 to – 70  13 C of CH 4 production (CO2 red., acetate ferment.)  13 C of CH 4 in incubations (instantaneous, and integrated) reflects variation of pathways, and substrate  13 C

Controls on H 2 in anoxic sediments. Hoehler et al., – 14 day slurry incubations to estimate steady-state H 2 concentrations. For SR, methanogenesis, and acetogenesis, the observed [H 2 ] levels are low enough to limit the next process.

Zinder, 1993 Greater energy yield (more negative  G) allows sulfate reducers to outcompete methanogens for H 2.

Hoehler et al., 1998 Porewater sulfate and H 2 in Cape Lookout Bight sediments Estimated porewater H 2 turnover times are very short (0.1 to 5 s); profile H 2 gradients don’t reflect transport, but “local” production rate variations.

Hoehler et al. – Microbial communities maintain porewater H 2 concentrations at a minimum useful level (based on the energy they require to form ATP from ADP). The bulk H 2 may reflect the geometry of the H 2 producer / H 2 consumer association. Higher bulk H 2 Lower bulk H 2 H 2 consumer – sulfate reducer H 2 producer – fermenter

What happens to all this methane? Diffusion (transport) up into oxic zone – aerobic methane oxidation Bubble ebullition followed by oxidation in atmosphere shallow seds, with strong temperature or pressure cycles Anaerobic methane oxidation coupled to sulfate reduction Gas hydrate formation

Anaerobic methane oxidation by a consortium, made up of: sulfate reducers (with H 2 as electron acceptor) SO H 2 => S = + 4H 2 0 And methanogens (running in reverse, due to low pH 2 ) CH 4 + 2H 2 O => CO 2 + 4H 2 Together yielding CH 4 + SO 4 -2 => HS - + HCO H 2 O (Hoehler et al., ‘94)

Boetius et al., 2000 Used fluorescent probes to label, image aggregates of archaea (methanogens, red) and sulfate reducers (green) in sediments from Hydrate Ridge (OR) – observed very tight spatial coupling.

Nauhaus et al., 2002 Sediment incubations (Hydrate Ridge) demonstrating anaerobic methane oxidation, strong response to CH 4 addition. CH 4 consumption H 2 S production

DeLong 2000 (N&V to Boetius et al.) SO H 2 => S = + 4H 2 0 CH 4 + 2H 2 O => CO 2 + 4H 2 Anaerobic methane oxidation coupled with sulfate reduction

Low T + high P + adequate gas (methane, trace other HC, CO 2 ) => gas hydrate formation Why do we care about methane hydrates? Resource potential Fluid flow on margins Slope destabilization / slope failure Chemosynthetic biological communities Climate impact potential

Kvenvolden, ‘88 1 m 3 hydrate => 184 m 3 gas m 3 water total hydrate = 10,000 x gC (a guess!) Total fossil fuel = 5000 x gC DIC = 980 Terr bio = 830 Peat = 500 Atm = 3.5 Mar bio = 3

Methane hydrate stability Methane hydrate Methane gas

permafrostContinental margin

Known global occurance of gas hydrates

Geophysical signature of gas hydrates: presence of a “bottom simulating reflector” in seismic data, due to velocity contrast (hydrate / free gas). water sediment hydrate free gas

Porewater evidence of hydrate dissociation: low Cl- in zone of hydrate dissociation (during core recovery; decompression, warming)

Abrupt, global low- 13 C event in late Paleocene (benthic foraminifera, planktic foraminifera, terrestrial fossils): A gas hydrate release? Warming to LPTM – Late Paleocene thermal maximum

Dickens et al., 1997 High-resolution sampling of the 13 C event. Magnitude, time-scales, consistent with sudden release of 1.1 x g CH 4 with  13 C of –60 o/oo, and subsequent oxidation. Did warming going into LPTM drive hydrate dissociation, and methane release? Did similar (smaller) events occur during the last glaciation (MIS 3)? (Kennett)

Blair – aerobic methane oxidation in CLB

What controls the  13 C of biogenic methane? (strongly depleted, with a wide range) N. Blair – link to organic C flux?

Hoehler et al., 1998 Porewater sulfate and H 2 in Cape Lookout Bight sediments Estimated porewater H 2 turnover times are very short (0.1 to 5 s); profile gradients don’t reflect transport, but “local” production rate variations.

Can imagine a Redfield-type sulfate reduction stoichiometry: (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 53SO 4 -2 => 106(HCO 3 - ) + 16NH 3 + H 3 PO (H 2 S) Or even just: 2(CH 2 O) + SO 4 -2 => 2(HCO 3 - )+ H 2 S Production of ammonia, H 2 S, and alkalinity at the depth of SR. If NH 3 and H 2 S diffuse up and are reoxidized; consume O 2, release H + close to sediment-water interface If H 2 S reacts with Fe ++, reduced sulfur and Fe are buried.

But sulfate reducers can only oxidize a limited suite of simple organic substrates. They typically function as part of a community that includes fermenters, acetogens, and methanogens, as well as sulfate reducers. (Fenchel and Finlay, Ecology and Evolution in Anoxic Worlds)

The pathways are complex and variable, the processes are tightly linked, production and consumption of intermediates are rapid and in balance, … Jorgensen, 1983

Alperin, AMO-SR Skan Bay AK