Methane production - Methanogenesis Substrates / pathways Isotopic studies Hydrogen cycling Methane consumption - Ana

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

2. 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.

3. Two main methanogenic pathways: CO2 reduction Acetate fermentation Both pathways found in both marine and freshwater systems Many other substrates now recognized

4. CO2 reduction Acetate fermentation Zinder, 1993

5. Acetate fermentation CO2 reduction

6. 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 H2 partial pressure too low for methanogens.

7. Syntrophy between an acetogen and a methanogen

8. Zinder, 1993

10. Dominant pathway for methanogenesis? Stable isotope approaches. 4H2 + HCO3- + H+ => CH4 + 3H2O All H from water Distinct dD (stable hydrogen isotope) values for CO2 reduction and acetate fermentation, based on source of the hydrogen atoms. 3 of 4 H from acetate CH3COO- + H2O => CH4 + HCO3- Whiticar et al.

11. CO2 reduction - Slope near 1 Overlap in d13C; separation in dD Fermentation - Slope much lower

12. CO2 reduction - Slope near 1 Methanogenesis in freshwater systems dominated by acetate fermentation; in (sulfate-free) marine systems, by CO2 reduction Fermentation - Slope much lower

13. What controls the d13C of biogenic methane? (strongly depleted, with a wide range) N. Blair – link to organic C flux? -100 -50 Alperin et al., 1992: 120 day sediment incubations Measure concentrations and rates; Infer pathways and fractionations

14. 120 day sediment incubations Alperin et al., 1992 Gas leak SO4-2 < 0.2 mM Acetogenesis Methanogenesis

15. Total CH4 production d13C DIC Increase due to CO2 reduction d13C CH4 Shifting pathways, and source d13C Fraction from acetate

16. d13C of CH4 production (CO2 red., acetate ferment.) d13C of CH4 from DIC, E = –50 to – 70 d13C of CH4 in incubations (instantaneous, and integrated) reflects variation of pathways, and substrate d13C

18. Hoehler et al., 1998 Controls on H2 in anoxic sediments. 7 – 14 day slurry incubations to estimate steady-state H2 concentrations. For SR, methanogenesis, and acetogenesis, the observed [H2] levels are low enough to limit the next process.

19. Greater energy yield (more negative DG) allows sulfate reducers to outcompete methanogens for H2. Zinder, 1993

20. Porewater sulfate and H2 in Cape Lookout Bight sediments Estimated porewater H2 turnover times are very short (0.1 to 5 s); profile H2 gradients don’t reflect transport, but “local” production rate variations. Hoehler et al., 1998

21. Hoehler et al. – Microbial communities maintain porewater H2 concentrations at a minimum useful level (based on the energy they require to form ATP from ADP). The bulk H2 may reflect the geometry of the H2 producer / H2 consumer association. H2 consumer – sulfate reducer H2 producer – fermenter Higher bulk H2 Lower bulk H2

23. 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

24. Anaerobic methane oxidation by a consortium, made up of: sulfate reducers (with H2 as electron acceptor) SO4-2 + 4H2 => S= + 4H20 And methanogens (running in reverse, due to low pH2) CH4 + 2H2O => CO2 + 4H2 Together yielding CH4 + SO4-2 => HS- + HCO3- + H2O (Hoehler et al., ‘94)

25. 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. Boetius et al., 2000

26. H2S production CH4 consumption Sediment incubations (Hydrate Ridge) demonstrating anaerobic methane oxidation, strong response to CH4 addition. Nauhaus et al., 2002

27. Anaerobic methane oxidation coupled with sulfate reduction CH4 + 2H2O => CO2 + 4H2 SO4-2 + 4H2 => S= + 4H20 DeLong 2000 (N&V to Boetius et al.)

29. Low T + high P + adequate gas (methane, trace other HC, CO2) => 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

30. Kvenvolden, ‘88 1 m3 hydrate => 184 m3 gas + 0.8 m3 water DIC = 980 Terr bio = 830 Peat = 500 Atm = 3.5 Mar bio = 3 Total fossil fuel = 5000 x 1015 gC total hydrate = 10,000 x 1015 gC (a guess!)

31. Methane hydrate stability Methane gas Methane hydrate

32. permafrost Continental margin

33. Known global occurance of gas hydrates

34. 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

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

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

38. Dickens et al., 1997 High-resolution sampling of the 13C event. Magnitude, time-scales, consistent with sudden release of 1.1 x 1018 g CH4 with d13C 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)

46. Blair – aerobic methane oxidation in CLB