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.
CO2 reduction
Acetate fermentation

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

9.
Whiticar et al.
Dominant pathway for methanogenesis?
Stable isotope approaches.
Distinct dD (stable
hydrogen isotope) values
for CO2 reduction and
acetate fermentation, based
on source of the hydrogen
atoms.
All H from water
3 of 4 H from acetate
CH3COO- + H2O => CH4 + HCO3
-
4H2 + HCO3
- + H+ => CH4 + 3H2O

10.
CO2 reduction - Slope near 1
Fermentation - Slope much
lower
Overlap in d13C; separation in dD

11.
CO2 reduction - Slope near 1
Fermentation - Slope much
lower
Methanogenesis in freshwater
systems dominated by acetate
fermentation; in (sulfate-free)
marine systems, by CO2 reduction

12.
What controls the d13C of biogenic methane?
(strongly depleted, with a wide range)
-50
-100
N. Blair – link to organic C flux?
Alperin et al., 1992:
120 day sediment incubations
Measure concentrations and rates;
Infer pathways and fractionations

13.
Acetogenesis
SO4
-2 < 0.2 mM
120 day sediment incubations
Methanogenesis
Gas leak
Alperin et al., 1992

14.
Fraction
from
acetate
Total CH4 production
d13C DIC
Increase
due to CO2
reduction
d13C CH4
Shifting
pathways,
and source
d13C

15.
d13C of CH4 from
DIC,
E = –50 to – 70
d13C of CH4
production (CO2 red.,
acetate ferment.)
d13C of CH4 in incubations (instantaneous, and
integrated) reflects variation of pathways, and
substrate d13C

16.
Controls on H2 in anoxic sediments.
Hoehler et al., 1998
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.

17.
Zinder, 1993
Greater energy yield (more
negative DG) allows sulfate
reducers to outcompete
methanogens for H2.

18.
Hoehler et al., 1998
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.

19.
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.
Higher bulk H2 Lower bulk H2
H2 consumer –
sulfate reducer
H2 producer –
fermenter

20.
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

21.
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)

22.
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.

23.
Nauhaus et al., 2002
Sediment
incubations (Hydrate
Ridge)
demonstrating
anaerobic methane
oxidation, strong
response to CH4
addition.
CH4 consumption
H2S production

24.
DeLong 2000
(N&V to Boetius et al.)
SO4
-2 + 4H2 => S= + 4H20
CH4 + 2H2O => CO2 + 4H2
Anaerobic methane oxidation coupled with sulfate reduction

25.
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

26.
Kvenvolden, ‘88
1 m3 hydrate => 184 m3 gas + 0.8 m3 water
total hydrate =
10,000 x 1015
gC (a guess!)
Total fossil fuel =
5000 x 1015 gC
DIC = 980
Terr bio = 830
Peat = 500
Atm = 3.5
Mar bio = 3

27.
Methane
hydrate stability
Methane hydrate
Methane gas

28.
permafrost Continental margin

29.
Known global
occurance of
gas hydrates

30.
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

31.
Porewater evidence of hydrate dissociation:
low Cl- in zone of hydrate dissociation
(during core recovery; decompression, warming)

32.
Abrupt, global low-13C event in late Paleocene
(benthic foraminifera, planktic foraminifera,
terrestrial fossils): A gas hydrate release?
Warming to LPTM –
Late Paleocene
thermal maximum

33.
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)

34.
Blair – aerobic methane oxidation in CLB

35.
What controls the d13C of biogenic methane?
(strongly depleted, with a wide range)
N. Blair – link to organic C flux?
-100 -50

36.
Hoehler et al., 1998
Porewater sulfate and
H2 in Cape Lookout
Bight sediments
Estimated porewater H2 turnover times are very
short (0.1 to 5 s); profile gradients don’t reflect
transport, but “local” production rate variations.

37.
Can imagine a Redfield-type
sulfate reduction stoichiometry:
(CH2O)106(NH3)16(H3PO4) + 53SO4
-2 =>
106(HCO3
-) + 16NH3 + H3PO4 + 53(H2S)
Or even just:
2(CH2O) + SO4
-2 => 2(HCO3
-)+ H2S
Production of ammonia, H2S, and alkalinity at the depth of SR.
If NH3 and H2S diffuse up and are reoxidized;
consume O2, release H+ close to sediment-water interface
If H2S reacts with Fe++, reduced sulfur and Fe are buried.

38.
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)

39.
The pathways are complex and variable, the processes are tightly linked,
production and consumption of intermediates are rapid and in balance, …
Jorgensen, 1983

40.
Alperin, AMO-SR
Skan Bay AK