Methane

1
Methane

• CH4
• Greenhouse gas (20x more powerful than CO2)
• Formed biologically (methanogenesis)
• Huge reservoir as methane clathrate hydrate in
  cold soils and ocean bottom stable structure at
  low T, high P

2

• 2x1016 kg of C in these deposits
• What happens if the oceans warm??
• Clathrate gun hyothesis warming seas melt
  these clathrates, CH4 released en masse to
  atmosphere

3
Microbes and methane production

• Methanogenesis Reduction of CO2 or other
  organics to form CH4 (also CH4 generation from
  special fermentative rxns)
• Only certain groups of Archaea do this,
  specifically with the Euryachaeota subdivision
• Called methanogens
• These organisms do not compete well with other
  anaerobes for e- donors, thus they thrive where
  other alternate e- acceptors have been consumed

4
Methane cycle
5
Microbial methane oxidation

• Organisms that can oxidize CH4 are Methanotrophs
  mostly bacteria
• All aerobic methanotrophs use the enzyme methane
  monooxygenase (MMO) to turn CH4 into methanol
  (CH3OH) which is subsequently oxidized into
  formaldehyde (HCHO) on the way to CO2
• Anaerobic methane oxidation use SO42- as the e-
  acceptor this was long recognized chemically,
  but only very recently have these microbes been
  more positively identified (though not cultured)

6
Phosphorus cycle

• P exists in several redox states (-3, 0, 3, 5)
  but only 5, PO43-, stable in water
• 1 microbe to date has been shown to grow on PO33-
  (phosphite, P3) as a substrate
• P is a critical nutrient for growth, often a
  limiting nutrient in rivers and lakes
• Most P present as the mineral apatite
  (Ca5(PO4)3(F,Cl,OH)) also vivianite
  (Fe3(PO4)28H2O)

7
P sorption

• P strongly sorbs to FeOOH and AlOOH mineral
  surfaces as well as some clays
• P mobility thus inherently linked to Fe cycling
• P sorption to AlOOH is taken advantage of as a
  treatment of eutrophic lakes with excess P (alum
  is a form AlOOH) AlOOH is not affected by
  microbial reduction as FeOOH can be.

8
P cycling linked to SRB-IRB-MRB activity
9
Redox Fronts

• Boundary between oxygen-rich (oxic) and more
  reduced (anoxic) waters
• Oxygen consumed by microbes which eat organic
  material
• When Oxygen is gone, there are species of
  microbes that can breathe oxidized forms of
  iron, manganese, and sulfur

10
St. Albans Bay Sediments
Mn2 2e- --gt Mn0(Hg)
H2O2 2e- 2H ? 2H2O
O2 2e- 2H ? H2O2
Fe3 1e- ? Fe2
FeS(aq)
11
Results Seasonal Work

• Sediments generally become more reduced as summer
  progresses
• Redox fronts move up and down in response to
  Temperature, wind, biological activity changes

12
Seasonal Phosphorus mobility

• Ascorbic acid extractions of Fe, Mn, and P from
  10 sediment cores collected in summer 2004 show
  strong dependence between P and Mn or Fe
• Further, profiles show overall enrichment of all
  3 parameters in upper sections of sediment
• Fe and Mn would be primarily in the form of Fe
  and Mn oxyhydroxide minerals ? transformation of
  these minerals is key to P movement

13
P Loading and sediment deposition

• Constantly moving redox fronts affect Fe and Mn
  minerals, mobilize P and turn ideal profile into
  what we actually see