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Methane

Methane. CH 4 Greenhouse gas (~20x more powerful than CO 2 ) Formed biologically (methanogenesis) Huge reservoir as methane clathrate hydrate in cold soils and ocean bottom – stable structure at low T, high P. 2x10 16 kg of C in these deposits What happens if the oceans warm??

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Methane

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  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)2*8H2O)

  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 PO43- PO43- PO43- PO43- PO43- PO43- PO43- PO43- Org C + SO42- FeOOH FeS2 H2S Blue Green Algae blooms Sulfate Reducers

  9. Oxic Anoxic 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- --> 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…

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