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

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
Anoxic
Oxic

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…