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

• 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…

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

Methane cycle

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)

Phosphorus cycle

• P exists in several redox states (-3, 0, +3, +5) but only +5, PO4

3-, 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)

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.

P cycling linked to SRB-IRB-MRB

activity

Blue Green Algae blooms

FeOOH

PO43- PO4

3-

PO43-PO4

3-

Org C + SO42-

H2SFeS2

PO43- PO4

3-

PO43-

PO43-

Sulfate Reducers

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

AnoxicOxic

St. Albans Bay Sediments

0.341

-0.058

0.000

0.050

0.100

0.150

0.200

0.250

0.300

-0.100-1.800 -1.600 -1.400 -1.200 -1.000 -0.800 -0.600 -0.400

0.106

-0.019

-0.010

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

0.100

-0.100-1.800 -1.600 -1.400 -1.200 -1.000 -0.800 -0.600 -0.400

Mn2+ + 2e- --> Mn0(Hg)

Fe3+ + 1e- Fe2+

0.304

-0.004

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.180

0.200

0.220

0.240

0.260

0.280

-0.100-1.800 -1.600 -1.400 -1.200 -1.000 -0.800 -0.600 -0.400

O2 + 2e- + 2H+ H2O2

H2O2 + 2e- + 2H+ 2H2O

FeS(aq)

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

6-23-04 Core 2 Profile 2

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 20 40 60 80

Current (nA)

Dep

th (

mm

)

O2 (nA)

7-19-04 Core 1 Profile 1

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 50 100

Current (nA)

Dep

th (

mm

) Mn (nA)

O2 (nA)

Fe3+ (nA)

FeS (nA)

7-26-04 Core 2 Profile 1

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 20 40 60 80Current (nA)

Dep

th (

mm

)

O2 (nA)

Mn (nA)

8-12-04 Core 1 Profile 1

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 20 40 60 80

Current (nA)

Dep

th (

mm

)

O2 (nA)

Mn (nA)

FeS (nA)

Fe3+ (nA)

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

Profiles seasonal sample averages

0

1

2

3

4

5

6

7

8

9

0 100 200 300

Conc. (mg/g sediment)

Dep

th (

top

of

sect

ion

, cm

)

AA P

AA Fe / 10

AA Mn

NaOH P

Ascorbic AcidDepth P-Fe P-Mn0-1 cm 0.863 0.8941-2 cm 0.933 0.9212-3 cm 0.829 0.5673-4 cm 0.604 0.5594-5 cm 0.732 0.7775-6 cm 0.889 0.8956-8 cm 0.866 0.8048-10 cm 0.894 0.876

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…

Profiles seasonal sample averages

0

1

2

3

4

5

6

7

8

9

0 100 200 300

Conc. (mg/g sediment)D

epth

(to

p o

f se

ctio

n,

cm)

AA P

AA Fe / 10

AA Mn

NaOH P