Transcript Lecture 11- CH4 and P cycling

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, 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)
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
PO43-
PO43-
PO4
Org C + SO42-
FeS2
FeOOH
H2S
PO43-
3-
PO43-
Sulfate Reducers
PO43-
PO43- PO43Blue Green Algae blooms
Redox ‘Fronts’
Oxic
Anoxic
• 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
St. Albans Bay Sediments
Mn2+ + 2e- --> Mn0(Hg)
H2O2 + 2e- + 2H+  2H2O
O2 + 2e- + 2H+  H2O2
Fe3+ + 1e-  Fe2+
FeS(aq)
7-19-04 Core 1 Profile 1
10
10
5
5
0
0
-5
-5
Mn (nA)
-10
O2 (nA)
-15
Fe3+ (nA)
-20
-20
FeS (nA)
-25
-25
-30
-30
-10
O2 (nA)
-15
Depth (mm)
Depth (mm)
6-23-04 Core 2 Profile 2
-35
-35
0
20
40
60
80
0
Current (nA)
10
5
5
0
0
-5
-5
-15
Mn (nA)
Depth (mm)
Depth (mm)
10
O2 (nA)
100
8-12-04 Core 1 Profile 1
7-26-04 Core 2 Profile 1
-10
50
Current (nA)
-10
-15
O2 (nA)
-20
-20
Mn (nA)
-25
-25
FeS (nA)
-30
-30
Fe3+ (nA)
-35
-35
0
20
40 60
Current (nA)
80
0
20
40
60
Current (nA)
80
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
Seasonal Phosphorus mobility
Depth
0-1 cm
1-2 cm
2-3 cm
3-4 cm
4-5 cm
5-6 cm
6-8 cm
8-10 cm
Ascorbic Acid
P-Fe
P-Mn
0.863
0.894
0.933
0.921
0.829
0.567
0.604
0.559
0.732
0.777
0.889
0.895
0.866
0.804
0.894
0.876
Profiles
seasonal sample averages
0
Depth (top of section, cm)
1
2
AA P
3
AA Fe / 10
4
AA Mn
5
NaOH P
6
7
8
9
0
100
200
Conc. (mg/g sediment)
300
• 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
P Loading and sediment deposition
Profiles
seasonal sample averages
0
• Constantly moving redox fronts
affect Fe and Mn minerals,
mobilize P and turn ideal profile
into what we actually see…
Depth (top of section, cm)
1
2
AA P
3
AA Fe / 10
4
AA Mn
5
NaOH P
6
7
8
9
0
100
200
Conc. (mg/g sediment)
300