1. Microbes, e- flow Catabolism – breakdown of any compound for energy
Anabolism – consumption of that energy for biosynthesis
Transfer of e- facilitated by e- carriers, some bound to the membrane, some freely diffusible

2. The Redox ladder
H2O H2 O2
NO3- N2
MnO2
Mn2+
Fe(OH)3
Fe2+
SO42-
H2S
CO2
CH4
Oxic
Post - oxic
Sulfidic
Methanic
Aerobes
Dinitrofiers
Maganese reducers
Sulfate reducers
Methanogens
Iron reducers
The redox-couples are shown on each stair-step, where the
most energy is gained at the top step and the least at the bottom step. (Gibb’s free energy becomes more positive going down the steps)

3. Profiles and microbial habitats
Minerals
Expected? 3 2
Fe2+
depth
H2S 4
H2S 1
Org. C
Org. C
Concentration

4. Diffusion, Fickian Diffusion from high to low levels..
Where D is the diffusion coefficient, dc/dx is the gradient, and J is the flux of material

5. Other nutrients needed for life
Besides chemicals for metabolic energy, microbes need other things for growth.
Carbon
Oxygen
Sulfur
Phosphorus
Nitrogen
Iron
Trace metals (including Mo, Cu, Ni, Cd, etc.)
What limits growth??

6. Nutrients
Lakes are particularly sensitive to the amount of nutrients in it:
Oligotrophic – low nutrients, low photosynthetic activity, low organics  clear, clean…
Eutrophic – high nutrients, high photosynthetic activity, high organics  mucky, plankton / cyanobacterial population high
Plankton growth:
106 CO NO3- + HPO H2O + 18 H+ + trace elements + light  C106H263O110N16P O2 (organic material composing plankton)
This C:N:P ratio (106:16:1) is the Redfield Ratio
What nutrients are we concerned with in Lake Champlain?

7. Nutrient excess can result
in ‘blooms’

8. Lake Champlain
Phosphorus limited?
Algal blooms
What controls P??

9. Nutrient cycling linked to SRB-IRB-MRB activity
Blue Green Algae blooms
FeOOH
PO43-
Org C + SO42-
H2S
FeS2
Sulfate Reducers

10. Redox ‘Fronts’
Boundary between oxygen-rich (oxic) and more reduced (anoxic) waters
Oxygen input through entrainment (wind, wave action, photosynthesis)
Oxygen consumption from heterotrophic consumption, reaction with reduced forms of Fe, Mn, S
Anoxic
Oxic

11. Methods
Voltammetric microelectrodes  In-situ pore water measurements to determine redox front by real time analysis of O2, Fe2+/Fe3+, Mn2+, H2S
DGT (Diffusive Gradients in Thin films) to monitor P fluxes through sediment
Gravity Coring and chemical extractions of iron, manganese, and phosphorus in the sediment
Inductively-coupled plasma optical emission spectroscopy (ICP-OES) to measure iron, manganese, and phosphorus from extractant

12. Voltammetric data - St. Albans Bay Sediments
Mn2+ + 2e- --> Mn0(Hg)
H2O2 + 2e- + 2H+  2H2O
O2 + 2e- + 2H+  H2O2
Fe3+ + 1e-  Fe2+
FeS(aq)

13. Results: Sediments generally become more reduced as summer progresses
Redox fronts move up and down in response to temperature, wind, biological activity changes
This movement of the redox front invokes changes in mineralogy and nutrient dynamics on short time scales

14. Seasonal Phosphorus mobility
Redox front movement mobilizes P in the sediment over time
Movement is EVENT based – fluxes should vary significantly in time. We currently have little idea what P fluxes are out of the sediment
Profiles show overall enrichment of P, Mn, and Fe in upper sections of SAB 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

15. 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…

16. Redox changes and Nutrients
We have linked at least some component of P dynamics to redox changes and Fe, Mn mineralogy – this is one of several processes that would affect nutrient balance in these bay systems however.
Nitrogen speciation and availability may also be affected by these processes, but potentially in very different ways – hence there may be some background control on N/P ratio which may affect algal community dynamics

17. Geochemical niches and microbial community changes in Saint Albans Bay, Lake Champlain, sediments
PCA analysis of Restriction Fragment Length Polymorphism analysis from DNA extracted from 4 geochemically distinct zones and a 1mm buffer zone between
1 – Oxic, top mm of sediment
2 – Buffer – 1 mm
3 – Nitrate reduction
4 – buffer 2 – 1 mm
5 – manganese reduction
6 – buffer – 1 mm
7 – iron reduction
8 - bottom of sediment column

18. Can reducing conditions force P out of the sediment and into the overlying water column? Incubation experiments:
Oxic
Anoxic
From the volumetric analyses and visually (red is FeOOH  oxidized iron, black is FeS  reduced iron and sulfur) we see the difference in the redox condition between the two cores.

19. DGT films in incubation experiments
Diffusive Gradients in Thin films (DGT) probes put into sediment (½ in sediments, ½ in water above) to measure time-averaged P release in 2 incubation experiments
DGT films definitively show potential for significant release of P into pore water and overlying water column on reduction of the Fe minerals present