REDOX:
IRON, SULFUR, & SILICA
Redox Potential
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pE = -log free electrons
When corrected to pH 7 (H+ = OH-), called Eh
When Eh positive – oxidizing environment
When Eh negative – reducing environment
Redox vs. pH
Redox (mv) of common liquids
Example of Redox data
pH and ORP in Lake Skiennungen
Fe & Mn in oligotrophic and eutrophic lakes
Iron and Manganese in a Productive Lake
Redox: Sulfur and Iron
Particulate Fe and Fe (II) through the year
Sulfur and Iron in Mud
Fe & Mn in
Crooked lake
(mesotrophic)
Fe & Mn in
Little Crooked
lake
(eutrophic)
Transport of Fe in a lake
Iron Oxidizing Bacteria
• Gallionella (an iron-oxidizing bacterium)
• 4Fe (HCO3)2 + O2 + 6H2O
4Fe (OH)3 + 4H2CO3 + 4CO2 + 58 kcal
Ferrobacillus iron bacteria
• 4FeCO3 + O2+ 6H2O
Fe(OH)3 + 4CO2
Mn cycling
relative to O2
REDOX and Sulfur
S in oligotrophic and eutrophic lakes
SO4 in a
mesotrophic
hardwater
lake
SO4 budget for Linsley Pond
ORP, pH, and Sulfur bacteria
Autotrophic Sulfur-Oxidizing Bacteria
and Iron Transformations
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Thiobacillus
Some of the reactions that it can mediate
FeS2 + 3½O2+H2O
FeSO4 + H2SO4
2FeSO4 + ½O2+ H2SO4
Fe2(SO4)3 + H2O
Bacterial Transformations of Sulfur
• Sulfate-Reducing Bacteria (heterotrophic and
anaerobic)
• Sulfur-Oxidizing Bacteria
– Chemosynthetic
– Pigmented Autotrophic
Sulfate-Reducing Bacteria
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Ex: Desulfovibrio
Heterotrophic and anaerobic
H2SO4 + 2(CH2O)
2CO2 2H2O + H2S
H2SO4 + 4H2
H2S + 4H2O
Chemosynthetic Sulfur-Oxidizing Bacteria
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Ex: Beggiatoa
Deposit elemental S inside cell
H2S + ½O2
S + H2O
S + 1½O2 + H2O
H2SO4
Ex: Thiobacillus
Deposit elemental S outside
2Na2S2O3 + O2
2S + 2Na2SO4
Green Sulfur Bacteria
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Ex: Chlorobium
Uses a pigment similar to chlorophyll
CO2 + 2H2S LIGHT Food + H2O + 2S
2CO2 + 2 H2O + H2S LIGHT Food + H2SO4
Purple Sulfur Bacteria
• Ex: Chromatium
• They deposit sulfur
intracellularly
• Same reactions as Chlorobium
Purple Non-Sulfur Bacteria
• Ex: Rhodobacter
• Na2S2O3+2CO2 +3H2O LIGHT Food + Na2SO4 +H2SO4
Microbial-mediated Sulfur cycle in a lake
Sulfate reduction in Lake Gek Gel and
the Black Sea
Distribution of
sulfur in
meromictic
Lake Belovod
Abandoned Mine Drainage
Microbial Oxidation of Pyrite
AMD Discharges
Acid Precipitation
Si in lakes
Some Common Diatoms
Si in oligotrophic Lawrence Lake
Si and diatom population dynamics in
Lake Windermere
Dynamics of diatom periphyton and plankton
Green Algae and Diatoms