1. Iron and Manganese Cycling
Metals Cycling
reduction
Fe+2
(ferrous)
Mn+2
(manganous)
Mn+4
(manganic)
Fe+3
(ferric)
oxidation
Feº
(metalic)
Iron and Manganese Cycling
Iron Reducers
Iron Oxidizers
Acid Mine Drainage
Manganese Nodules

2. Iron Chemistry Neutral to alkaline; all insoluble.
Very acidic; Fe+2 and Fe+3 both soluble.
Anoxic and pH < 7; only Fe+2 soluble.
Organics may chelate; soluble.
depth
Fe+3

3. Iron Requirements
All life requires iron (cytochromes, heme groups, other proteins).
Not very bioavailable in oxic environments.
Some microbes produce siderophores (e.g. enterochelin).

4. Iron Reduction Photochemical Biological
Enhanced by hydroxyl radical formation from organic mater such as humic acids.
Biological
Anaerobic Respiration
Requires absence of O2 and Nitrate
Often important in aquatic sediments and water saturated soils (anoxic habitats).

5. Aerobic respiration yields greatest energy due to very positive O2 redox potential.
Without O2, anaerobic respiration uses alternate terminal electron acceptors in the order of decreasing redox potential.
E = +820 mV
E = +420 mV
E = -180 mV
E = -200 mV
E = -240 mV
Methanogenesis

6. Iron Reducing Bacteria in Anaerobic Decomposition
What’s Soil Gleying?

7. Magnetosomes Greigite (Fe3S4) or Magnetite (Fe304)

8. Microaerophilic Magnetotactic (Need the Oxic Anoxic Transition Zone)
Dashed arrows are Earth’s inclined geomagnetic field lines.

9. Metalic Iron Oxidation Corrosion of Steel
Abiotic Aerobic: rust!
2Feº + 1½ O2 + 3 H2O → 2Fe(OH)2
Anaerobic with Sulfate Reducing Bacteria (SRB):
Fe + H2O → Fe(OH)2 + H2
4H2 + SO4-2 → H2S + 2OH- + 2H2O
H2S + Fe → FeS + H2
4Fe + 4H2O + SO4-2 → FeS +3Fe(OH)2 + 2OH-

10. Microbial Influenced Corrosion (MIC)
Desulfovibrio spp., and SRB

11. Ferrous Iron Oxidation
Abiotic oxidation is low at pH < 4.
Microbial catalysis faster.
Different prokaryotes depending on:
pH range sulfide content;
organic matter content

12. There are four commonly accepted chemical reactions that represent the chemistry of pyrite weathering to form AMD. An overall summary reaction is as follows:
4 FeS O H2O → 4 Fe(OH)3 ¯ + 8 H2SO4
Pyrite + Oxygen + Water à "Yellowboy" + Sulfuric Acid
1) 2 FeS2 + 7 O2 + 2 H2O → 2 Fe SO H+
Pyrite + Oxygen + Water → Ferrous Iron + Sulfate + Acidity
2) 4 Fe2+ + O2 + 4 H+ → 4 Fe H2O
Ferrous Iron + Oxygen + Acidity → Ferric Iron + Water
{Thibacillus ferrooxidans; acidophilic pH < 3.5; consumes protons intracellularly to create PMF for ATP synthesis; other bacteria and archaea}
3) 4 Fe H2O → 4 Fe(OH)3 ¯ + 12 H+
Ferric Iron + Water → Ferric Hydroxide (yellowboy) + Acidity
4) FeS Fe H2O → 15 Fe SO H+
Pyrite + Ferric Iron + Water → Ferrous Iron + Sulfate + Acidity

13. PA Coal Field (Sources of AMD)

14. Circumneutral Fe+2 Oxidizers
Microaerophiles
Heterotrophic
No energy yield from ferrous ion
Morphology of iron oxides
Ribbons (Gallionella)
Sheaths (Sphaerotilus-Leptothrix Group)
Amorphous ppt coating (Siderocapsa)
Selective pressures for Fe(OH)3 ppt covering or attached to the bacteria cell surface:
Fe+2 toxicity
O2 toxicity
Protist predation
Viral attack
Autotrophs
Some facultative autotrophic Gallionella spp.
Some obligate lithoautotrophs

15. Emerson et al., 2000