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Metals Cycling Iron and Manganese Cycling Iron Reducers Iron Oxidizers Acid Mine Drainage Manganese Nodules Fe +2 (ferrous) Fe +3 (ferric) oxidation reduction.

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Presentation on theme: "Metals Cycling Iron and Manganese Cycling Iron Reducers Iron Oxidizers Acid Mine Drainage Manganese Nodules Fe +2 (ferrous) Fe +3 (ferric) oxidation reduction."— Presentation transcript:

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

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 –Enhanced by hydroxyl radical formation from organic mater such as humic acids. Biological –Anaerobic Respiration –Requires absence of O 2 and Nitrate –Often important in aquatic sediments and water saturated soils (anoxic habitats).

5 Aerobic respiration yields greatest energy due to very positive O 2 redox potential. Without O 2, anaerobic respiration uses alternate terminal electron acceptors in the order of decreasing redox potential. E = -240 mV Methanogenesis E = +820 mV E = +420 mV E = -200 mV E = -180 mV

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

7 0.5 μm Magnetosomes Greigite (Fe 3 S 4 ) or Magnetite (Fe 3 0 4 )

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 Corrosion Abiotic Aerobic: rust! 2Feº + 1½ O 2 + 3 H 2 O → 2Fe(OH) 2 Anaerobic with Sulfate Reducing Bacteria (SRB): Fe + H 2 O → Fe(OH) 2 + H 2 4H 2 + SO 4 -2 → H 2 S + 2OH - + 2H 2 O H 2 S + Fe → FeS + H 2 4Fe + 4H2O + SO4-2 → FeS +3Fe(OH) 2 + 2OH -

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

11 Ferrous Iron Oxidation Abiotic oxidation is low at pH < 4. Microbial catalysis 10-1000 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 FeS2 + 15 O2 + 14 H2O → 4 Fe(OH)3 ¯ + 8 H2SO4 Pyrite + Oxygen + Water à "Yellowboy" + Sulfuric Acid 1)2 FeS2 + 7 O2 + 2 H2O → 2 Fe2+ + 4 SO42- + 4 H+ Pyrite + Oxygen + Water → Ferrous Iron + Sulfate + Acidity 2) 4 Fe2+ + O2 + 4 H+ → 4 Fe3+ + 2 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 Fe3+ + 12 H2O → 4 Fe(OH)3 ¯ + 12 H+ Ferric Iron + Water → Ferric Hydroxide (yellowboy) + Acidity 4)FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 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 O 2 toxicity Protist predation Viral attack Autotrophs –Some facultative autotrophic Gallionella spp. –Some obligate lithoautotrophs

15 Emerson et al., 2000


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