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Metals Cycling

Metals Cycling . reduction. Fe +2 (ferrous). Mn +2 (manganous). Fe +3 (ferric). Mn +4 (manganic). Iron and Manganese Cycling Iron Reducers Iron Oxidizers Acid Mine Drainage Manganese Nodules. oxidation. Fe º (metalic). Iron Chemistry. Neutral to alkaline; all insoluble.

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Metals Cycling

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  1. Metals Cycling reduction Fe+2 (ferrous) Mn+2 (manganous) Fe+3 (ferric) Mn+4 (manganic) Iron and Manganese Cycling Iron Reducers Iron Oxidizers Acid Mine Drainage Manganese Nodules oxidation 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 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. MagnetosomesGreigite (Fe3S4) or Magnetite (Fe304) 0.5 μm

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

  9. Metalic Iron OxidationCorrosion 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 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 • O2 toxicity • Protist predation • Viral attack • Autotrophs • Some facultative autotrophic Gallionella spp. • Some obligate lithoautotrophs

  15. Emerson et al., 2000

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