Metals Cycling

Iron and Manganese CyclingIron ReducersIron Oxidizers

Acid Mine DrainageManganese Nodules

Fe+2

(ferrous)Fe+3

(ferric)

oxidation

reduction

Mn+2

(manganous)Mn+4

(manganic)

Feº(metalic)

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. dept

hFe+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).

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).

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 = -240 mV

Methanogenesis

E = +820 mV

E = +420 mV

E = -200 mV

E = -180 mV

Iron Reducing Bacteria in Anaerobic Decomposition

What’s Soil Gleying?

0.5 μm

MagnetosomesGreigite (Fe3S4) or Magnetite (Fe304)

Microaerophilic Magnetotactic(Need the Oxic Anoxic Transition Zone)

Dashed arrows are Earth’s inclined geomagnetic field lines.

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-

Desulfovibrio spp., and SRB

Microbial Influenced Corrosion (MIC)

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

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 H2SO4Pyrite + 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 H2OFerrous 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

PA Coal Field (Sources of AMD)

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

Emerson et al., 2000