Transcript Metals Cycling - Penn State York Home Page

Metals Cycling
reduction
Fe+2
(ferrous)
Mn+2
(manganous)
Mn+4
(manganic)
oxidation
Feº
(metalic)
Iron and Manganese Cycling
Iron Reducers
Iron Oxidizers
Acid Mine Drainage
Manganese Nodules
Fe+3
(ferric)
• 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.
Fe+3
depth
Iron Chemistry
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 = +820 mV
E = +420 mV
E = -180 mV
E = -200 mV
E = -240 mV
Methanogenesis
Iron Reducing Bacteria in
Anaerobic Decomposition
What’s Soil
Gleying?
Magnetosomes
Greigite (Fe3S4) or Magnetite (Fe304)
0.5 μm
Microaerophilic Magnetotactic
(Need the Oxic Anoxic Transition Zone)
Dashed arrows are Earth’s inclined geomagnetic field lines.
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-
Microbial Influenced Corrosion
(MIC)
Desulfovibrio spp., and SRB
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 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
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