FeMn Biogeochemistry Overview

1
Fe/Mn Biogeochemistry Overview

• Fe is the 4th most abundant element in the
  Earths crust, with an average concentration of
  4.3
• Mn is on average 60 time less abundant than Fe
  (0.072)but still relatively abundant compared
  to other metals
• Fe Mn are generally considered to be trace
  elements in open-water aquatic systems, but major
  elements in soils and sedimentary environments

2
Fe/Mn Biogeochemistry - Overview

• Fe and Mn are required by organisms in small
  quantities as nutrients for biochemical processes
• Fe is a major component of cytochromes and other
  oxidation-reduction proteins
• Mn is a cofactor for various enzyme systems
• Although Fe and Mn are abundant in the Earths
  crust, unlike other elements (e.g. C, N, S), they
  are often not readily available for organisms
• Plants and microorganisms produce chelating
  agents (siderophores) which facilitate
  solubilization and uptake of Fe/Mn from insoluble
  minerals

3
Fe/Mn Biogeochemistry - Overview

• In addition to the role of Fe and Mn as nutrients
  in biochemistry
• Bacteria mediate redox transformations in natural
  environments which are associated with energy
  generation
• Fe/Mn redox processes are dominant in cycling of
  metals in natural environments

4
Fe-Containing Minerals
Primary (Igneous) Secondary /
Authigenic .

Magnetite, Fe3O4 Iron-Sulfides Monosulfide,
FeS (many variants) Pyrite, FeS2
Pyroxenes Amphiboles Olivines Micas
Silicates, e.g. Illite Ky(Al4Fe4Mg4Mg6)- (Si8-yAl
y)O20(OH)4 Carbonates Ferroan
calcite/dolomite ankerite, siderite Fe(III)
Oxides Hematite, Fe2O3 Goethite, a-FeOOH
Lepidocrocite, g-FeOOH Ferrihydrite, Fe(OH)3
5
Mn-Containing Minerals
Primary (Igneous) Secondary
Authigenic

Pyroxenes Amphiboles Olivines Micas
Rhodonite, MnSiO3 Braunite, (Mn,Si)2O3 Oxides
Birnessite, d-MnO2 Pyrolusite, MnO2
Manganite, MnOOH
Hausmannite, Mn3O4 Rhodochrosite, MnCO3 No
Mn-sulfides!
Rhodocrosite MnCO3
Rhodonite MnSiO3
6
Birnessite / todorokite from Pacific Ocean
Pyrolusite (MnO2)
Manganite MnO(OH)
7
Basic Redox Cycles of Fe Mn
Ferric Fe(III)
Manganic Mn(IV)
O2, NO3-, Mn(IV)
O2
Smelting
Mn(III)
Mn(IV) reduction
Fe0
Fe(II) oxidation
Fe(III) reduction
Mn(II) oxidation
Corrosion
Smelting
Mn(II) Manganous
Fe(II) Ferrous
8
Oxidants and Reductants for Fe Mn in Natural
Environments
Abiotic reactions
Microbially-catalyzed reactions
Not yet documented reactions
Canfield et al. (2003)
9
Dissimilatory Fe(III) Reduction An Ancient Form
of Microbial Respiration?
Million Years Ago
Precambrian
Peak of BIFs
Banded Iron Formations
10
Acidophilic Fe(II)-Oxidizing Bacteria

• Mesophiles (lt 40 ºC)
• Thiobacillus, Leptospirillum, Ferromicrobium
• Moderate thermophiles (40-60 ºC)
• Sulfobacillus, Acidimicrobium, Leptospirillum,
  Ferroplasma (an extreme acidophile which grows at
  pH 0!)
• Extreme thermophiles (gt 60 ºC)
• Acidianus, Metallosphaera, Sulfurococcus

11
Microbial Oxidation of Fe(II)-Bearing Solid-Phase
Minerals at Circumneutral pH
Weathered FeS2-Bearing Basalt from the Juan de
Fuca Ridge

• Edwards et al. (2003) (Appl. Environ. Microbiol.)
  raise possibility, based on microbiological
  culturing, for low-temperature (2-4 C)
  lithoautotrophic energy metabolism coupled to
  oxidation of Fe(II) (and S(-II)) bound in
  silicate and sulfide minerals in basaltic rocks

Edwards et al. (2003)
Pure culture isolates are spread throughout
alpha and gamma Proteobacteria, with no obvious
close Fe(II)-oxidizing relatives
12
Edwards et al. incubated selected mineral
substrates at ambient seafloor temperatures near
a hydrothermal vent system. Findings Colonizatio
n densities by over an order of
magnitude Preference elemental sulfur gt chimney
sulfide gt marcasite gt pyrite gt sphalerite gt
chalcopyrite (corresponds well with the abiotic
oxidation kinetics of these materials, except
elemental sulfur, which is both the least
reactive to oxidizing species and the most
heavily colonized. Colonization densities
correspond with apparent degree of reaction
(dissolution pitting accumulation of secondary
alteration products). Heavy accumulations of
secondary Fe oxides on Fe-bearing minerals, most
notably on the chimney sulfide, form in situ as
the result of mineral dissolution and the
activity of neutrophilic Fe-oxidizing bacteria.
Results suggest that mineral-oxidizing bacteria
play a prominent role in weathering of seafloor
sulfide deposits, and that microbial utilization
of mineral substrates contributes to biomass
production in seafloor hydrothermal environments.
13
Fe/Mn Reduction - Overview

• Key concept Fe and Mn are generally highly
  insoluble under aerobic conditions, e.g. when
  present as oxides such as Fe(OH)3 or MnO2
• solubility of Fe(OH)3 is lt 1 nM at pH 7
• Under anaerobic conditions, Fe/Mn oxides are
  subject to reduction to Fe(II) and Mn(II), which
  are much more soluble/mobile
• Process is termed reductive dissolution

14
Environmental Significance of Microbial Fe/Mn
Reduction Role in Carbon/Energy Flow

• Fe/Mn reduction may contribute significantly to
  organic carbon oxidation in soils/sediments where
  Fe and/or Mn are abundant
• Reactions with sulfide (biotic and abiotic)
  dominate in organic-rich, high-S environments,
  e.g. shallow-water marine sediments
• Fe(III) is often the most abundant oxidant
  available for organic matter oxidation in
  aquifers after O2 is depleted Fe reduction plays
  an important role in oxidation of both natural
  organic carbon as well as hydrocarbon contaminants

15
Fe/Mn Reducing Organisms
Group 1. Fermentative bacteria 2. Circumneutral
organic carbon/H2 oxidizing Fe/Mn reducers 3.
Organic carbon/H2 oxidizing sulfate-reducing
bacteria 4. Acetate oxidizing methanogens 5.
Circumneutral S oxidizing Mn reducers 6.
Acidophilic organic carbon/H2 oxidizing Fe
reducers 7. Acidophilic S oxidizing Fe
reducers 8. Magnetotactic bacteria 9.
Thermophilic organic carbon/H2 oxidizing
archaea 10. Thermophilic S oxidizing archaea
Representative Organisms Bacillus,
Clostridium Geobacter, Shewanella, Geothrix,
Desulfuromonas, Geovibrio Desulfovibrio,
Desulfobacterium, Desulfobulbus,
Desulfotomaculum Methanosarcina Desulfovibrio,
Desulfobacterium, Geobacter, Desulfuromonas Acidip
hillium Thiobacillus, Leptospirillum Magnetospiri
lum, Magnetococcus Archaeoglobus, Geoglobus,
Pyrrococcus, Pyrodictium, Methanococcus Sulfulobus
, Acidianus, Sulfurococcus
No evidence for generation of energy for growth
from Fe/Mn reduction
16
Environmental Significance of Microbial Fe/Mn
Reduction Reduction of Other Trace Metals and
Metalloids

• Fe/Mn reducing bacteria can carry out enzymatic
  reduction of a wide variety of oxidized metals,
  metalloids, and radionuclides, e.g.
• Reduction generally (but not always) leads to
  precipitation/immobilization gt potential role in
  bioremediation of sediment