The Weathering of Nickeldoped Silicate Glass in Anaerobic Groundwater Microcosms

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The Weathering of Nickel-doped Silicate Glass in
Anaerobic Groundwater Microcosms
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Research Question

• How do increases in silicate-Ni concentrations
  impact microbially-mediated silicate weathering
  by a native, anaerobic consortia?

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Hypotheses

• Accelerated dissolution of Si will occur at low
  Ni concentrations due to Ni micronutrient
  requirements by methanogens.
• High Ni concentrations are toxic to the native
  microbial consortia and will cause a decrease in
  biomass.

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Lake
Well 9014
Oil Pool
Railroad
Spray Zone
Wetland
U.S. Geological Survey Bemidji Research Site
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(No Transcript)
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Methanogenesis

• Methanogenesis is carried out by a group of
  strict anaerobes from the Archaea known as the
  methanogens
• Most methanogens use CO2 as a terminal electron
  acceptor in anaerobic respiration, reducing it to
  CH4 and H2O

Madigan and Martinko 2006.
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Methanogenesis and Syntrophy

• Conversion of most organic compounds to CH4
  requires the partnership of methanogens with
  fermenters, which supply the substrates
• H2, CO2, fatty acids and alcohols
• Syntrophic relationship
• The H2 produced in primary fermentation is
  removed by H2-consumers (methanogens)

Madigan and Martinko 2006.
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Substrates for methanogenesis

• CO2-type substrates
• CO2 4H2 ?CH4 2H2O
• Methyl substrates
• CH3OH H2 ? CH4 H2O (methyl reduction)
• 4CH3OH? 3CH4 CO2 2H2O (methyl oxidation)
• Acetotrophic substrates
• CH3COO- H2O ? CH4 HCO3-

Madigan and Martinko 2006.
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DOC biodegradation products
Lovley and Phillips 1988, Madigan and Martinko
2006.
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Variables to be tested

• Silica flux
• Iron reduction
• pH
• Change in alkalinity
• Metabolic activity
• CO2 and CH4
• Biomass

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Field methods
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Field measurements
Alkanility 10.6 mmol/L
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Silica solubility through time
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Silica solubility at varying nickel concentrations
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Fe II concentration comparison between averaged
replicates and sterile controls (day 35 vs. day
50)
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pH
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Change in alkalinity
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Solution carbon dioxide Ni-doped glass (dayn -
day10)
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Solution carbon dioxide Cu/Ni-doped glass vs.
Ni-doped glass (dayn - day10)
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Change in solution methane (dayn - day10)
? CH4 (?mol/L)
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Change in solution methane (dayn - day10)
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Biomass comparison of microcosms (day 2 vs. day
50)
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Interpretation of Results
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Silica and Iron

• Si(aq) ? Fe(II)(aq), therefore precipitation
  of silica may be a result of complexation with
  ferric iron precipitation
• Confirm by XRD and SEM analyses

day 35 and day 50
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Ni concentration in silicate glass
Contrary to the hypotheses, lower Ni
concentrations do not yield higher rates of
silica dissolution.
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Biomass and CO2 Ni-doped glass

• With the exception of Ni-1, biomass does not
  increase over time
• Relative increase in CO2 over time, which
  reflects an increase in microbial respiration

day 2 vs. day 50
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Biomass and CO2 Cu/Ni-doped glass

• There is an increase in biomass, which coincides
  with increasing CO2

day 2 vs. day 50
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Methane contribution

• Increasing CH4 is negligible compared to CO2
• Methanogenic bacteria represent a small fraction
  of microbial consortia
• Ni proportion in glass inconsistently impacts
  methane production

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Iron, pH, and alkalinity

• Decreasing ferrous iron concentrations not
  expected in anaerobic environment
• Evidence of O2 contamination
• Fermenters, methanogens, and iron reducers
  stunted
• Insignificant changes in pH and alkalinity

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Conclusions

• The relative rate of silica dissolution is not
  highest for low Ni concentration in glasses
• Concentration of Ni-5 (2 mol ) does not appear
  to be toxic to the native microbial consortia
• More time is needed to evaluate microcosms
• Contamination has interfered with the
  experimental processes

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References

• Lovley DR, Phillips EJP. Novel mode of microbial
  energy metabolism Organic carbon oxidation
  coupled to dissimilatory reduction of iron or
  manganese. Applied and Environmental
  Microbiology, 54(6) 1472-1480.
• Madigan MT, Martinko JM. 2006. Brock biology of
  microorganisms. - 11th edition. Pearson Prentice
  Hall, Upper Saddle River, NJ.

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Extra stuff
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pH discussion

• Fe Reduction
• CH2O Fe(OH)3 ? Fe2 CO2 OH-
• Fe Oxidation
• Fe2 H2O O2 ? Fe(OH)3 H

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Fe(III) reduction CH4 production in anaerobic
environments

• e- donor competition among Fe(III)-reducing and
  methanogenic bacteria
• Simultaneous when e- donors (H2) and acceptors
  are not limiting
• The rate of organic matter fermentation limits
  the supply of e- donors to the TEAPs
• When sufficient microbially-reducible Fe(III) is
  available, Fe(III) reducers outcompete
  methanogens for e- donors

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Ni as a micronutrient

• Ni is required as the central atom of the
  tetrapyrole coenzyme F430
• Coenzyme F430 is in involved in the terminal step
  of methanogenesis as part of the methyl reductase
  enzyme complex

www.jaun.chem.ethz.ch/res/res_F430.html
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Fe II Concentration Comparison between Averaged
Replicates (day 35 vs. day 50)