Chapter 21 Microorganisms and Metal Pollutants

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Chapter 21Microorganisms and Metal Pollutants
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Metals Defined

• Metals
• Gold, silver, copper,
• Metaloids
• Arsenic, boron, germanium tellurium
• Heavy metals (environmental toxicity)
• Arsenic, cadmium, copper, chromium, mercury,
  lead, zinc

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Oligodynamic Effect of Metals

• Many metals are required for normal biological
  functions
• Iron, cobalt, nickel, copper, zinc
• Many of the metals with known biological function
  can be toxic at high concentrations
• Toxic metals exert their toxicity in a number of
  ways
• Displacement of other essential metals from their
  binding site on biological molecules
• Arsenic and cadmium compete with phosphate and
  zinc, respectively
• Bioavailability A metal that can be taken up by
  an organisms is considered to be bioavailable

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Total versus soluble metal

• In the environment, the soluble form of a metal
  is usually a small fraction of the total metal.
• Add 100 mg/L of a metal to a water sample and
  you retain only 20 mg/L Cd and 0.12 mg/L Cu in
  solution

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Summary of various toxic influences of metals on
the microbial cell
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Mechanisms of metal resistance and detoxification
Most common way microbes prevent metal toxicity
is to pump the metal back out of the cell
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Mercury

• Why are we concerned with mercury?
• High toxicity due to the affinity of Hg to sulfur
  disruption of protein structure and function
• Resistance in eukaryotes biosynthesis of
  sulfhydryl-rich compounds (metallothioneins,
  phytochelatins). They contain SH groups
• Prokaryotes detoxify by reduction of Hg(II) to
  Hg(0) and subsequent volatilization

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Anthropogenic Emission of Hg into the environment

• Burning of fossil fuels
• Coal fired power plants contributes 65 of
  anthropogenic emissions
• Metal mining operations
• gold and silver
• Metal smelting and refining
• Cement manufacture
• Chemical manufacture
• Production of goods
• Disposal of Hg-waste
• Municipal landfills

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Natural Emission of Hg into the Biosphere

• Deep-sea vents

• Volcanoes

Terrestrial hot springs
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Hg(II)
Hg(0) ?Hg(II)
(via precipitation)
photochemical
Microbes
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Biomagnification of methyl-Hg
Small fish
Large fish
5000 ng/g
Humans
180 ng/g
1000 ng/g
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Bioaccumulation of methyl-Hg

• Accumulates in tissues over time
• Concentrates in the muscle tissue of fish
• Accumulates in the envelopes of nerve cells
• 100x more toxic than Hg0 and Hg2
• Destroys muscle proteins and enzymes essential to
  cell function

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The mercury resistance (mer) system in microbes

• Common among bacteria in soils and natural
  waters
• Applications in bioremediation and in
  monitoring of mercury in the environment

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Radionuclides

• Radioactive elements contribute to environmental
  contamination
• Department of Energy has been releasing
  radionuclides produced during nuclear bomb
  manufacturing into the environment since the
  1940s
• Plutonium, uranium, cesium, technicium
• These elements have a long half-life
• Their concentration in environment is typically
  low, but the radiation produced by low
  concentrations is still toxic to higher life
  forms

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Dissimilatory Metal Reduction

• Some microorganisms can use metals and
  radionuclides as terminal electron acceptors.
  This is an enzymatic process and is termed
  dissimilatory metal reduction. It is also
  sometimes called direct metal reduction
• Other microorganisms can reduce metals and rads
  indirectly through non-enzymatic mechanisms,
  usually involving a reaction between a microbial
  end product and the metal

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Schematic of Dissimilatory (Direct) Reduction
2 Acetyl CoA
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Bacterial Fe(III) Reduction

• Not all Fe(III) reducing bacteria generate usable
  cellular energy via Fe(III) reduction. However,
  under certain conditions, iron reduction can
  potentially generate substantial energy. For
  example, when acetate is oxidized the standard
  free energy change at pH 7 is

-193.4 kcal/mol for Fe(III) -201 kcal/mol for
O2 -5.5 kcal/mol for Fe(OH)3
This shows that insoluble iron oxides and
hydroxides are less favored electron acceptors
at neutral or alkaline pHs
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Indirect (Non-enzymatic) Metal Reduction

• Hydrogen sulfide produced by sulfate reducing
  bacteria may reduce ferric to ferrous iron via
  the following equation

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Indirect Metal Reduction
goethite
greigite mackinawite pyrrhotite pyrite
Adapted from Geomicrobiology by H. L. Erhlich
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• Microbial dissimilatory metal/rad reduction is a
  rapidly evolving area of study. In the past
  several years investigators have discovered that
  microbes are capable of directly reducing a wide
  variety of metals/rads.
• These discoveries are of considerable importance
  because they provide
• Information on natural metal cycling and
  deposition in nature
• Potential bioremediation options metals and
  radionuclides

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Metabolism of a Fe(III)-Reducing Bacterium
Fe(III)
ACETATE
U(VI) Co(III) Cr(VI) Se(VI) Pb(II) Tc(VII)
Benzoate Toluene Phenol p-Cresol Benzene
ATP
CO2
Fe(II)
CCl4 Cl-ethenes Cl-aromatics Nitro-aromatics
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Metals Known to be Reduced via Dissimilatory
Microbial Reduction

• Cr(VI)
• Fe(III)
• U(VI)
• Mn(IV)
• Se(VI), (IV), (0)
• Tc(VII)
• Hg(II)
• Cu(II)

• Co(III)
• Pd(II)
• Np(V)
• Pu(IV)
• Mo(VI)
• V(V)
• Au(III), (I)
• Ag(I)

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Uranium reduction leads to uranium precipitation
and immobilization
U6sol
U6sol
U4insol
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Mineral particle-associated metal reducing
bacteria as catalysts of uranium reduction
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Biostimulation
Goal to promote uranium reduction and
immobilization

• increase microbial biomass to increase
  enzyme-mediated transformation
• avoid excess biomass production that leads to
  formation plugging (impedes nutrient delivery to
  U reduction zone)

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Avoid stimulation that leads to excess bacterial
biomass accumulation in flow path of fluid
delivering nutrients to stimulate metal
reduction
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NABIR Field Research Center

• Located on the Oak Ridge Reservation
• S-3 Ponds consist of 4 unlined ponds constructed
  in 1951 at the Y-12 Plant
• Ponds received liquid wastes composed of nitric
  acid plating wastes containing nitrate and
  various metals and radionuclides (U, Tc) from
  1951-1983.

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Groundwater

• Contains gt40,000 mg/L total dissolved solids
• S-3 Pond plume contains elevated levels of
  nitrate, bicarbonate, Al, U, Tc, and
  tetrachloroethylene
• Plume is stratified
• Mobile nitrate and Tc are extensively distributed
• Nitrate has migrated approx. 1 km in the
  Nolichucky Shales since 1951 via preferential
  pathways.
• Less mobile U and other metals are more
  restricted in distribution

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Are indigenous bacteria capable of uranium
reduction?
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Evaluate ability of subsurface microbial
communities at this site to reduce metals

• Assess expression of genes known to be involved
  in metal reduction
• Look for evidence of metal reduction during
  stimulation

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AREA 3
Nitrate8200mg/L pH3.1 U650 mg/L Tc-9940,000
pCi/L Al438 mg/L SO41,000mg/L TCE3.1 mg/L
Nitrate N
pH Al U
2
Reactor effluent
Chemical extraction
Lactate or EtOH
FBR
surface
Side-stream coupon reactor
subsurface
Down
-
hole
U reduction zone
coupon reactor
in MLS wells
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Summary

• Metals can be used by microbes for essential
  metabolism or be toxic to cells, depending on
  availability and concentration in the
  environment.
• Most common way for microbes to avoid toxicity is
  to pump metal back out of cell into the
  environment.
• Some metals are becoming increasingly
  bioavailable in the environment and this
  increases exposure and health risks to organisms
  up the food chain.
• Microbes can transform redox active metals from
  soluble toxic forms to insoluble less toxic forms.