Microbial transformations of mercury in the environment
lipophilic
2e-
Hg0
[CH3-]
[CH3-]
Hg2+
CH3Hg+
(CH3)2Hg
HS-, -S-R
HgS,
Hg+-S-R
Algae
Fish
Children
Women
Fetus
Infants
Minnamata Bay
Hg2+, Hg0
CH3-Hg+, (CH3)2Hg
Anoxic sediment
Pathology: constricted visual field, impairment of vibration sense,
total blindness, ataxia, necrosis of Glia cells
“Life is Redox Chemistry”
AH2 + B
A + BH2
Energy
DG= -nFDE, DE= Eacceptor - Edonor
DG<0
Redox pot ent ials for bact eria-mediat ed redox sequences
( St andard st at e condit ions at pH 7 )
DG o' = - nFDE o'
Elect ron donor
o'
E [ mV]
CH2 O
CO2
H2
+
DE o '=Eo' acceptor - Eo'
donor
Elect ron accept or
o'
E [ mV]
-5 0 0
-4 7 0
-50 0
-47 0
-4 2 0
-42 0
CH2 O
H2
CO2
H+
H
2+
CH4
H2 S
H2 S
CO2
So
24
SO
Succinat e
-25 0
-2 4 0
-2 2 0
-2 5 0
-2 2 0
Fe
H2 S
0
+30
0
+ 30
Fe( OH)3
24
Succinat e
Fumarate
FeCO3
FeCO3
Fe( OH) 3 + HCO -3
+ 2 00
+ 2 00
+
NO3-
-
Fe( OH) 3 + HCO3+
NH4
NH 4
+ 3 60
+ 3 60
+ 4 20
+ 4 20
NO3-
NO2-
NO3
NO3
Mn
2+
Mn
+ 5 50
+ 5 50
Mn4 +
H2 O
O2
NO3-
2+
4+
Mn
N2
N2
+ 7 50
+ 7 50
+ 8 20
+ 8 20
CH4
CO2
So
SO
Fumarate
NO2-
H2 S
NO3-
H2 O
O2
Carbon and electron flow in sulfidogenic environments
Complex organic matter
Carbohydrates, nucleic acids
proteins, lipids
1
Lactate
Propionate
Higher fatty acids
Alcohols
Aromatic compounds
H2, CO2
Acetate
2
SO42-
H2S, CO2
1
2
= fermentative bacteria, e.g., lactic acid bacteria, clostridia, enteric bacteria, propionibacteria
= sulfate-reducing bacteria, e.g., Desulfovibrio spp, Desulfobacterium spp, Desulfobacter spp,
Desulfotomaculum spp.
Carbon and electron flow in methanogenic environments
Complex organic matter
Carbohydrates, nucleic acids
proteins, lipids
1
Lactate
Propionate
Higher fatty acids
Alcohols
Aromatic compounds
2
H2, CO2
3
Acetate
4
1
CH4, CO2
2
= fermentative bacteria, e.g., lactic acid bacteria, clostridia, enteric bacteria, propionibacteria
= proton-reducing, syntrophic bacteria, e.g., Syntrophomonas spp, Syntrophobacter
3
4
= homoacetogenic bacteria, e.g., Acetobacterium woodii, Clostrodium aceticum
= methanogenic archaea, e.g., Methanosarcina spp, Methanobacterium spp., Methanospirillum spp.
Carbon and electron flow in sulfidogenic environments
Complex organic matter
Carbohydrates, nucleic acids
proteins, lipids
1
Lactate
Propionate
Higher fatty acids
Alcohols
Aromatic compounds
H2, CO2
Acetate
2
SO42-
H2S, CO2
1
2
= fermentative bacteria, e.g., lactic acid bacteria, clostridia, enteric bacteria, propionibacteria
= sulfate-reducing bacteria, e.g., Desulfovibrio spp, Desulfobacterium spp, Desulfobacter spp,
Desulfotomaculum spp.
Sulfate-reducing bacteria
- all sulfate-reducing bacteria are strict anaerobes
- phylogenetically heterogeneous group:
gram + : Desulfotomaculum spec., some Clostridia
gram - : Desulfovibrio spec., Desulfobacterium spec., Desulfobacter spec.,
Thermodesulfobacterium spec.
archaea: Archaeoglobus spec.
General characteristics of catabolism in sulfate-reducing bacteria
2-
SO 4
nXH 2
[H]
ATP
nX
H2 S
N.B.: All sulfate-reducers are specialized to use products of the primary fermenting bacteria
-
XH 2 = H2 , HCOO , Acetate, Propionate, Butyrate, Stearate (C 18 ), Fumarate, Succinate, Malate,
Ethanol, Methanol, Alkane
Benzoate, Phenylacetate, Toluene,
Alanine, Glutamate, Cholin, Nicotinate, Indole
Based on their metabolic properties sulfate-reducing bacteria can be subdivided into
2 groups:
A) Incomplete oxidizers : organic substrates
Acetate + CO2
e.g.: Desulfovibrio spec., Desulfobulbus spec., Desulfotomaculum (some)
B) Complete oxidizers:
organic substrates
CO 2
(some acetate can be excreted)
e.g.: Desulfobacter spec., Desulfococcus spec., Desulfosarcina spec., Desulfonema spec.,
Desulfobacterium spec., Desulfotomaculum acetoxidans
N.B.: The complete oxidizers can use the citric acid cycle or the carbon
monoxide dehydrogenase pathway for complete oxidation of acetyl-CoA.
Oxidative carbon monoxide pathway in Sulfate-reducing bacteria
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Mercury in the environment
lipophilic
2e-
Hg0
[CH3-]
[CH3-]
Hg2+
CH3Hg+
(CH3)2Hg
HS-, -S-R
HgS,
Hg+-S-R
Algae
Fish
Children
Women
Fetus
Infants