biogeochemistry of Mn (Sunda and Huntsman 1990). Mn
3+ being a strong oxidant
and a reductant, it has been largely ignored due to its property to disproportionate to
Mn
2+ and MnO 2 (Johnson 2006). However, recent improvements in the understanding of Mn chemistry indicate that dissolved Mn exist mostly as Mn
3+ in sub-oxic
regions (Trouwborst et al. 2006). Mn being one of the strongest oxidant in the
natural environment, eventually participates in redox reactions and due to its
sorptive characteristics controls the distributions and bioavailability of several
toxic and essential trace elements (Tebo et al. 2004). Microorganisms like bacteria
and fungi are known oxidizers of Mn
2+ and reducers of Mn oxide-containing
minerals. They carry out oxidation/reduction of Mn as a way to conserve energy
for growth or oxidation of carbon (Nealson and Myers 1992; Tebo et al. 2005). The
oxidation of Mn
2+ under natural conditions is catalyzed only by microbes under pH
range of 5.5–8.0, Eh value above +200 mV and oxygen concentration of 3–5 mg L
À1
(Schweisfurth et al. 1978).
The three representative stoichiometric relationships that typically follow the
oxidation of Mn
2+ depending on the oxide product formed according to Nealson
et al. (1988) are
Mn
2þ
þ
1
2
O 2 þ H 2 O ! MnO 2 þ 2H
þ
Mn
2þ
þ
1
4
O 2 þ
3
2
H 2 O ! MnOOH þ 2H
þ
3Mn
2þ
þ
1
2
O 2 þ 3H 2 O ! Mn 3 O 4 þ 6H
þ
A number of bacteria capable of mixotrophic and autotrophic growth could
derive useful energy from Mn
2+ oxidation. The oxidation of Mn
2+ by bacteria
could yield DF r of +2.79 and DF
0
r of À16.31 kcal (Ehrlich 1976, 1978) in the
reaction
Mn
2þ
þ
1
2
O 2 þ H 2 O ! MnO 2 þ 2H
þ
and yield DG À18.5 kcal in the reverse reaction
MnO 2 þ 4H
þ
þ 2e ! Mn
2þ
þ 2H 2 O
when allowance for physiological pH of 7.0 is made (Ehrlich 1987).
The key steps in the reduction of Mn
4+ to Mn
2+ involve the reactions of strongly
bound Mn
3+ {{Mn
3+ }} and weakly bound Mn
2+ {Mn
2+ } on the surface of MnO 2
(Ehrlich 2002b). The following steps explain the process
fMn
2þ
g þ MnO 2 þ 2H 2 O ! 2ffMn
3þ
gg þ 4OH
À
2ffMn
3þ
gg þ 2e
À
! fMn
2þ
g þ Mn
2þ
52
P.P. Sujith and P.A. Loka Bharathi
3+ being a strong oxidant
and a reductant, it has been largely ignored due to its property to disproportionate to
Mn
2+ and MnO 2 (Johnson 2006). However, recent improvements in the understanding of Mn chemistry indicate that dissolved Mn exist mostly as Mn
3+ in sub-oxic
regions (Trouwborst et al. 2006). Mn being one of the strongest oxidant in the
natural environment, eventually participates in redox reactions and due to its
sorptive characteristics controls the distributions and bioavailability of several
toxic and essential trace elements (Tebo et al. 2004). Microorganisms like bacteria
and fungi are known oxidizers of Mn
2+ and reducers of Mn oxide-containing
minerals. They carry out oxidation/reduction of Mn as a way to conserve energy
for growth or oxidation of carbon (Nealson and Myers 1992; Tebo et al. 2005). The
oxidation of Mn
2+ under natural conditions is catalyzed only by microbes under pH
range of 5.5–8.0, Eh value above +200 mV and oxygen concentration of 3–5 mg L
À1
(Schweisfurth et al. 1978).
The three representative stoichiometric relationships that typically follow the
oxidation of Mn
2+ depending on the oxide product formed according to Nealson
et al. (1988) are
Mn
2þ
þ
1
2
O 2 þ H 2 O ! MnO 2 þ 2H
þ
Mn
2þ
þ
1
4
O 2 þ
3
2
H 2 O ! MnOOH þ 2H
þ
3Mn
2þ
þ
1
2
O 2 þ 3H 2 O ! Mn 3 O 4 þ 6H
þ
A number of bacteria capable of mixotrophic and autotrophic growth could
derive useful energy from Mn
2+ oxidation. The oxidation of Mn
2+ by bacteria
could yield DF r of +2.79 and DF
0
r of À16.31 kcal (Ehrlich 1976, 1978) in the
reaction
Mn
2þ
þ
1
2
O 2 þ H 2 O ! MnO 2 þ 2H
þ
and yield DG À18.5 kcal in the reverse reaction
MnO 2 þ 4H
þ
þ 2e ! Mn
2þ
þ 2H 2 O
when allowance for physiological pH of 7.0 is made (Ehrlich 1987).
The key steps in the reduction of Mn
4+ to Mn
2+ involve the reactions of strongly
bound Mn
3+ {{Mn
3+ }} and weakly bound Mn
2+ {Mn
2+ } on the surface of MnO 2
(Ehrlich 2002b). The following steps explain the process
fMn
2þ
g þ MnO 2 þ 2H 2 O ! 2ffMn
3þ
gg þ 4OH
À
2ffMn
3þ
gg þ 2e
À
! fMn
2þ
g þ Mn
2þ
52
P.P. Sujith and P.A. Loka Bharathi