faster in the oxidized surface sediment where the Mn-oxidizing bacteria are
abundant.
In an attempt to describe the general mechanism of Mn
2+ oxidation, Tebo and
Emerson (1986) describe a model where they conclude that Mn oxidation rate is not
dependent on Mn concentration in the water column but is rather a function of the
total number of surface-binding sites available. Similarly, de Vrind et al. (1986b)
showed with experimental evidence that mature spores of marine Bacillus SG-1
could oxidize Mn
2+ . On the other hand, de Vrind et al. (1986a) demonstrated
that vegetative cells of the same organism could reduce manganese. The reduction
of Mn by the vegetative cells was thought to make Mn
2+ available for sporulation
in a manganese-limited environment. Accumulation of MnO 2 by the spore
coat prevented further oxidation of bound Mn
2+ as the active sites get masked
when oxygen gets consumed and protons get liberated. In another approach,
the marine Pseudomonas sp. Strain S-36 when grown in continuous culture
was found to obtain energy for CO 2 fixation from Mn
2+ oxidation (Kepkay and
Nealson 1987).
In a different approach, Sunda and Huntsman (1987) used radiotracers to
determine the kinetics of particulate Mn formation in seawater. According to
them, Mn
2+ oxidation is microbially catalyzed and the rates depend upon the
increase in temperature with respect to the ratio of particulate to dissolved Mn in
estuarine water. The indirect process of Mn
2+ oxidation by Chlorella sp at high pH
(>9.0) resulting from photosynthesis was reported by Richardson et al. (1988).
They demonstrated that growth of photosynthetic organisms as aggregates or as
concentrated cell suspension in pelagic waters could generate microenvironments
with steep gradients of oxygen and pH conducive for Mn
2+ oxidation. Radiotracer
studies on hydrothermal vent locations showed the scavenging of Mn
2+ at higher
rates under in situ incubations compared to onboard studies (Mandernack and Tebo
1993). Their results could suggest that bacteria not only enhance the scavenging of
Mn within vent waters, but also facilitated Mn deposition to the sediments. In
another interesting observation by Hansel and Francis (2006), the unrecognized role
of Roseobacter like planktonic bacteria in Mn
2+ oxidation and cycling in coastal
waters was identified suggesting an alternative means of Mn
2+ oxidation in the
photic zone. The bacterium showed the ability to oxidize Mn
2+ in the presence of
light through photooxidation pathway and by direct enzymatic action in the dark.
Based on a kinetic model of the oxidative pathway, Webb et al. (2005) stated that
Mn
3+ is a transient intermediate and the rate-limiting step in the oxidation of Mn
2+ .
They suggested that oxidation of Mn
2+ could involve a unique multicopper
oxidases (MCOs) system capable of two-electron oxidation of its substrate.
MCOs are a class of enzymes that have metallocentre assembly containing four
Cu atoms (Brouwers et al. 2000b). They couple the four-electron reduction of
dioxygen to water with the oxidation of substrate. The well-defined MCOs are
laccase, ascorbate oxidase, and ceruloplasmin. The others include phenoxazinone
synthase, bilirubin oxidase, dihydrogeodin oxidase, sulochrin oxidase, and FET3
(Solomon et al. 1996).
56
P.P. Sujith and P.A. Loka Bharathi
abundant.
In an attempt to describe the general mechanism of Mn
2+ oxidation, Tebo and
Emerson (1986) describe a model where they conclude that Mn oxidation rate is not
dependent on Mn concentration in the water column but is rather a function of the
total number of surface-binding sites available. Similarly, de Vrind et al. (1986b)
showed with experimental evidence that mature spores of marine Bacillus SG-1
could oxidize Mn
2+ . On the other hand, de Vrind et al. (1986a) demonstrated
that vegetative cells of the same organism could reduce manganese. The reduction
of Mn by the vegetative cells was thought to make Mn
2+ available for sporulation
in a manganese-limited environment. Accumulation of MnO 2 by the spore
coat prevented further oxidation of bound Mn
2+ as the active sites get masked
when oxygen gets consumed and protons get liberated. In another approach,
the marine Pseudomonas sp. Strain S-36 when grown in continuous culture
was found to obtain energy for CO 2 fixation from Mn
2+ oxidation (Kepkay and
Nealson 1987).
In a different approach, Sunda and Huntsman (1987) used radiotracers to
determine the kinetics of particulate Mn formation in seawater. According to
them, Mn
2+ oxidation is microbially catalyzed and the rates depend upon the
increase in temperature with respect to the ratio of particulate to dissolved Mn in
estuarine water. The indirect process of Mn
2+ oxidation by Chlorella sp at high pH
(>9.0) resulting from photosynthesis was reported by Richardson et al. (1988).
They demonstrated that growth of photosynthetic organisms as aggregates or as
concentrated cell suspension in pelagic waters could generate microenvironments
with steep gradients of oxygen and pH conducive for Mn
2+ oxidation. Radiotracer
studies on hydrothermal vent locations showed the scavenging of Mn
2+ at higher
rates under in situ incubations compared to onboard studies (Mandernack and Tebo
1993). Their results could suggest that bacteria not only enhance the scavenging of
Mn within vent waters, but also facilitated Mn deposition to the sediments. In
another interesting observation by Hansel and Francis (2006), the unrecognized role
of Roseobacter like planktonic bacteria in Mn
2+ oxidation and cycling in coastal
waters was identified suggesting an alternative means of Mn
2+ oxidation in the
photic zone. The bacterium showed the ability to oxidize Mn
2+ in the presence of
light through photooxidation pathway and by direct enzymatic action in the dark.
Based on a kinetic model of the oxidative pathway, Webb et al. (2005) stated that
Mn
3+ is a transient intermediate and the rate-limiting step in the oxidation of Mn
2+ .
They suggested that oxidation of Mn
2+ could involve a unique multicopper
oxidases (MCOs) system capable of two-electron oxidation of its substrate.
MCOs are a class of enzymes that have metallocentre assembly containing four
Cu atoms (Brouwers et al. 2000b). They couple the four-electron reduction of
dioxygen to water with the oxidation of substrate. The well-defined MCOs are
laccase, ascorbate oxidase, and ceruloplasmin. The others include phenoxazinone
synthase, bilirubin oxidase, dihydrogeodin oxidase, sulochrin oxidase, and FET3
(Solomon et al. 1996).
56
P.P. Sujith and P.A. Loka Bharathi