Environmental Engineering Reference
In-Depth Information
H
2
O
2
due to sunlight effects on algae are 0.04-1.7
×
10
6
M h
−
1
for five algae at a
concentration of 0.097-1.0
×
10
−
3
mg m
−
3
Chl
a
(Zepp et al.
1987
).
4.3.1 Mechanism of Microbial Decomposition of H
2
O
2
and ROOH
Decay of peroxides (H
2
O
2
and ROOH) by phytoplankton, algae and microbes is a
reverse effect of peroxide production in natural waters. Peroxides (H′OOH, H′
=
H
or R) may be decomposed by catalase, peroxidase and superoxide dismutase, pro-
duced by phytoplankton, algae and microbes to generate energy for their growth
and to eliminate excessive intracellular levels of H
2
O
2
and O
2
•
−
(Fujiwara et al.
1993
; Moffett and Zafiriou
1990
; Zepp et al.
1987
; Mostofa et al. (Manuscript in
preparation); Wong et al.
2003
). Such a decomposition effect induced by phyto-
plankton, algae and microbes would usually occur constantly, until the concen-
tration of peroxides reaches a minimum level that would afford inefficient further
decomposition. Catalase enzymatically activates the peroxides (H′OOH
*
) to use
them as oxidants (electron acceptors) and reductants (electron donors). Afterwards,
disproportionation of activated H′OOH
*
converts them into water or alcohols and
oxygen. A reaction scheme (Eqs.
4.2
,
4.3
) for the decomposition of peroxides by
catalase can be generalized as follows (Moffett and Zafiriou
1990
):
H
′
OOH
+
Catalase
→
H
′
OOH
∗
+
Catalase
#
(4.2)
2H
′
OOH
∗
+
Catalase
#
→
H
′
−
O
−
H
+
O
2
+
Catalase
(4.3)
where Catalase
#
is the activated state of catalase.
Peroxidase enzymatically activates the peroxides (H′OOH
*
) to detoxify them
to H
2
O or any other end product. As reducing species it uses organic compounds
(H
2
R) other than H′OOH. A reaction scheme (Eqs.
4.4
,
4.5
) for the decomposition
of peroxides is presented below (Moffett and Zafiriou
1990
):
H
′
OOH
+
Peroxidase
→
H
′
OOH
∗
+
Peroxidase
#
(4.4)
H
′
OOH
∗
+
H
2
R
+
PEROXIDASE
#
→
H
′
−
O
−
H
+
H
−
O
−
H
+
R
+
PEROXIDASE
(4.5)
where Peroxidase
#
is the activated state of peroxidase. It has been shown that the
percentage decay of H
2
O
2
was 65-80 % by catalase and 20-35 % by peroxidase, as
estimated by isotopic measurements in seawater (Moffett and Zafiriou
1990
). The
sources of catalase and peroxidase in natural waters are bacteria and marine phyto-
plankton (Kim and Zobell
1974
), but these enzymes are also part of the dissolved
organic matter (Serban and Nissenbaum
1986
). Similarly, chloroplasts have a per-
oxidase-mediated H
2
O
2
scavenging system (Tanaka et al.
1985
). Natural marine
peroxidases are also capable of catalyzing H
2
O
2
-mediated halogenation reactions
in the oceanic environments (Theiler et al.
1978
; Baden and Corbett
1980
). The
decay of H
2
O
2
is usually low (12 % after 5 h incubation) in upstream waters due to
the presence of few bacteria (some 10
5
cells mL
−
1
), and much higher in polluted
