Environmental Engineering Reference
In-Depth Information
tryptophan-like compounds (Mostofa and Sakugawa
2009
). Thus, production of
H
2
O
2
and ROOH significantly depends on the molecular nature and composition
of FDOM or CDOM rather than on DOC concentration.
4.3 Production and Decay Affected by Phytoplankton,
Algae and Microbes
Production and decay of H
2
O
2
and ROOH are greatly influenced by marine
phytoplankton, algae and microbes. Two phenomena are involved. First, marine
phytoplankton, algae and microbes may produce autochthonous DOM, which is
then involved into the photoinduced or microbiological (the latter being highlighted
under dark incubation) generation of H
2
O
2
and ROOH compounds in natural waters.
Second, the decay of H
2
O
2
and ROOH compounds may be caused by catalase, per-
oxidase and superoxide dismutase produced by phytoplankton, algae and microbes.
A variety of marine organisms or phytoplankton can produce or excrete
organic compounds such as riboflavin (Dunlap and Susic
1985
; Mopper and
Zika
1987
), amino acids including tryptophan, proteins, carbohydrates and satu-
rated and unsaturated fatty acids (McCarthy et al.
1997
; Rosenstock and Simon
2001
; Nieto-Cid et al.
2006
). All of these organic compounds are photolytically
reactive. For example, 1 nM riboflavin added to seawater can produce approxi-
mately 10 nM H
2
O
2
(Mopper and Zika
1987
), and tryptophan can produce H
2
O
2
at a rate of 648 nM h
−
1
in aqueous media (Table
2
). The organisms, 10
5
cocco-
lithophorid cells L
−
1
, can produce H
2
O
2
at a rate of 1-2 nM h
−
1
in oligotrophic
waters (Palenic et al.
1987
). Production of H
2
O
2
by the eukaryotic phytoplankton
species
Hymenomonas
carrterae
is induced by amino acid oxidation by cell-sur-
face enzymes (Palenic et al.
1987
). The photorespiration cycle of phytoplankton
involves production of H
2
O
2
during glycolate oxidation (Lehninger
1970
), which
can be expressed as follows (Eq.
4.1
):
CH
2
OHCOOH
+
O
2
glycolate oxidase
(4.1)
−→
CHOCOOH
+
H
2
O
2
The rate of photorespiration increases with high light intensity, possibly as a
way to dissipate the excess light energy (Harris
1979
), but its exact role is unclear
(Ogren
1984
).
The exposure of algae suspensions to sunlight can produce H
2
O
2
((Johnson et
al.
1989
; Collen et al.
1995
; Zepp et al.
1987
), possibly by photoinduced excita-
tion of DOM released under photo- and microbial assimilation of algae (Mostofa
et al.
2009b
; Medina-Sánchez et al.
2006
; Fu et al.
2010
; Takahashi et al.
1995
;
Marañòn et al.
2004
). This hypothesis is supported by the fact that the H
2
O
2
pro-
duction from algal suspensions is low in the initial two hours of irradiation, and is
greatly increased with further irradiation (Zepp et al.
1987
). It can be assumed that
the high production of H
2
O
2
after two hours occurs because of the photodegrada-
tion of organic substances newly released from algal suspensions in the reaction
media during the initial irradiation period. For example, the production rates of
