Environmental Engineering Reference
In-Depth Information
is decomposed more slowly (Mostofa et al.
2009
; Zhang et al.
2009
; Mostofa
KMG et al.
2008
; Ogawa et al.
2001
).
Low concentrations of Chl
a
during the summer stratification period in upper sur-
face waters might be the effect of photoinduced degradation of Chl
a
by sunlight.
Degradation of Chl
a
presumably involves two facts. First of all, cyanobacteria can
generate internally reactive oxygen species (ROS) such as superoxide radical anion
(O
2
•
−
) in PSII, which can
all be involved into cells decomposition (see chapter
“
Photosynthesis in Nature: A
tion of ROS from DOM (of both allochthonous and autochthonous origin), NO
2
), hydrogen peroxide (H
2
O
2
) and hydroxyl radical (HO
−
−
and NO
3
(see also chapters
“
Photoinduced and Microbial Generation of Hydrogen
of Hydroxyl Radical in Natural Waters
”). These ROS can decompose Chl
a
that is
involvement can be justified by the observation that autoxidation is substantially
enhanced in the presence of a peroxide or hydroperoxide initiator (Fossey et al.
1996
;
Wilson et al.
2000
; Kwan and Voelker
2003
). Dissolved O
2
is substantially varied
(from 6.0 to 12.0 mg L
−
1
) in a variety of surface waters, whereas the saturated dis-
solved O
2
concentration in pure water is 7.5 mg L
−
1
at 30 °C (Falkner et al.
2005
;
Garcia et al.
2005
; Schmittner et al.
2007
; Araoye
2009
; Abowei
2010
; Keeling et al.
2010
; Hatcher
1987
). High contents are generally found at low temperature, particu-
larly in the Arctic and Antarctic Oceans. Such high contents of dissolved O
2
prompt
the rapid absorption of electrons released from either chromophoric DOM (CDOM)
or POM (e.g. phytoplankton or algae) upon light illumination, which enhances pro-
duction of O
2
•
−
and H
2
O
2
. Dissolved O
2
in water is the ultimate electron acceptor
upon illumination by light, forming O
2
•
−
that is a long-suspected first intermediate
in photoinduced reactions that take place in natural surface waters (Baxter and Carey
1983
; Bielski et al.
1985
; Petasne and Zika
1987
; Micinski et al.
1993
). The involve-
ment of dissolved O
2
in H
2
O
2
production can be justified by the experimental obser-
vation that 5-40 % of the oxygen produced by photosynthetically active organisms
can be fixed through photochemical reactions in natural waters (Laane et al.
1985
).
Experimental studies show that H
2
O
2
can affect cyanobacteria at concentra-
tion values that are 10 times lower than for green algae and diatoms. Strong light-
dependent toxicity can enhance the difference, for which reason H
2
O
2
can act as
a limiting factor for cyanobacterial growth (Drábková et al.
2007
). H
2
O
2
concen-
trations of approximately 2-8
μ
M, which are produced during light exposure of
aquatic macrophyte leachates or DOM, can inhibit microbial growth or bacterial
carbon production (Farjalla et al.
2001
; Anesio et al.
2005
). The addition of 0.1
μ
M
H
2
O
2
to humic lake water can inhibit BCP by as much as 40 % (Xenopoulos and
Bird
1997
). Photobleaching and CO
2
production in irradiated waters can be signifi-
cantly decreased upon addition of ROS scavengers, whilst post-irradiation bacte-
rial growth in samples containing a ROS scavenger can be significantly increased
Scully et al. (
2003
). The decrease of ROS activity (CO
2
production) can likely
cause an accumulation of bioavailable DOM and enhance microbial processes
(Scully et al.
2003
). Chl
a
is more susceptible to photochemical decomposition than
