730
K. M. G. Mostofa et al.
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 surface 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
− ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (HO
• ) in PSII, which can
all be involved into cells decomposition (see chapter “Photosynthesis in Nature: A
New Look” for a detailed description). The second fact is the photoinduced generation of ROS from DOM (of both allochthonous and autochthonous origin), NO 2
−
and NO 3
− (see also chapters “Photoinduced and Microbial Generation of Hydrogen
Peroxide and Organic Peroxides in Natural Waters” and “Photoinduced Generation
of Hydroxyl Radical in Natural Waters”). These ROS can decompose Chl a that is
found outside the cells (see chapter “Photosynthesis in Nature: A New Look”). H 2 O 2
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 dissolved 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, particularly 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 production 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 involvement of dissolved O 2 in H 2 O 2 production can be justified by the experimental observation 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 concentration values that are 10 times lower than for green algae and diatoms. Strong lightdependent 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 concentrations of approximately 2–8 μM, 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 μM
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 significantly decreased upon addition of ROS scavengers, whilst post-irradiation bacterial 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
K. M. G. Mostofa et al.
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 surface 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
− ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (HO
• ) in PSII, which can
all be involved into cells decomposition (see chapter “Photosynthesis in Nature: A
New Look” for a detailed description). The second fact is the photoinduced generation of ROS from DOM (of both allochthonous and autochthonous origin), NO 2
−
and NO 3
− (see also chapters “Photoinduced and Microbial Generation of Hydrogen
Peroxide and Organic Peroxides in Natural Waters” and “Photoinduced Generation
of Hydroxyl Radical in Natural Waters”). These ROS can decompose Chl a that is
found outside the cells (see chapter “Photosynthesis in Nature: A New Look”). H 2 O 2
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 dissolved 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, particularly 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 production 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 involvement of dissolved O 2 in H 2 O 2 production can be justified by the experimental observation 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 concentration values that are 10 times lower than for green algae and diatoms. Strong lightdependent 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 concentrations of approximately 2–8 μM, 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 μM
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 significantly decreased upon addition of ROS scavengers, whilst post-irradiation bacterial 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