Environmental Engineering Reference
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observations that UV-B radiation can inhibit the oxygen-evolving complex of
PSII in M. aeruginosa (Jiang and Qiu 2011 ). The whole electron-transport activ-
ities are significantly varied: the transfer from water to methyl viologen being
inhibited by 27.9 % under UV-B, that from diphenylcarbazide to methyl viologen
by 13.3 % (Jiang and Qiu 2011 ).
Cyanobacterial blooms in freshwater have apparently increased over the last
few decades all over the world (Xu et al. 2000 ; Chen et al. 2003 ; McCarthy et
al. 2007 ). UV-B influences the CO 2 -uptake mechanism of M. aeruginosa , and this
cyanobacterium has many adaptive strategies to cope with prolonged UV-B expo-
sure (Jiang and Qiu 2005 ; Song and Qiu 2007 ). It has been shown that maximum
quantum yield and maximum electron transport rate in seaweeds collected from
the Red Sea decreased largely due to the combined effects of increased irradiance
(PAR) and presence of UV radiation (Figueroa et al. 2009 ). A 33-kDa protein of
the water-splitting complex is sensitive to UV-B. Therefore, its degradation con-
tributes importantly to the decline of the electron transport rate (Jiang and Qiu
2011 ; Prabha and Kulandaivelu 2002 ). Short-term UV-B exposure can severely
inhibit photosynthetic capability, which could be quickly restored upon exposure
to PAR (Jiang and Qiu 2011 ). Quite surprisingly, UV-A can assist the photo repair
of UV-damaged DNA and enhance carbon fixation under reduced levels of solar
radiation or fast mixing conditions (Gao et al. 2007 , 2007 ; Karentz et al. 1991 ;
Barbieri et al. 2002 ; Helbling et al. 2003 ). Recent study reveals that the PSII of
M. aeruginosa FACHB 854 is more sensitive to UV-B exposure than PSI, and the
oxygen-evolving complex of PS II is an important target for UV-B damage (Jiang
and Qiu 2011 ).
The mechanisms behind the photoinhibition effects of strong sunlight, UV light
or high irradiance (drought/heat stress) on aquatic microorganisms are presumably
involving two facts: First, there are direct effects in which a high number of elec-
trons is released from chlorophylls (Chl) (P680) in PSII of microorganisms, upon
excitation by strong light or strong UV light (Eq. 5.1 ). The release of many elec-
trons can produce elevated amounts of reactive oxygen species (ROS) such as 1 O 2 ,
O 2
(Eq. 5.2 ). Among the ROS, H 2 O 2 can be used in photosyn-
thesis whilst the remaining ROS including H 2 O 2 can react with the Chl + (P680 + )
functional groups bound to PSII, killing the cells (Eq. 5.3 ). These reactions can be
schematically depicted as follows:
, H 2 O 2 and HO
Chl ( or P680 ) + h υ → Chl +
or P680 +
+ e
(5.1)
E + O 2 + H υ → 1 O 2 / O 2 •− / H 2 O 2 + H υ → HO
(5.2)
+ Chl +
or P680 +
Chl +
or P680 +
HO
1 O 2 / O 2 •− / H 2 O 2
damage
(5.3)
ROS production in cells of aquatic microorganisms has generally been detected in
earlier studies, which are extensively discussed in earlier sections. The process is sup-
ported by the earlier observation that chlorophylls can easily undergo photooxidation,
involving attack of singlet oxygen and enzymatic degradation (Brown SB and Hendry
1991 ; Gossauer and Engel 1996 ). Experimental studies show that H 2 O 2 can affect the
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