Environmental Engineering Reference
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PSII in the S 2 and S 3 states indicates that UV absorption by the Mn(III) and Mn(IV) ions
could be the primary sensitizer of UV-induced damage of the water-oxidizing
machinery.
Damage by UV-A radiation
In contrast to the wealth of information available on the damaging mechanism of
UV-B radiation our knowledge is more limited on the effects of UV-A (315-400 nm)
radiation. The intensity of this spectral range in the natural sunlight is at least 10 times
higher than that of UV-B, and its penetration to the Earth is not attenuated significantly
by the ozone layer or other components of the atmosphere 29 . Thus, the damaging effects
of UV-A radiation could be highly significant 30 .
UV-A radiation has been shown to damage PSII to a considerably larger extent
than PSI 31 . Within PSII, the slow rise of variable chlorophyll fluorescence together with
the modification of the oscillatory pattern of flash-induced oxygen evolution indicates a
damage of PSII donor side components 31 . This conclusion is supported by the
appearance of a fast decaying phase of flash-induced chlorophyll fluorescence, which
arises from the recombination of Q A - with intermediate electron donors, such as Tyr-Z + ,
which are stabilized by the inactivation of the water-oxidizing complex. Further support
for the primary effect of UV-A on the water-oxidizing complex is provided by low
temperature EPR measurements. As Figure 5 shows, the so-called multiline signal,
100
SII slow
80
Fe 2+ Q A
-
60
40
S2-multiline
20
Ox. rate
0
0
30
60
90
120
150
180
UV-A irradiation time (min)
Figure 5. The effect of UV-A radiation on the functioning of PSII redox components. Isolated
PSII membrane particles were irradiated with UV-A. The rate of oxygen evolution is compared with
the amplitude of EPR signals arising from the S 2 state of the water-oxidizing complex (S 2 -multiline),
the reduced Q A acceptor (Q A - Fe 2+ ) and the oxidized Tyrosine-D donor (Tyr-D + ).
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