Environmental Engineering Reference
In-Depth Information
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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