625
Photosynthesis in Nature: A New Look
the iron-sulfur cluster Fe-S A /Fe-S B of PS1. This process could induce the production of superoxide radical (O 2
•−
) and H 2 O 2 (Fujii et al. 1990). When catalytic free
metals [e.g., Fe(II), Cu(I) and Mn(II)] are present near the production site of O 2
•−
and H 2 O 2 , the strong oxidizing agent OH
• can be produced via a Fenton reaction
or by direct photodissociation (Zepp et al. 1992; Nakatani et al. 2007; Zellner
et al. 1990; Goldstein and Rabani 2008; Halliwell and Gutteridge 1984; Stadtman
1993). The photo-Fenton reaction could proceed as follows (Eqs. 5.31, 5.32):
The HO
• radical thus generated kills the cells of microorganisms in natural
waters. Fe concentration and pH can significantly affect both the growth and the
reactive oxygen species (ROS) production in Chattonella marina, a harmful algal
bloom species (Liu et al. 2007). The rapid photoinduced release of electrons from
the outer shell of metal ions in PSII metal-protein complexes can be involved in
chronic toxicity. It has recently been shown that exposure of PSII to Sb 3+ and
Cr 4+ in Synechocystis sp. can increase the dissipated energy flux and decrease the
performance index and the maximum quantum yield for primary photochemistry (ϕP O ) (Zhang et al. 2010; Pan et al. 2009). It can also cause damage to cellular components and to the overall photosynthetic driving force. The fluorescence
yield at 684 nm, which is connected to the maximum quantum yield for primary
photochemistry (Zhang et al. 2010) is affected by the metal-functional groups of
PSII. It can be the easiest way by which electrons are released, which can subsequently result into high production of HO
• via O 2
•− or H 2 O 2 . Such a process
can decressee photosynthetic efficiency or damage the PSII or DNA. Along with
this effect, complexation between metal ions and the functional groups of PSII
can inhibit electron transport and cause the overall photosynthetic efficiency
to decline. Severe damage in the water-splitting site of PSII can produce an
increased ratio of F O ∕ F V (fluorescence level before and after flash) (Pan et al.
2009), which may occur because of HO
• photoproduction. Note that Sb 3+ compounds are approximately ten times more toxic than Sb 5+ ones, possibly because
of the two unpaired electrons in Sb 3+ species. In contrast, Sb 5+ species have no
outer shell electrons.
Another possible effect linked with complex formation between metals and the
functional groups bound to PSII is the blockage of the normal function of electron
release by PSII-bound functional groups. This can also significantly decrease the
photosynthetic efficiency of aquatic organisms.
The stimulating effect of Cu 2+ (1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 8 ) on PSII is presumably caused by the partially and completely filled outer shell of its s- and d-orbitals. Therefore, the complexes formed between Cu 2+ and the PSII functional
groups are not as strong as those involving other metal ions with empty orbitals. This effect can roughly recover the normal function of PSII in organisms.
In addition, the electrons released from the metal-protein complexes of PSII can
induce the generation of relatively low amounts of H 2 O 2 , which might enhance
(5.31)
Fe
3+ + hυ/H 2 O
• /O 2
•− → Fe
2+ + O 2 + H
+
(5.32)
Fe
2+ + H 2 O 2 + hυ → Fe
3+ + HO
• + OH
−
Photosynthesis in Nature: A New Look
the iron-sulfur cluster Fe-S A /Fe-S B of PS1. This process could induce the production of superoxide radical (O 2
•−
) and H 2 O 2 (Fujii et al. 1990). When catalytic free
metals [e.g., Fe(II), Cu(I) and Mn(II)] are present near the production site of O 2
•−
and H 2 O 2 , the strong oxidizing agent OH
• can be produced via a Fenton reaction
or by direct photodissociation (Zepp et al. 1992; Nakatani et al. 2007; Zellner
et al. 1990; Goldstein and Rabani 2008; Halliwell and Gutteridge 1984; Stadtman
1993). The photo-Fenton reaction could proceed as follows (Eqs. 5.31, 5.32):
The HO
• radical thus generated kills the cells of microorganisms in natural
waters. Fe concentration and pH can significantly affect both the growth and the
reactive oxygen species (ROS) production in Chattonella marina, a harmful algal
bloom species (Liu et al. 2007). The rapid photoinduced release of electrons from
the outer shell of metal ions in PSII metal-protein complexes can be involved in
chronic toxicity. It has recently been shown that exposure of PSII to Sb 3+ and
Cr 4+ in Synechocystis sp. can increase the dissipated energy flux and decrease the
performance index and the maximum quantum yield for primary photochemistry (ϕP O ) (Zhang et al. 2010; Pan et al. 2009). It can also cause damage to cellular components and to the overall photosynthetic driving force. The fluorescence
yield at 684 nm, which is connected to the maximum quantum yield for primary
photochemistry (Zhang et al. 2010) is affected by the metal-functional groups of
PSII. It can be the easiest way by which electrons are released, which can subsequently result into high production of HO
• via O 2
•− or H 2 O 2 . Such a process
can decressee photosynthetic efficiency or damage the PSII or DNA. Along with
this effect, complexation between metal ions and the functional groups of PSII
can inhibit electron transport and cause the overall photosynthetic efficiency
to decline. Severe damage in the water-splitting site of PSII can produce an
increased ratio of F O ∕ F V (fluorescence level before and after flash) (Pan et al.
2009), which may occur because of HO
• photoproduction. Note that Sb 3+ compounds are approximately ten times more toxic than Sb 5+ ones, possibly because
of the two unpaired electrons in Sb 3+ species. In contrast, Sb 5+ species have no
outer shell electrons.
Another possible effect linked with complex formation between metals and the
functional groups bound to PSII is the blockage of the normal function of electron
release by PSII-bound functional groups. This can also significantly decrease the
photosynthetic efficiency of aquatic organisms.
The stimulating effect of Cu 2+ (1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 8 ) on PSII is presumably caused by the partially and completely filled outer shell of its s- and d-orbitals. Therefore, the complexes formed between Cu 2+ and the PSII functional
groups are not as strong as those involving other metal ions with empty orbitals. This effect can roughly recover the normal function of PSII in organisms.
In addition, the electrons released from the metal-protein complexes of PSII can
induce the generation of relatively low amounts of H 2 O 2 , which might enhance
(5.31)
Fe
3+ + hυ/H 2 O
• /O 2
•− → Fe
2+ + O 2 + H
+
(5.32)
Fe
2+ + H 2 O 2 + hυ → Fe
3+ + HO
• + OH
−