Photosynthesis in Nature: A New Look - Photobiogeochemistry of Organic Matter - page 624

of Synechococcus cells with 0.5 M NaCl can suppress the reduction of P700 +
(Allakhverdiev et al. 2000a ). Because P700 + is reduced by plastocyanin, it is sug-
gested that the association of this compound with the PSI complex is disturbed by
the presence of NaCl (Allakhverdiev et al. 2000a , b ).
In cyanobacteria, the oxygen-evolving machinery of PSII located on the
luminal side of thylakoid membranes is stabilized by three extrinsic pro-
teins. They are PsbO (33-kD protein), PsbV (cytochrome c 550 ), and PsbU
(Allakhverdiev and Murata 2008 ; Shen et al. 1998 ; Nishiyama et al. 1999 ). Cyt
c 550 and PsbU are loosely bound to the donor side of the core complex of PSII
(Nishiyama et al. 1997 , 1999 ). These proteins could be easily dissociated from
the cyanobacterial PSII complex in the presence of elevated concentrations of
NaCl (Shen et al. 1998 , 1992 ). Moreover, pulse-chase experiments revealed that
salt stress can inhibit the de novo synthesis of D1 in Synechococcus (Ohnishi
and Murata 2006 ).
Light is an important factor in restoring the activity of PSII and PSI during dark
incubation of cyanobacterial cells under salt stress (Allakhverdiev et al. 2005 ). When
light is applied to Synechococcus cells, protein synthesis occurs for the recovery of
the photosystems from salt stress (Allakhverdiev and Murata 2008 ; Hagemann et al.
1991 ; Allakhverdiev et al. 1999 , 2005 ). Weak light at 70 mE m − 2 s − 1 is sufficient to
generate ATP, which seems to support recovery (Allakhverdiev and Murata 2008 ).
Such conditions are sufficient to induce the necessary excitation, because of the for-
mation of complexes between cations (e.g. Na + and other cations from salts) and the
functional groups bound to PSII and PSI. Recent studies of PSII photoinhibition in
cyanobacteria suggest that oxidative stress due to reactive oxygen species (ROS) can
inhibit protein synthesis and the repair of PSII. However, it does not stimulate pho-
todamage to PSII (Nishiyama et al. 2005 , 2006 ; Takahashi and Murata 2008 ; Murata
et al. 2007 ). Note that salinity in marine waters is accounted for various salts includ-
ing NaCl (86 %), but comparison of river and sea water shows that Na + , Ca 2 + ,
Mg 2 + , K + , HCO 3 , Cl − and SO 4 2 − in the sea are typically 1,670, 27, 330, 170, 2.4,

2,400 and 245 times, respectively, higher than in rivers (see chapter “ Complexation
of Dissolved Organic Matter With Trace Metal Ions in Natural Waters ” for more
discussion). Also note that the occurrence of these salts can cause changes in the
absorption properties of chromophoric dissolved organic matter (CDOM), and in the
fluorescence properties of fluorescent dissolved organic matter (FDOM). A change
in the optical properties (generally shifting from shorter towards longer wave-
lengths) and in the complexation behavior of both CDOM and FDOM can be linked
to an enhanced photoinduced generation of H 2 O 2 (see chapters “ Photoinduced and
Microbial Generation of Hydrogen Peroxide and Organic Peroxides in Natural
Waters ” “ Colored and Chromophoric Dissolved Organic Matter in Natural Waters ”,
“ Fluorescent Dissolved Organic Matter in Natural Waters ”, and “ Complexation of
Dissolved Organic Matter With Trace Metal Ions in Natural Waters ” , respectively for
their detailed discussion).
A proposed mechanism for the decline of photosynthesis of microorganisms
is that cations (e.g. Na + , Ca 2 + , Mg 2 + , Sr 2 + ) of various salts occurring in marine
waters can form complexes with functional groups bound to microorganisms (or

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