STRESS RESPONSES - Cyanobacteria - page 618

recovery (86-92% of the initial level after 4 h). In the second phase, the decrease in PSII activity was
directly proportional to the intensity of light without any decrease in the dark. However, the changes
in O 2 evolution activity were independent of light intensity during both phases (Lu and Zhang,
2000). Cytochrome c oxidase ( ctaI )-defective mutant of Synechocystis sp. strain PCC 6803 showed a
substantial decrease in photosynthesis due to reduction in photochemical effi ciency of PSII and of
the chlorophyll in the reaction centre of the photooxidizable form of PSI (Ryu et al ., 2003). Incubation
of S . platensis cells in sodium chloride (0.8 M) under moderate light intensity caused a decrease in
PSII mediated oxygen evolution activity and increase in PSI activity along with increase of P700.
Western blot analysis revealed a 40% loss in the thylakoid membrane protein D1 in the PSII reaction
centre. SDS-PAGE analysis of cell-free extracts of cells treated with NaCl also showed alterations in
thylakoid membrane proteins. Of these a 47-kDa chlorophyll protein and 94-kDa protein showed
a decline in their amounts with an increase in 17-kDa protein. The diminishing of the former two
proteins was associated with a decreased energy transfer from light-harvesting antenna to PSII in
the extracts as well as alterations in chlorophyll fl uorescence of whole cells and isolated thylakoids
(Sudhir et al ., 2005). This has been further confi rmed by Zhang et al. (2010) who reported a decrease
in the excitation energy transfer from phycobilisomes to PSII with a concomitant increase in PSI
electron transport activity in the salt-stressed cells of S . platensis . Salt stress in the same organism not
only inhibited PSII activity but also caused alteration in the proteins of thylakoid membrane. The
PsbO protein got dissociated and accumulated in the soluble fraction. This in turn resulted in the
modifi cation of Q B niche from the acceptor side to an increase at the donor side (Gong et al ., 2008).
An osmotic stress protein of Anabaena sp. strain PCC 7120 that is immunologically related
to plant dehydrins has been identifi ed after an osmotic upshift due to the presence of sucrose,
sorbitol and polyethylene glycol. This protein is a 40 kD osmotic-stress-induced protein when
osmotic potential shift of 0.5 MPa (sucrose or PEG) or 1.2 MPa (sorbitol) was present in the external
medium. The synthesis of plant dehydrin-like polypeptides was also noted in Calothrix sp. strain
PCC 7601 and in Nostoc sp. strain PCC 7911 (Mac-R2) (Close and Lammers, 1993). The involvement
of water channels in osmotic stress due to the presence of sorbitol (1.0 M) has been indicated in
Synechocystis sp. strain PCC 6803 and the decrease in the cytoplasmic space by 50% suggested that
water loss from cells constituted the primary event. Secondly, the O 2 evolution rate was reduced by
50% of the original level in presence of benzoquinone and DCBQ (an artifi cial electron acceptor).
The transport of electrons by PSI from reduced DCPIP to MV decresed to 70% of the original level.
This emphasizes that the activities of both PSII and PSI have declined in intact cells. The presence
of p-chloromercuriphenyl-sulfonic acid (at100 µM), a water channel blocker, prevented the effl ux of
water and suppressed the inactivation of PSI and PSII and protected the cells against osmotic-stress
induced by sorbitol. Further the inactivation of both PSII and PSI is suggested to be due to an increase
in intrathylakoid concentration of K + ions that may have caused a dissociation of plastocyanin or
cytochrome c-553 from the PSI complex. The release of osmotic stress caused an infl ow of water
through water channels with a simultaneous effl ux of K + ions and restoration of activities of PSI
and PSII (Allakhverdiev et al ., 2000b).
ii) Compatible solutes : The synthesis of compatible solutes is one of the mechanisms adopted by
cyanobacteria in overcoming salt and osmotic stress (Empadinhas and da Costa, 2008; Klähn and
Hagemann, 2011). In a comprehensive study on carbohydrate accumulation during salt stress,
Reed et al . (1986) tested 71 strains of cyanobacteria from freshwater and marine environments.
Of the 22 marine isolates, 41% accumulated glucosylglycerol [(2-O-α-D-glucopyranosyl)-glycerol;
GG; principal among them being Spirulina subsala , Synechococcus sp. (4 strains)], 36% accumulated