626
K. M. G. Mostofa et al.
photosynthesis in aquatic organisms. Further studies will be needed to evaluate the exact mechanism behind this phenomenon. The formation of complexes
between metals and the functional groups of either PSII or PSI is a relatively
new hypothesis, which could greatly help improving the present understanding of the effects of metal ions on the photosynthetic efficiency of aquatic
organisms.
5.8 Effect of Salinity or Salt Stress
Salinity is an important controlling factor for photosynthesis, its effect depending
on the organisms such as higher plants or aquatic microorganisms. Therefore, the
effect of salinity can be divided into two classes: (i) effect on aquatic microorganisms; (ii) effect on higher plants.
Effects of Salinity on Aquatic Microorganisms
Water salinity or salt stress has a significant impact on the photosynthetic capability of organisms, but the actual effect is highly dependent on the different
kinds of microorganisms (Liu et al. 2007; Satoh et al. 1983; Ahel et al. 1996;
Moisander et al. 2002; Marcarelli et al. 2006; Segal et al. 2006; Demetriou et al.
2007; Allakhverdiev and Murata 2008). It has been shown that salinity in marine
waters can alter the PSI and PSII of organisms, which is connected to salt stress.
However, some organisms such as cyanobacteria or phytoplankton can overcome the salt stress and are capable of growing under salinity conditions which
would be harmful to most other organisms. The basic physiological responses of
cyanobacterial cells to salt stress occur in three phases (Hagemann and Erdmann
1997). First, within seconds an increase of the ambient concentration of NaCl
can cause an influx of Na + and Cl − ions into the cytoplasm. Second, within an
hour it starts the replacement of Na + by K + ions, leading to a decline in the toxic
effects of high Na + concentrations. Third, within several hours the cells become
acclimatized to the elevated ion concentrations. During this phase, the synthesis
or uptake of compatible solutes/components mitigates the toxic effects of salts
and preserves the structures of complex proteins and cell membranes (Bhagwat
and Apte 1989; Reed et al. 1985; Hagemann et al. 1990, 1991; Hayashi and
Murata 1998; Chen and Murata 2002). The cyclic electron transport via PSI is
also activated (Joset et al. 1996; Hibino et al. 1996). However, over a longer
period of time, such as within several days, salt stress can inhibit cell division
(Ferjani et al. 2003).
The increase in the intracellular concentrations of Na + and Cl – ions can cause
irreversible inactivation of the oxygen-evolving machinery and of the electrontransport activity of PSI (Kuwabara and Murata 1983; Miyao and Murata 1983;
Murata and Miyao 1985; Allakhverdiev et al. 2000a, b). For instance, incubation
K. M. G. Mostofa et al.
photosynthesis in aquatic organisms. Further studies will be needed to evaluate the exact mechanism behind this phenomenon. The formation of complexes
between metals and the functional groups of either PSII or PSI is a relatively
new hypothesis, which could greatly help improving the present understanding of the effects of metal ions on the photosynthetic efficiency of aquatic
organisms.
5.8 Effect of Salinity or Salt Stress
Salinity is an important controlling factor for photosynthesis, its effect depending
on the organisms such as higher plants or aquatic microorganisms. Therefore, the
effect of salinity can be divided into two classes: (i) effect on aquatic microorganisms; (ii) effect on higher plants.
Effects of Salinity on Aquatic Microorganisms
Water salinity or salt stress has a significant impact on the photosynthetic capability of organisms, but the actual effect is highly dependent on the different
kinds of microorganisms (Liu et al. 2007; Satoh et al. 1983; Ahel et al. 1996;
Moisander et al. 2002; Marcarelli et al. 2006; Segal et al. 2006; Demetriou et al.
2007; Allakhverdiev and Murata 2008). It has been shown that salinity in marine
waters can alter the PSI and PSII of organisms, which is connected to salt stress.
However, some organisms such as cyanobacteria or phytoplankton can overcome the salt stress and are capable of growing under salinity conditions which
would be harmful to most other organisms. The basic physiological responses of
cyanobacterial cells to salt stress occur in three phases (Hagemann and Erdmann
1997). First, within seconds an increase of the ambient concentration of NaCl
can cause an influx of Na + and Cl − ions into the cytoplasm. Second, within an
hour it starts the replacement of Na + by K + ions, leading to a decline in the toxic
effects of high Na + concentrations. Third, within several hours the cells become
acclimatized to the elevated ion concentrations. During this phase, the synthesis
or uptake of compatible solutes/components mitigates the toxic effects of salts
and preserves the structures of complex proteins and cell membranes (Bhagwat
and Apte 1989; Reed et al. 1985; Hagemann et al. 1990, 1991; Hayashi and
Murata 1998; Chen and Murata 2002). The cyclic electron transport via PSI is
also activated (Joset et al. 1996; Hibino et al. 1996). However, over a longer
period of time, such as within several days, salt stress can inhibit cell division
(Ferjani et al. 2003).
The increase in the intracellular concentrations of Na + and Cl – ions can cause
irreversible inactivation of the oxygen-evolving machinery and of the electrontransport activity of PSI (Kuwabara and Murata 1983; Miyao and Murata 1983;
Murata and Miyao 1985; Allakhverdiev et al. 2000a, b). For instance, incubation