STRESS RESPONSES - Cyanobacteria - page 612

and β-glutamate, ectoine, hydroxyectoine, trehalose, glycine betaine, glucosylglycerol and proline
(Fig. 2). Hence increasing the concentration of trehalose, proline and other neutral molecules does
not hurt enzymes much whereas increasing concentration of ions such as K + does. Glycine betaine
(and to a lesser extent proline and trehalose) are not merely just “extra” osmotic pressure, they
are actual osmoprotectants and positively counteract the effect of increased ionic strength. For
E. coli growing in a typical mineral salts medium plus glucose, the internal osmolality (or OP) is ~300
milli-osmolar. The pressure outwards on the cell wall is approximately 3.5 atmospheres. E.coli can
adapt to grow in media containing up to about 4% NaCl if provided with osmoprotectants. Marine
bacteria live at much greater osmotic upshifts than this.
All the mesophilic prokaryotes generally accumulate the above compatible solutes under various
stress conditions. In contrast, thermophilic and hyperthermophilic organisms generally accumulate
very unusual compatible solutes such as di-myo-inositol-1-1-phosphate, di-mannosyl-di-myoinositol-
1-1-phosphate, di-glycerol phosphate, mannosylglycerate and mannosylglycerimide (Fig. 3). The
accumulation of these unusual compatible solutes has not been encountered in organisms inhabiting
low or moderate temperatures. The protective roles of mannoglycerate and di-glycerol phosphate
during thermal denaturation of enzymes in vitro is another interesting point that goes in their
favour to confer protection from high temperature related metabolic adjustments (Santos and da
Costa, 2002). The osmoadaptive mechanisms of prokaryotes have been summarized (Empadinhas
and da Costa, 2008). Two types of strategies have been envisaged. Of these, the fi rst one known as
“salt-in” strategy involves an infl ux of ions from the surrounding environment. This strategy seems
to be restricted to the extreme halophilic Archaea such as Halomicrobium , Haloarcula , Haloquadratum ,
Halorhabdus , Natronobacterium and Natronococcus (all belonging to the family Halobacteriaceae), the
halophilic bacteria of the order Haloanaerobiales and the bacterium Salinibacter . The major ion that
is accumulated by these bacteria is K + . The second strategy involves accumulation of low molecular
weight organic compatible solutes. Large majority of the microorganisms rely on this strategy to
overcome different stresses and this constitutes a versatile mode of osmotic adaptation in an otherwise
osmotically changing environment.
I. SALT STRESS
Knowledge that has emanated from the studies on salt stress in cyanobacteria can be studied under
the following heads: (1) Biochenmical and physiological studies (Reed et al ., 1986; Schubert et al ., 1993;
Jeanjean et al ., 1993; Hagemann et al ., 1994, 1999; Marin et al ., 2002; Singh et al ., 2002; Ferjani et al .,
2003); (2) Synthesis of compatible solutes; (3) Na + /H + antiporters; (4) Salt intake and cell signalling
(Elanskaya et al ., 2002; Shoumskaya et al ., 2005); (5) Gene level responses that deal with salt stress
tolerance; (6) Genome- and proteome-based studies (Hagemann et al ., 1991; Fulda et al ., 2000, 2006;
Huang et al ., 2006) including those of microarray analysis of genes and their products (Kanesaki et
al ., 2002; Marin et al ., 2004; Hagemann, 2011). These are presented below.
i) Biochemical and physiological studies : The requirement of sodium for cyanobacterial growth
at high pH (Allen and Arnon, 1955; Espie et al ., 1988), uptake of several inorganic nutrients such
as inorganic carbon, nitrate, phosphate (Lara et al. , 1993; Avendaño and Fernández.-valiente, 1994)
and photosynthetic electron transport at the O 2 evolving complex (Zhao and Brand, 1988) has
been reported. Freshwater cyanobacteria tolerate low levels of sodium chloride (0.5 M) whereas
cyanobacteria from hypersaline environments can tolerate up to concentrations as high as 3 M.
Apte et al . (1987) examined the relationship between sodium uptake and salt tolerance in Anabaena