STRESS RESPONSES - Cyanobacteria - page 715

cAMP regulates respiratory levels during rehydration in such a manner that the damages caused
by oxidative stress are kept at minimum.
Cyanobacteria growing beneath pieces of gypsum rock, mainly composed of Chroococcidiopsis
sp., after 2-3 days of rehydration exhibited nitrogenase activity aerobically in light. Chroococcidiopsis
sp. strain PCC 7203, on the other hand, showed nitrogenase activity under microaerophilic and low
light conditions. The presence of molybdenum nitrogenase 2 was confi rmed by sequencing of nifH
gene. It is suggested that the gypsum shards provide a good microhabitat for Chroococcidiopsis to fi x
nitrogen under low light and oxygen tensions (Boison et al ., 2004).
iii) EPS and its role in desiccation tolerance: Two types of cyanobacterial EPS can be recognized
viz., the EPS associated with cell surface and the EPS released into the external medium and known
as released polysaccharides (RPS). The EPS on cell surface are circumscribed in the form of sheaths,
capsules and slimes depending on the thickness, consistency and appearance (De Philippis and
Vincenzini, 1998, 2003). While the sheaths and capsules generally refl ect the shape of the organism,
the slimes form a dispersed material around the organism and do not refl ect the shape of cells.
Studies on chemical composition of EPS revealed that it can be composed of homoplysachharides
or heteropolysaccharides. The number of monosaccharides in the latter range from six to twelve
in nearly 75% of the cyanobacterial EPS. The hexoses (glucose, galactose, mannose and fructose),
pentoses (ribose, xylose and arabinose), deoxyhexoses (fucose, rhamnose and methylrhamnose) and
the acidic hexoses (glucoronic and galacturonic acids) constitute the subunits. Besides these, the
presence of acetylated aminosugars and other non-carbohydrate constituents such as phosphate,
lactate, acetate, sulphate and glycerol groups has been demonstrated in the cyanobacterial EPS.
The presence of sulphate and uronic acids contribute to the overall negative charge of the EPS and
makes it 'sticky' in nature (Pereiro et al ., 2009). Chi et al . (2007) for the fi rst time reported the presence
of a homopolysaccharide consisting of a single monosaccharide (α D-1,6-glucose) in Cyanothece sp.
113, a marine, diazotrophic cyanobacterium. In two strains of Synechocystis , the presence of large
number of monosaccharides (11 to 12) has been reported. Synechocystis sp. strain PCC 6803 showed
the presence of xylose, fucose, rhamnose, 3-O-methyl-deoxyhexose, mannose, glucose, galactose,
4-O-methylhexose and methylhexose in its EPS whereas Synechocystis sp. strain PCC 6714 contained
arabinose and 3-O-methylpentose in addition to those present in the former strain. Uronic acids
(16.4-16.7%), glucosamine and galactosamine (10-15%) and proteins (20-40% w/w) and sulphated
residues are also present (Panoff et al . 1988).
The EPS of N . commune is a high molecular weight substance that accumulates to more than 60%
of the dry weight of the colonies (Hill et al ., 1999).The biochemical and structural properties of the
sheath of N . commune were examined in the fi eld material as well as its isolate in cultures ( N . commune
DRH1). The sheath around the colonies possessed a pellicle consisting of Ca/Si rich material that
acts as a barrier to the epiphytes. There appears to be a variation in the number of monosaccharides
present in the EPS of species of Nostoc as for example three monosaccharide subunits ( N . fl agelliforme
a desert species of China, Huang et al ., 1998), 6 monosaccharide subunits (freshwater species of
N . commune from China, Brüll et al ., 2000) and seven monosaccharide subunits ( N . commune from
mountain area, Huang et al ., 1998; N . commune DIH1 from deserts of Mongolia, Helm et al ., 2000)
have been reported in literature. Huang et al . (1998) compared hot-water soluble polysaccharides
in the sheath material of fi eld samples as well as RPS of laboratory grown cultures of N . commune ,
N . flagelliforme and N . sphaeroides . The sheath of the field samples consisted of three main
monosacharides glucose, xylose and galactose in the ratio of 2:1:1. The presence of mannose to
varying levels in the three species and of arabinose only in N . fl agelliforme is signifi cant. But the