STRESS RESPONSES - Cyanobacteria - page 717

This has been confi rmed by Wright et al . (2005) who cloned the wspA gene that existed as a single
copy in a monocistronic operon in the genome of N . commune DRH1. The transcription of the genes
wspA and sodF and the synthesis and release of WspA into EPS have been induced upon desiccation
or UV-A/B irradiation. The binding of the purifi ed WspA protein, expressed in E . coli, to the UV-A/B
absorbing pigment scytonemin emphasizes the predicted role of WspA in global stress response of
N . commune . Morsay et al . (2008) identifi ed two novel glycosidases in the EPS of N . commune . One
of these proteins (20 kDa) with β-D-glucosidase activity has been identifi ed as a cyanobacterial
fasciclin proteins secreted on the cell surface of N . punctiforme PCC 73102, A . variabilis ATCC 29413
and Anabaena sp. strain PCC 7120. The second major water-soluble protein (36 kDa) was found to
be identical to the Wsp that showed heat-resistant β-D-glucosidase activity. The presence of these
two glycosidases has been suggested to dissolve the EPS material facilitating the dispersal of the
fi laments. The existence of stable SodF mRNA in abundance in the desiccated cells for prolonged
periods (13 years) and its turnover within 15 min of rehydration followed by the release of Fe-SOD
enzyme into the surrounding medium enabled N . muscorum to tolerate oxidative stress contributed
by rehydration as well as UV-A/B exposure.
In agreement with the idea that desiccation-tolerant cyanobacteria also exhibit freezing-
tolerance, desiccation-tolerant N . commune resisted freezing stress whereas desiccation-sensitive
Synechocystis sp. strain PCC 6803 and Fischerella muscicola could not tolerate freezing-stress. This
mostly depended on the ability of N . commune to convert the excess light energy absorbed by it
to heat and the protection of the cells from photoinactivation (Lin et al ., 2004). The role of EPS in
conferring desiccation and freezing tolerance was investigated by subjecting the EPS-depleted
cells of N . commune to these stresses (Tamaru et al ., 2005) and by the addition of extracted EPS of
cryptoendolithic Nostoc sp. to the cultures of Chlorella sp. and Chroococcidiopsis sp. (Knowles and
Castenholz, 2008). Desiccated colonies did not possess the ability to evolve O 2 , a property which
they quickly regained by rehydration. However, in air-dried colonies (with 10% w/w water) and
natural colonies of N . muscorum with EPS the capacity to evolve O 2 was not damaged. Removal
of EPS by mechanical means of either the naturally occurring colonies or laboratory cultures of
N . commune KU002 made them more desiccation- and heat- and freezing-sensitive (Tamaru et al .,
2005). The RPS of a cryptoendolithic Nostoc sp. CCMEE 6160 when added to the cultures of other
naturally occurring algae in the community ( Chlorella sp. CCMEE 6038 and Chroococcidiopsis sp.
CCMEE 5056) protected them from desiccation tolerance. The staining of the cultures with SYTOX
Green (a fl uorescent stain) clearly differentiated the live (appear red due to autofl uorescence) and
dead cells (appear bright green due to the fl uorescence of the dye). Protection from freezing stress
by the addition of RPS to the two test cultures was not to the extent observed in desiccation stress.
The differential staining by SYTOX Green of living and dead cells has been attributed to the integrity
of the membranes, the former cells retained the integrity of the membranes and in the latter cells
the integrity of membranes is damaged (Knowles and Castenholz, 2008).
iv) Membrane modifi cation: During dehydration as the matric water potential inside the cells
decreases, the hydration shell around the macromolecules is disturbed. At least in case of lipids the
physical state of water molecules is not well understood but the loss of the water molecules infl uences
greatly the physical state of the lipid molecules. The lipid bilayer of the cytoplasmic membrane as
well as thylakoid membranes in cyanobacteria face the change in physical state of the lipid molecules
at the time of desiccation as well as rehydration. During desiccation when water molecules around
the lipid molecules are removed the packing density of the head groups increases in the lipid bilayer.
Due to increase of packing density, there is every possibility for increase in vander waals interactions