STRESS RESPONSES - Cyanobacteria - page 713

derived from such cultures, rehydrated and desiccated materials was tested for the amplifi cation of
10 gene loci. The genomic DNA existed in a covalently modifi ed state and required pre-treatment
with N-phenacylthiazolium bromide for amplifi cation of gene loci (a reagent that is generally used
to cleave DNA- and protein-linked advanced glycosylation end-products). In vitro desiccation of
covalently closed circular DNA resulted in the loss of supercoiling, aggregation, transformation
and transfection effi ciency but these effects were minimized in presence of trehalose. It is expected
that trehalose present in the Nostoc colonies provides the required milieu for protecting the cellular
components including DNA from oxidative damage and degradation. The mechanism of damage
due to desiccation and rehydration is mainly due to the production of ROS and the consequent
oxidative stress induced in the model organisms under study. These have already been highlighted
within the preceding section on oxidative stress.
ii) Physiological studies: During dehydration, photosynthetic systems undergo changes leading
to cessation in photosynthetic activity. This is refl ected at the level of (i) dissipation of absorbed
light energy as heat, (ii) deactivation of PSI and PSII activities, (iii) delay in quenching of PSII
fl uoresecence, and (iv) inhibition of energy transfer from phycobilisomes to the anchor protein and
the anchor protein to the PSII core complex. Inhibition of photosynthetic activity, in presence of high
light intensity and temperature, leads to the formation of ROS and oxidative stress is inevitable. So
it must be pretty harsh and very critical to survive under such conditions.
Most of the physiological studies conducted relate to understanding the recovery of
photosynthesis, respiration and nitrogen fi xation during rehydration. Photosynthetic rates of four
strains of Chroococcidiopsis and one Chroococcus (all strains isolated from arid desert rocks) decreased
with decreasing water potential caused by matric treatment when compared to osmotic control
method. This was confi rmed by 14 CO 2 -uptake studies (Potts and Friedmann, 1981). The effects of
rehydration on the recovery of photosynthesis in Scytonema geitleri (growing on roof tops of buildings)
indicated that photochemical reactions of photosynthesis recover fi rst followed by increase in ATP
pool size. Subsequently, CO 2 fi xation and nitrogen fi xation recovered. A reversal in the sensitivity
of the processes was noted during dehydration (Tiwari and Tripathi, 1998). These results were
confi rmed by an assay of fl uorescence measurements, 14 C-fi xation, O 2 evolution/consumption
and dichlorophenolindophenol reduction in S . geitleri where osmotic water potential (0 MPa) in
presence of sucrose during rehydration showed more recovery of PSI than PSII than with matric
treatments (Tiwari and Tripathi, 2001). The recovery of photosynthetic activity of N . commune during
rehydration very much corresponded with the rate of absorption kinetics of water. This very much
depended on the amount and structure of sheath and the extent of slime present outside the cells.
The absorption kinetics of water uptake (at 1 min, 2 h and 9 h) by desiccated colonies of N . commune
during rehydration was determined and the maximum absorption of water by a colony was 23-fold
its initial dry weight after 26 h in dark. The fl uorescence of phycobiliproteins and PSI complex were
recovered within 1 min whereas it took 2 min for the recovery of PSI and cyclic electron fl ow around
PSI. But the recovery of PSII occurred after a lag of 5 min and that too in two phases with half-life times
of about 20 min and 2 h. Carbon fi xation corresponded with the fi rst phase of PSII recovery. Although
the colonies absorbed 20 times water of their initial dry weight, water equivalent to twice to its initial
dry weight was suffi cient to maintain PSII activity (Satoh et al ., 2002). Activation of photosynthesis
in the rehydrated sand crusts from Negev Desert, Isreal (mainly consisting of Microcoleus sp.) took
place within less than 5 min which did not require de novo protein synthesis. During this time, over
50% of PSII activity, assembly of phycobilisomes and PSI antennae could be recovered. Another
remarkable feature is that the soil crust algae did not show photoinhibition, a property generally