STRESS RESPONSES - Cyanobacteria - page 714

exhibited by model organisms like Synechocystis sp. strain PCC 6803 when exposed to high light
conditions. The rate of PSII repair was dependent on the intensity of light and its duration. So this
explains how the crust organisms are able to readily adjust to the extreme environmental conditions
of temperature, high light intensity and the daily desiccation and rehydration cycles prevalent in
deserts (Harel et al ., 2004). Ohad et al . (2005) reported inactivation of photosynthetic electron fl ow
during desiccation in native biological sand crusts and those crusts cultivated on nitrocellulose fi lters
that contained fi laments of Microcoleus sp. The morphological features of the cyanobacterium were
confi rmed by scanning-transmission and atomic force microscopy. Desiccation caused a complete
loss of fl uorescence in the native crusts accompanied by a decrease in 77 k PSII fl uorescence emission
relative to that of PSI. Further, a decrease of energy transfer from phycobilisomes to PSII was noted.
The addition of trehalose (1.5 M) to the native crusts retrieved the loss of fl uorescence by 50% but
was accompanied by a decrease of 77 k PSI fl uorescence induced by chlorophyll excitation. This
is explained as a survival strategy of Microcoleus by showing alterations in energy transfer from
antennae to reaction centres. Ohad et al . (2010) monitored high light-induced changes in Mirocoleus -
dominated sand crusts maintained in the fi eld (under natural light intensity of 200 µmol photons
m -2 s -1 ) in dry and wet experimental plots. A dramatic decline in chlorophyll fl uorescence yield
(50%) was observed. As it was not possible to measure the rates of uptake of CO 2 and release of
O 2 in these natural crusts, nitrocellulose fi lter-grown Microcoleus was used to measure chlorophyll
fl uorescence at high light intensity (500 µmol photons m -2 s -1 ). The loss of chlorophyll fl uorescence
was up to 18-23% of its initial value but the decrease in O 2 evolution was marginal suggesting an
uncoupling between variable fl uorescence and the photosynthetic electron transport. Laboratory
grown liquid cultures of Microcoleus when exposed to very high light intensities (2000 µmol photons
m -2 s -1 ), the chlorophyll fl uorescence and thermoluminescence emission were reduced to 85% and
90% of their initial values but O 2 evolution, O 2 uptake and PSI activity remained unaffected. It means
that there was no inhibition of forward electron fl ow activity of Microcoleus PSII. As light-induced
variable fl uorescence was not inhibited by electron transfer inhibitors 3-(3,4-dichlorophenyl)-1,1-
dimethylurea (DCMU), 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DMMIB) or the uncoupler
carbonyl cyanide-p-trifl uoromethoxyphenylhydrazone (FCCP), it ruled out the possibility of
reduction of plastoquinone or O 2 or dissipation of excess light excitation as heat (via modulation of
energy transfer from antenna to the reaction centres, known as non-photochemical quenching). So
it was concluded that a greater back fl ow of electrons from Quinone A to phaeophytin took place,
as the temperature required for thermoluminescence emissions was quite low when compared
to other cyanobacteria used as models in photosynthesis experiments. Microcoleus thus has an
effi cient protective mechanism against light-induced oxidative stress and is able to thrive under
the harsh desert conditions. When desiccated N . fl agelliforme (for 2 years and 8 years dry storage)
was rehydrated, recovery of photosynthesis and respiration in the 2-year sample was faster as it
absorbed water much faster. But the 8-year sample failed to recover both the processes and showed
PSI activity alone (Liu et al ., 2010). A dramatic increase in cAMP levels in the cells of Anabaena sp.
strain PCC 7120 upon rehydration prompted Higo et al . (2008) to study the respiratory O 2 uptake and
photosynthetic O 2 evolution in the wild-type and the adenyl cyclase gene ( cyaC ) disruptant mutant.
During rehydration, the cyaC disruptant mutant showed enhanced O 2 uptake than wild-type which
resulted in more oxidative stress as evidenced by lipid peroxidation and protein carbonylation. The
cyaC disruptant mutant took more time for the recovery of photosynthesis as 70% of the O 2 -evolving
activity was reached after one day of rehydration. The growth of the cyaC mutant was perceptible
only when it was able to recover from the damages caused by rehydration. It was suggested that