STRESS RESPONSES - Cyanobacteria - page 706

do so by simultaneously getting additional exposure to accompanying stress conditions such as
freezing and/or halophilic or high light intensity conditions. As all these stress conditions produce
ROS and oxidative stress is a consequence of desiccation as well as rehydration. The extent of water
present inside the desiccated prokaryotic cells, the methods of water removal (preferential exclusion
hypothesis in relation to replacement of water by the osmoprotectant substances, the differences
between drying and salting or sugaring, the sensitivities of prokaryotes to air drying, the effects of
freezing-thawing, the differences between freeze-drying and air drying) and the targets of desiccation
damage caused in prokaryotes to proteins, nucleic acids and lipids and membranes are the important
areas that received attention in studies on desiccation tolerance of prokaryotes (Potts, 1994, 1999,
2001; Potts et al ., 2005).
One of the desiccation tolerant and radioresistant bacterium isolated from irradiated meat more
than 50 years ago is Deinococcus radiodurans (Deinococcaceae; Anderson et al ., 1956). More than 30
species of this genus have now been isolated from a variety of habitats, i.e. upper sand layers of
deserts, Antarctic rocks, soil, water, faecal matter and hot springs (Ferreira et al ., 1997; de Groot et
al ., 2005; Blasius et al ., 2008). Genome sequences of D . radiodurans (White et al ., 1999), D . geothermalis
(Makarova et al ., 2007; Fig. 20 A) and D . deserti (deGroot et al ., 2009; Fig. 20 B and C) have already been
published. Like Deinococcus , among the cyanobacteria the most dominant photosynthetic organism of
the extreme arid and cold deserts is the coccoid Chroococcidiopsis (Fig. 21). Species of this genus have
been found from diverse extreme habitats such as thermal springs (Vasishta, 1968; Anagnostidis and
Pantazidou, 1988) , shady caves (Friedmann, 1961), rocks, walls and paddy fi elds (Tiwari, 1972), air
spaces of porous rocks or underneath rock-soil surfaces of hot deserts (Stolz, 1990; Grilli-Caiola et al .,
1993; Billi et al. , 2001; Boison et al ., 2004; Wierzchos et al ., 2006) or ice-free Ross Deserts of Antarctica
( Friedmann et al ., 1988; Friedmann, 1993; Nienow and Friedman, 1993) and hypersaline ponds and
lakes (Dor and Paz, 1989; Dor et al ., 1991; Řezanka et al ., 2003). Endolithic cyanobacteria inhabiting
harsh environments of cold and hot deserts are generally dominated by species of Chroococcidiopsis ,
Myxosarcina and Gloeocapsa (van Thielen and Garbary, 1999; Whitton and Potts, 2000; de la Torre et
al ., 2003). However, endolithic cyanobacteria inhabiting non-desert environments like limestone of
Niagara Escarpment and travertine deposits in Yellowstone National Park are dominated by both
fi lamentous and unicellular types such as Leptolyngbya , Nostoc and Synechocystis (Gerrath et al .,
1995, 2000). Molecular characterization of endolithic cyanobacteria inhabiting exposed dolomite
rocks in central Switzerland (accession No. AY153458) led to the identifi cation of the organism as
Chroococcidiosps sp. (91% identity; Sigler et al ., 2003). Endolithic biofi lms colonizing Antarctic granite
rocks also showed the presence of Chroococcidiopsis -like organism bearing resemblance (98% identity)
to AY153458 (de los Ríos et al ., 2007). Cryptoendolithic cyanobacteria dominated by species of
Chroococcidiopsis have been reported to cause chemical weathering in sandstones by bioalkalization in
different parts of S. Africa, Australia, N. America and S. America (Büdel et al ., 2004). Chroococcidiopsis
is morphologically simple but exhibits wide variability. It is a unique ancient type that resembles
certain Proterozoic microfossils (Friedmann and Ocampo-Friedmann, 1995).
Ultrastructural studies on desiccated Chroococcidiopsis revealed the presence of multilayered
envelopes and the ability to tolerate desiccation by this organism was correlated with the synthesis
of these multilayered envelopes of extracellular polysaccharides (EPS) and production of spores.
Moreover, the amounts of acid-, sulphate- and beta-linked polysaccharides increased in response to
desiccation (Grilli-Caiola, 1993; 1996). Phylogenetic analysis of the desert strains of Chroococcidiopsis
based on 16S rRNA suggested that four strains (057, 123, 171 and 029) were distinct from the type
species Chroococcidiopsis thermalis PCC 7203 and one of the strains (029, from Negev Desert, Israel)
has been identifi ed as suitable organism for genetic studies on desiccation tolerance (Billi et al ., 2001).