STRESS RESPONSES - Cyanobacteria - page 607

IV. DESICCATION 705

i) Targets of desiccation damage

ii) Physiological studies

iii) EPS and its role in desiccation tolerance

iv) Membrane modifi cation

v) Trehalose synthesis

vi) MAAs

vii) The role of scytonemin

viii) Transcriptional regulation

Microorganisms grow in the environments suitable for them and make a niche for themselves.
Due to the fl uctuations in the nutritional or environmental conditions they face various adverse
situations. In order to adjust to such conditions, the microorganisms adapt to the new environment
by altering their metabolic profi le. Due to this plasticity, they are able to grow under the changing
environment without detriment to growth and development. The availability of excess nutrients
or lack of proper nutrients, light (high or low and UV-light), salinity, high solute concentrations
(osmotic stress), temperature (high and low), hydrogen ion concentration (high or low pH) and lack
of water (drought leading to desiccation) are the common abiotic stresses which microorganisms
experience in nature.
The cyanobacteria grow in various types of habitats and gain a cosmopolitan distribution.
Majority of them are aerobic photoautotrophs but some of them show the distinctive property of
heterotrophic growth and survive for long periods in darkness. They grow in all types of terrestrial
habitats where they are suggested to play important role in terrestrial ecosystems (Whitton, 1992;
Whitton and Potts, 2000). They inhabit all types of water bodies that are salty, brackish or freshwater.
The picoplanktonic cyanobacteria grow in all types of oceans from surface waters to greater depths
(even extending up to 100 m). Freshwater bodies of differing trophic states support the growth of a
number of cyanobacteria where bloom-forming and non-bloom-forming forms are prevalent. Their
ability to survive in extremely high and low temperatures enables them to grow in hot springs,
Arctic and Antarctic Lakes, snow and ice (Sabacka and Elster, 2006; Seckbach, 2007). Their growth
under such harsh conditions is commonly associated with sharing their tasks and supporting each
other by the formation of crusts or mats. These have been variously designated as “microbiotic soil
crusts” (Elridge and Greene, 1994), “biological soil crusts” (also abbreviated to biocrusts; Belnap
et al ., 2001a), “cryptogamic crusts” (Strandling et al ., 2002) and “cryptobiotic crusts” (Pócs, 2009).
The biocrusts are dominated by either cyanobacteria or lichens or mosses (Fig. 1). Their symbiotic
association from fungi to higher plants provides the best example of their existence in extremely
desiccated state (Usher et al ., 2007). This is suggestive of the fact that the cyanobacteria can cope
up with a wide spectrum of abiotic stresses such as heat and cold, desiccation, salinity, high light
intensity, nutrient starvation, anaerobiosis and the presence of high concentrations of pesticides
and heavy metals.