STRESS RESPONSES - Cyanobacteria - page 705

IV. DESICCATION
Desiccation can be defi ned as the ability to lose water (or dry up) to equilibrium with air and remain
in a state of suspended metabolism for a very long time. Upon rehydration (or absorption of water)
the cells regain the functions of active metabolism. The ability to tolerate desiccation is met within
different groups of plants and animals. Among plants, bryophytes, pteridophytes and certain
angiosperms are capable to lead a desiccated life. However, the most important stages are represented
by the seeds, spores and pollen grains. Important animal species belonging to nematodes, rotifers and
tardigrates exhibit desiccation tolerance. Desiccation can be complete or partial. Complete desiccation
refers to drying to complete air dryness without any metabolism whereas during partial desiccation
basal metabolism is still maintained. The term “anhydrobiosis” was coined to represent the state of
complete desiccation (Crowe et al ., 1992). A quantitative defi nition of complete desiccation has also
been proposed and widely accepted. Complete desiccation represents drying to <0.1 g H 2 O g -1 of dry
weight or mass that is to 10% water content or less than that. In other words, it is roughly equivalent
to air-dryness at 50% relative humidity at 20ºC and corresponds to a water potential of -100 MPa
(Gaff, 1997; Haranczyk et al ., 1998; Proctor, 2003). The limits of complete desiccation in prokaryotes
seem to be low as bacteria can survive drying up to 2% of water content (Potts, 1994). It means drying
up to 10% or lower levels of water content would result in probably too little water left inside the
cells to hydrate the macromolecules. There are many examples of surviving complete desiccation for
considerable period of time. The longest duration of survival was represented by seeds of Nelumbo
nucifera which germinated successfully after 1100 years (Shen-Miller et al ., 1995). Other examples
are from mosses and liverworts which recovered after 20-25 years of complete desiccation. Many
bacteria, fungi, lichens and microalgae are also endowed with the capacity to tolerate desiccation.
The cyanobacterium N . commune survived complete desiccation without any oxidative damage to
genomic DNA (due to a covalent modifi cation) for 13 years (Shirkey et al ., 2003) and recovered after
13 years of herbarium storage (Shirkey et al ., 2000). Understanding the mechanism of desiccation
tolerance would greatly help in identifying and developing techniques for the storage of tissues,
cells and enzymes (Potts, 1994) and in the metabolic engineering of sensitive cells including those
of humans (Potts, 2001; Potts et al ., 2005).
Desiccation is a very complex process that occurs very rapidly. The matric water potential or
matric water pressure very much depends on the presence of particulate structures present in water
viz., proteins, ribosomes and even bacterial cells. At the interface of these particulate structures, the
water molecules tend to group themselves and have less tendency to react chemically. Thus these
particulate structures and their interfaces tend to decrease the activity of water ( a w ). The presence
of solutes also lowers a w . Depending on temperature, the rate at which water escapes as water
vapour varies. For example, if the relative humidity is 100% at 20ºC, it is decreased to 50% when
the temperature is increased to 32ºC at constant pressure. Likewise, water vaporization can also
be infl uenced by the relative amounts of solutes present in a solution. In nature, bright sunny days
result in completely dry atmosphere and the relative humidity drops nearly to 1 to 2%. The fl ora and
fauna of that region start experiencing loss of internal water as vapour. In case of higher plants, the
rate of transpiration will determine the rate at which water is lost as water vapour. In case of other
pro- and lower-eukaryotes, there is no such regulation but the organisms tend to experience the loss
of water as water vapour and start getting desiccated. But the formation of mist/dew in the early
morning will increase the relative humidity of the atmosphere and the organisms start acquiring
water or get rehydrated. The cycles of desiccation and rehydration assume greater signifi cance in
hot, arid and cold deserts. It is worth noting that as the organisms experience desiccation, they