The speed of water uptake by desiccated Nostoc commune was found to depend upon the duration of desiccation. The rehydration of desiccated colonies led to marked, time-dependent changes in structure and ultrastructure and fluctuations in the composition of the transcriptome. Physical evaporative water loss is an active process that was influenced by inhibitors of transcription and translation.Certain cells can survive in a dry, metabolically inactive state, sometimes for long periods (8, 10). Such aged and desiccated cells rapidly recover their physiological capacities following the addition of water and then resume active growth. There is some understanding of the mechanisms that permit cells to withstand extreme fluctuations in water availability, including a knowledge of factors that lead to the damaging of cell components, as well as a knowledge of repair processes (1-4, 6, 13). Despite much study of the different strategies used to overcome acute water deficit, it is still unclear how physiological responses to drying and rehydration are controlled at the whole-cell level or how a complement of gene products could interact synergistically, through four dimensions, to provide desiccation tolerance.Nostoc commune is an important source of fixed nitrogen in nutrient-depleted soils from the tropics to the polar regions (10). In these habitats, N. commune is subject to repeated cycles of desiccation and rewetting, with water availability being the critical modulator of the function and success of this microorganism. In order to match key physiological processes with the availability of water, we first measured rates of water uptake and evaporation from sample colonies. Desiccated colonies of N. commune were collected from Topsail Island, N.C. (N. commune TOP/1993), and were stored dry in sealed glass bottles in the dark until analysis (16). The colonies form thin, irregular, flattened, brittle fragments of thallus, which consist of filaments embedded within a complex extracellular polysaccharide (6, 10). When submerged in water, the colonies swell rapidly and assume a consistency comparable to that of an approximately 4-mm-thick, 10% (wt/vol) polyacrylamide gel. The rates of rewetting and drying of these colonies were evaluated with an automated analytical balance system consisting of a Mettler Toledo AB 2045 analytical balance connected through a serial interface to a personal computer operating Balance Talk version 4.0 software (Labtronics Inc.). The results were compared with those obtained with synthetic colonies (inanimate controls) made of cellulose and molded and compressed into the shape of Nostoc colonies.Twenty-seven N. commune TOP/1993 colonies of different shapes and weights were used to measure the kinetics of water uptake and loss. In one rehydration experiment, six desiccated colonies with dry weights between 0.072 and 0.194 g were used.At the time of the first rehydration, the colonies had been in desiccation storage for 8 years. Desiccated colonies were rehydrated in petri dishes, through the ad...
