Nostoc flagelliforme must undergo a dehydration/rehydration cycle during its growth stages; the mechanisms underlying this constraint are examined. The novel insights into N. flagelliforme's response to desiccation and rehydration at ultrastructural, physiological and proteomic levels were offered. The structure of colonies and cells remained unchanged in response to dehydration and rehydration treatments except that the sheath appeared shrunken, and both the quantity and volume of vacuoles were decreased when dehydrated compared with rehydration. A significant increase in photosynthesis, respiration, total Rubisco activity, superoxide anion level, SOD, CAT, POD, nitrogenase and glutamine synthetase (GS) activities in response to rehydration was noted, whereas H(2)O(2), ammonium, proline and glutamate contents all registered a decrease. 32 differentially expressed proteins between dehydrated and rehydrated colonies were categorized according to their predicted functions into secretion, signaling, transcription and translation, antioxidative processes, nitrogen metabolism, energy metabolism, lipid metabolism and chaperonin. The dehydration is a quiescent state in which metabolism is down-regulated, upon rehydration, a metabolic shift occurs from quiescent to active. The specific metabolic and regulated mechanisms to accommodate the dehydration/rehydration cycle in N. flagelliforme is reported here.