Rehydration response of nitrogenase activity and carbon fixation in terrestrial Nostoc commune from Stipa–Bouteloa grassland

Abstract

The effects of wetting–drying cycles on patterns of carbon dioxide exchange and acetylene reduction were examined for colonies of the terrestrial cyanophyte Nostoc commune collected from a semiarid grassland site in southern Alberta, Canada. At 14 and 21 °C acetylene reduction takes ca. 14 h to resume maximal rates, although activity is detected within minutes of rehydration. Net photosynthesis reaches compensation minutes after rehydration and is maximal in under 80 min at 21 °C. No respiratory burst is evident, either for replicates previously dried rapidly (0.5 h) or slowly (5 h). In marked contrast, however, at 7 °C only minimal recovery of acetylene reduction is evident after 48 h diurnal treatment, while net photosynthesis requires a recovery time of 6 h. Again no respiratory burst is evident. After rehydration, the magnitude of acetylene reduction is strongly dependent on both the previous illumination level and temperature. The rapid decline in activity on transfer to darkness or on addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea can be moderated by increasing the duration of the previous light exposure or by incubation at lower temperature. This suggests the presence of substrate pools, the replenishment of which by photosynthesis allows oxidative phosphorylative support of nitrogenase activity in the dark. The extreme resistance of N. commune to desiccation and heat stress combined with its ability to resume metabolic activity within minutes of wetting should allow very effective utilization of small precipitation events during the summer months. The slower recovery of nitrogenase activity upon rehydration at lower temperatures may restrict winter field activity on rare occasions when thalli become dehydrated between snowfall periods.