Freeze-Recovery Physiology of Nitrogenase Activity in Terrestrial Nostoc sp. Colonies.

Nostoc sp. colonies from field collections were cultured and propagated on silica sand with aqueous N-free BG-11 medium. Laboratory experiments were conducted to characterize the in vivo freeze-recovery physiology of nitrogenase activity. Nitrogenase activity was monitored by the acetylene reduction technique. Frozen Nostoc sp. colonies were thawed and warmed to 10, 15, 20, 25, or 30 degrees C. At 25 and 30 degrees C, nitrogenase activity was detected within 6 h after thawing. At 20 degrees C or lower, nitrogenase activity was not detected until 12 h after thawing. Optimum thawing temperature with respect to the recovery of nitrogenase activity was 25 degrees C. In subsequent experiments, laboratory-grown Nostoc colonies were used along with the following conditions: prefreezing treatment of 3 days of exposure to light or darkness, freezing, and then thawing to 25 degrees C in light or darkness with or without metabolic inhibitors [3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU), monofluoroacetate, or chloramphenicol]. Approximately 30% of the energy in the initial recovery of nitrogenase activity (to 12 h after thawing) appeared to be supplied via the utilization of carbon compounds stored before freezing. Photosynthetic conditions (i.e., light and without DCMU) were necessary for maximum recovery of nitrogenase activity. In the presence of the protein synthesis inhibitor chloramphenicol, nitrogenase activity was still detected at 12 to 48 h after thawing. Although damage may occur to nitrogenase, some of the enzyme was capable of surviving the freeze-thaw period in vivo. However, complete recovery of nitrogenase activity (equal to prefreezing activity) may entail some de novo synthesis of nitrogenase.