OSCILLATING BEHAVIOR OF CARBOHYDRATE GRANULE FORMATION AND DINITROGENFIXATION IN THE CYANOBACTERIUM CYANOTHECE SP STRAIN ATCC-51142

It has been shown that some aerobic, unicellular, diazotrophic cyanoba cteria temporally separate photosynthetic O-2 evolution and oxygen-sen sitive N-2 fixation. Cyanothece sp. ATCC strain 51142 is an aerobic, u nicellular, diazotrophic cyanobacterium that fixes N-2 during discrete periods of its cell cycle. When the bacteria are maintained under diu rnal light-dark cycles, N-2 fixation occurs in the dark. Similar cycli ng is observed in continuous light, implicating a circadian rhythm. Un der N-2-fixing conditions, large inclusion granules form between the t hylakoid membranes. Maximum granulation, as observed by electron micro scopy, occurs before the onset of N-2 fixation, and the granules decre ase in number during the period of N-2 fixation. The granules can be p urified from cell homogenates by differential centrifugation. Biochemi cal analyses of the granules indicate that these structures are primar ily carbohydrate, with some protein. Further analyses of the carbohydr ate have shown that it is a glucose polymer with some characteristics of glycogen. It is proposed that N-2 fixation is driven by energy and reducing power stored in these inclusion granules. Cyanothece sp. stra in ATCC 51142 represents an excellent experimental organism for the st udy of the protective mechanisms of nitrogenase, metabolic events in c yanobacteria under normal and stress conditions, the partitioning of r esources between growth and storage, and biological rhythms.