Aspects of the relation betweenCyanophora paradoxa(Korschikoff) and its endosymbiotic cyanellesCyanocyta korschikoffiana(Hall & Claus) - I. Growth, ultrastructure, photosynthesis and the obligate nature of the association

Ultrastructural studies of the symbiotic relation between the apochlorotic cryptomonad Cyanophora paradoxa and the cyanobacterium Cyanocyta korschikoffiana have demonstrated that the endosymbiotic cyanobacteria are within host cell vacuoles and possess a highly reduced prokaryotic cell wall which becomes particularly evident during binary fission. Division by the endosymbiotic cyanobacteria was found not to be in synchrony with division by the host cryptomonad. The cyanobacteria divided most rapidly when the host population had reached its asymptote, such that, in cultures in extended stationary phase, it is possible to observe as many as eight cyanobacteria in one host cell. Nutritionally, C. paradoxa was found to be an obligate phototroph; its auxotropic capabilities are confined to acetate and cyanocobalamin. Analysis of the rate of photosynthesis by the intact association demonstrated that saturation occurred at the 140 $\mu $E cm$^{-2}$ s$^{-1}$ (where E is einstein), and at saturating light intensities, the rate of CO$_{2}$ fixation was 30 $\mu $mol CO$_{2}$ (mg chl a)$^{-1}$ h$^{-1}$. The best rate of CO$_{2}$ fixation obtained by the isolated cyanobacteria represented 12% of the rate found in the intact association. Two independent methods of analysis showed that at best, only 15% of the carbon fixed by the cyanobacteria was translocated to the host, mostly as glucose and probably sucrose. Studies with metabolic inhibitors directed against the prokaryotic cyanobacteria demonstrated the close interdependence of the host with the endosymbionts, in that disruption of the endosymbionts always resulted in death of the host. Our inability to grow the cyanobacteria away from the host, combined with the apparent inability of the host to survive in the absence of the photosynthetic endosymbiotic cyanobacteria provide strong support for the concept that the association represents a well integrated mutually obligate relation, but the concept that the cyanobacteria are chloroplasts cannot be supported.