NITROGENASE ACTIVITY, PHOTOSYNTHESIS, AND THE DEGREE OF HETEROCYST AGGREGATION IN THE CYANOBACTERIUM ANABAENA-FLOS-AQUAE

The nitrogen-fixing cyanobacterium Anabaena flosaquae (Lyngb.) De Breb. exhibited aggregation of heterocysts from different filaments in a eutrophic lake and when grown in unialgal culture. The resulting aggregated filaments formed unialgal flocculent masses having a thickness of several centimeters that apparently resulted from cohesive mucilage surrounding heterocysts. We tested the effects of heterocyst aggregation on nitrogenase activity (NA) and photosynthesis in relation to microscale environmental O2 gradients. The redox indicator 2,3,5-triphenyl tetrazolium chloride showed that aggregated heterocysts had lower intracellular redox potential than those that were dispersed. Microelectrode measurements showed that heterocyst aggregates in actively photosynthesizing flocculent masses were surrounded by a microzone of O2 30% higher than in the surrounding water; dispersed cells exhibited no such elevated O2 microzone. Despite high levels of O2, NA was greater in aggregated than dispersed samples. Microscale irradiance measurements made with a fiber optic light probe showed that 40% of the incident light was absorbed within the first 3 mm of a 1-cm-thick flocculent mass. The microscale irradiance data, together with nitrogenase and photosynthesis versus irradiance data, imply that the ratio of N:C fixation is lowest in filaments on the outside of 1.5-2.0-cm masses and increases toward the center.