Direct evidence for the role of light‐mediated gas vesicle collapse in the buoyancy regulation of Anabaena flos‐aquae (cyanobacteria)1

Quantitative measurements were made of the changes in gas vacuole volume and the major components of cell mass (protein and carbohydrate) on cultures of a nabaena flos‐aquae which lost buoyancy as they were shifted from low to high light intensity. Assuming densities of 1,300 kg· m ‒3 for protein and 1,600 for carbohydrate, we calculated the change in ballast brought about by changes in these components and compared them with ballast changes resulting from gas vacuole collapse. We also compared the calculated excess density of filaments with direct measurements of filament density in gradients of p ercoll. t he results clearly demonstrated that increased density resulted from the loss of gas vacuoles. Small variations in density could be attributed to changes in protein and carbohydrate but these would not have caused a loss of buoyancy in the absence of a decrease in gas vacuole content. Analyses of the type described can be used to determine the cause of buoyancy change in other microorganisms and can be performed on phytoplankton collected from lakes.