Light limitation and lower nutrient availability lead to buoyancy loss in freshwater cyanobacterium Raphidiopsis raciborskii

Abstract Raphidiopsis raciborskii is a globally dominant cyanobacterium that is often distributed within subsurface layers. However, the mechanisms regulating its buoyancy, which are critical to its survival and distribution, are not well understood. This study aimed to elucidate the growth dynamics, changes in gas vesicle volume, and buoyancy responses of a strain of R. raciborskii under varying light and nutrient conditions. Results showed that low light conditions were more conducive to the growth and gas vesicle synthesis, whereas high light intensity induced the loss of buoyancy partly through the dilution of gas vesicles and partly from carbohydrate accumulation. Lower nutrient availability reduced buoyancy: the synthesis of gas vesicles was severely impaired, resulting in a marked reduction in gas vesicle volume per cell. Nevertheless, growth continued under low dissolved inorganic nitrogen availability, presumably supported by nitrogen fixation. Buoyancy regulation over several days was driven by changes in gas vesicle volume and carbohydrate storage, whereas the rapid response within hours was affected by light-dependent carbohydrate accumulation and utilization. R. raciborskii lost buoyancy rapidly in response to light may help explain why it rarely forms extensive surface blooms like other problematic freshwater cyanobacteria.