Buoyancy regulation of Microcystis flos-aquae during phosphorus-limited and nitrogen-limited growth

Abstract

The dominance of gas-vacuolate cyanobacteria is often attributed to their buoyancy and to their ability to regulate buoyancy in response to environmental conditions. Changes in absolute gas vesicles volume, carbohydrate content, protein content and colony buoyancy of Microcystis flos-aquae were investigated during nitrogen-limited, phosphorus-limited and nutrient-replete growth. When nutrient-replete, M. flos-aquae cells consistently had excess gas vesicles, which provided sufficient buoyancy that the influence of daily carbohydrate changes on cells upon floatation was negligible. However, during nitrogen-limited growth, gas vesicle volume per cell decreased significantly with nitrogen exhaustion. The maximum decrease of gas vesicle volume was up to 84–88%. At the same time, cellular carbohydrate content had an accumulation trend. The decrease of gas vesicle buoyancy together with the daily increase in carbohydrate are suggested to explain the daily changes in the cell floatation. During phosphorus-limited growth, gas vesicle volume per cell decreased slightly (maximum to 22–32%), and they still provided sufficient buoyancy that most cells kept floating even though there were significant daily carbohydrate changes. Since nitrogen limitation caused more significant buoyancy loss than phosphorus limitation did, surface water blooms may disappear or appear frequently in nitrogen limited water bodies while they may persist a longer time in phosphorus limited water bodies. The quantitative analysis in buoyancy change by gas vesicles, carbohydrate and protein suggested that long-term buoyancy regulation was mainly determined by changes of gas vesicle volume whereas short-term buoyancy regulation was mainly determined by carbohydrate accumulation and consumption. Both long-term and short-term buoyancy regulation were influenced by cell nutrient status. Furthermore, gas vesicle volume per cell and protein content changed in the same way in both nitrogen-limited and phosphorus-limited growth, which implied that the decrease of gas vesicles were associated with controls of total protein synthesis.