Limnol. Oceanogr., 44(2), 1999, 273–281

A mechanism of buoyancy regulation in cyanobacteria was investigated in a fluctuating light environment typical of the mixed layer in a lake. New measurements of density and cellular carbohydrate content were obtained from a field sample of the cyanobacterium Microcystis aeruginosa. The rate of increase in carbohydrate and density in this cyanobacterium does not instantaneously adjust to an increase in light intensity. There is a time lag, the response time, corresponding to physiological adjustment to the increase in light, before equilibration of the rate of carbohydrate and density increase. Also, the rate of density and carbohydrate increase is nonlinear over the response time. Based on these experimental findings, a new model for buoyancy regulation was developed that includes the response time. This model was applied to two fluctuating light regimes that simulate Langmuir circulation and turbulence. The simulations reveal that the response time is critical in determining if a cell equilibrates its rate of density change to a change in the light that it receives. If the response time is longer than the time scale of the light fluctuation, then the rate of density increase will be less than the optimal equilibrium rate. Models that do not include the response time will not correctly predict the buoyancy of cyanobacteria and their water column position after an episode of mixing. The existence of cyanobacteria that can regulate their buoyancy has been well documented (Walsby 1969a, 1970; Oliver and Walsby 1984; Utkilen et al. 1985; Thomas and Walsby 1986; Kromkamp et al. 1986; Humphries and Lyne 1988; Zohary and Robarts 1989; Kromkamp and Walsby 1990; Ibelings et al. 1991; Walsby et al. 1991). This characteristic is considered to be important in contributing to the success of cyanobacteria in a wide range of aquatic ecosystems (Reynolds and Walsby 1975; Reynolds 1984, 1987; Reynolds et al. 1987) because it provides the advantage of being able to alter position in the water column through physiological changes. Ganf and Oliver (1982) showed how buoyancy regulation allowed cyanobacteria to overcome the vertical separation of light and nutrients in a stratified lake. There are three mechanisms of buoyancy regulation. The first is changes in cell ballast (Oliver and Walsby 1984; Thomas and Walsby 1986; Kromkamp et al. 1988) resulting from the accumulation or consumption of storage molecules. The second mechanism is regulation of gas vesicle synthesis (Walsby 1969b), which depends on light (Deacon and Walsby 1990) and macronutrients such as nitrogen and carbon (Klemer 1978, 1991). If gas vesicle synthesis ceases, buoyancy may be lost as the cellular concentration of gas vesicles is diluted by cell growth and division (Walsby et al. 1983). The thrid mechanism is irreversible collapse of gas vesicles under rising turgor pressure (Walsby 1971) generated through the photosynthetic production of low-molecularweight carbohydrates (Grant and Walsby 1977) and by lightdependent uptake of potassium ions (Allison and Walsby 1981). However, this mechanism probably is not responsible for buoyancy regulation in most natural populations (Walsby