MODELING VERTICAL MIGRATION OF THE CYANOBACTERIUM MICROCYSTIS

Computer models can be helpful tools to provide a better understanding of the mechanisms responsible for the complex movements of cyanobacte ria resulting from changes in buoyancy and mixing of the water column in a lake. Kromkamp & Walsby (1990) developed a vertical migration mod el for Oscillatoria, that was based on the experimentally determined r elationship between the rates of density change and photon irradiance in this cyanobacterium. To adapt this model to Microcystis, we determi ned related changes in carbohydrate content in cultures of Microcystis . Samples were incubated at various constant values of photon irradian ce and then placed in the dark. The changes in carbohydrate content of the cells during these incubations were investigated. The relationshi p between the ratio of carbohydrate to protein and cell density in Mic rocystis was established to permit conversion of the rates of carbohyd rate change to rates of density change. By plotting the calculated rat es of density change against the values of photon irradiance experienc ed during the incubations, an irradiance-response curve of density cha nge was established. The curve showed a distinct maximum at 278 mu mol photons m(-2) s(-1). At higher values of photon irradiance, the rate of density change was strongly inhibited. A positive linear correlatio n was found between cell density and the rates of density decrease in the dark,. The validity of the use of rate equations of density change , which are based on short-term incubations at constant values of phot on irradiance, to predict density changes in Microcystis in fluctuatin g light regimes was tested. This was accomplished by measuring the tim e course of change in carbohydrate content of two continuous cultures of Microcystis, which were submitted to fluctuating light regimes, and comparing the results with the changes in the carbohydrate contents o f these cultures predicted by the rate equations of carbohydrate chang e. The results showed good agreement: the rate equations of density ch ange were therefore introduced into the model to simulate vertical mig ration of Microcystis. The model predicts that the maximum migration d epth of Microcystis will increase with colony size up to a maximum of 200 mu m radius. The effect of colony size on the net increase in cell density during the light period was also investigated with the model. It predicts that small colonies have a higher net increase in cell de nsity than large colonies, but are inhibited at high photon irradiance s at the surface.