Abstract
Two different modes of UV-B irradiation of the cyanobacterium Synechocystis sp. PCC 6803 are compared: turbidostatic control and additional physiostatic control. Under turbidostatic control, the cells were exposed to different constant UV-B irradiances, whereas under physiostatic control, an electronic control loop modulated UV-B irradiance in such a way that photosynthetic efficiency ϕ PSII was kept constant at a fixed set point. The UV-B-induced stimulation of the synthesis of pigments, α-tocopherol, and the antioxidative potential of methanolic soluble components of Synechocystis showed significant differences depending on the mode of irradiation, even though the overall doses were equal. For example, compared to the initial values, the concentrations of myxoxanthophyll and zeaxanthin increased to 226–244% and 453% upon constant UV-B irradiation in turbidostatic processes, whereas maxima of 600% and 740% were reached in turbidostatic process with additional physiostatic control. The α-tocopherol concentration increased under constant UV-B irradiances, up to a maximum of 150%. Under physiological control, however, maximum increases of 390% over the initial values were measured. Furthermore, a reaction scheme is given explaining the higher yield under physiostatic control.
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