Summary: Continuous cultures of DSM 185 were shifted from a high to a low irradiance (67 to 4 μE m s) and vice versa (4 to 67 μE m s). The kinetics of photoacclimation of the cultures were analysed during these transitions until steady state was reached. When irradiance was shifted from 4 to 67 μE m s, bacteriochlorophyll synthesis halted for 4 h. During this period, pigments were progressively diluted in the newly formed biomass, resulting in a lower specific pigment content. The specific growth rate of the organisms did not change immediately after the shift, but rather underwent a gradual increase during the following 10 h. This transition was accompanied by a transient increase in the levels of glycogen, indicating that CO fixation rates increased immediately after the shift, and that unused photosynthate was stored as glycogen. The shift from a high to a low irradiance was characterized by an immediate drop in the specific growth rate to virtually zero, and by comparatively sharp decreases in the specific rates of sulfur and sulfide oxidation and in the specific rate of glycogen accumulation. The specific content of bacteriochlorophyll a increased during the first 10 h. During the same period the specific content of glycogen decreased.


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