Photosynthetic membrane and chlorophyll content of the cyanobacterium Anacystis nidulans were determined after growth at different light intensities in Mg2+-limited and non-Mg2+limited continuous cultures. Decreasing the light intensity from 445 to 3 1 pE m-2 s-' resulted in a threefold increase of chlorophyll per unit cell volume. The photosynthetic lamellar structure, on the other hand, increased twofold under the same conditions. The specific chlorophyll content of the photosynthetic membrane increased when the light intensity was decreased from 445 to 282 pE m-2 s-' and was constant at lower light intensities. The results indicate that the maximum chlorophyll content in the thylakoids of A . nidulans was about 1.5 fg pm-2, which means that the membrane contained 1 chlorophyll molecule per nm2 under these conditions.

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The effect of photon fluence rate on Anacystis nidulans grown in Mg2+-and non-Mg2+-limited continuous cultures was studied. A decline in photon fluence rate from 445 to 30 yE mW2 s-' resulted in a decrease of the mean cell volume from 2 to 0.8 pm3 in the Mg2+-limited culture. Compared with non-Mg2+-limited cultures, this was the only obvious difference found when decreasing the photon fluence rate. Altering the photon fluence rate for a Mg2+-limited chemostat therefore seems to be a suitable system for studies of the Mg2+-controlled events in cell division of A . nidulans. At constant dilution rate, the DNA content per unit cell volume was independent of photon fluence rate, while the RNA content decreased with this factor. The RNA/protein ratio decreased by a factor of 3 in both Mg2+-and non-Mg2+-limited cultures, when decreasing the photon fluence rate from 445 to 100 pE m-2 s-l. The RNA efficiency therefore varied with photon fluence rate and the organism seemed to have surplus RNA at high light intensities.