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Microcystis aeruginosa, a unicellular cyanobacterium, contains small phycobilisomes consisting of C-phycocyanin, allophycocyanin, and linker polypeptides. SDS-polyacrylamide gels of the phycobilisomes were examined for fluorescent bands before and after spraying with a solution of ZnSO(4), followed by Coomassie brilliant blue staining for protein. This procedure provides a rapid and sensitive method for detecting small amounts of phycobilin-containing polypeptides and distinguishing them from other tetrapyrrole-containing polypeptides and from ;colorless' ones. Three polypeptide bands, in addition to the alpha and beta phycobiliprotein subunits, have been detected under these conditions. An 85 kilodalton polypeptide was identified as a phycobiliprotein due to its enhanced fluorescence in the presence of ZnSO(4). The other polypeptides do not contain chromophores and are colorless. They are approximately 34.5 and 30 kilodaltons in size.

Phycobilisomes isolated from Microcystis aeruginosa grown to midlog at high light (270 microeinsteins per square meter per second) or at low light intensities (40 microeinsteins per square meter per second) were found to be identical. Electron micrographs established that they have a triangular central core apparently consisting of three allophycocyanin trimers surrounded by six rods, each composed of two hexameric phycocyanin molecules. The apparent mass of a phycobilisome obtained by gel filtration is 2.96 x 10(6) daltons. The molar ratio of the phycobiliproteins per phycobilisome is 12 phycocyanin hexamers:9 allophycocyanin trimers. The electron microscopic observations combined with the phycobilisome apparent mass and the phycobiliprotein stoichiometry data indicate that M. aeruginosa phycobilisomes are composed of a triangular central core of three stacks of three allophycocyanin trimers and six rods each containing two phycocyanin hexamers. Adaptation of M. aeruginosa to high light intensity results in a decrease in the number of phycobilisomes per cell with no alteration in phycobilisome composition or structure.

Light intensity adaptation (20 to 565 microeinsteins per square meter per second) of Microcystis aeruginosa (UV-027) was examined in turbidostat culture. Chlorophyll a and phycocyanin concentrations decreased with increasing light intensity while carotenoid, cellular carbon, and nitrogen contents did not vary. Variation in the number but not the size of photosynthetic units per cell, based on chlorophyll a/P(700) ratios, occurred on light intensity adaptation. Changes in the numbers of photosynthetic units partially dampened the effects of changes in light intensity on growth rates.