Light Regulation of Phycobilisome Structure and Gene Expression in Spirulina platensis C1 (Arthrospira sp. PCC 9438)

Light intensity is the most significant environmental factor influencing the light harvesting complexes (phycobilisomes) in cyanobacteria. Spirulina platensis C1, a filamentous cyanobacterium, was grown in batch culture under low (50 μE m−2s−1) and high (500 μE m−2s−1) photosynthetic photon flux density. Phycobilisomes were isolated, purified and analyzed by spectrophotometry, gel electrophoresis and electron microscopy. The absorption spectra in conjunction with the electron micrographs revealed that they are hemidiscoidal with three allophycocyanin cylindrical cores and six phycocyanin rods radiating from the core. The 33 kDa linker polypeptide normally found in low light cells disappeared under high light resulting in smaller phycobilisomes. The decrease in the number of phycobilisomes correlated well with the decrease in phycocyanin and allophycocyanin content from cells shifted from low to high light. The phycobiliprotein transcripts exhibited different pattern in response to different light intensities. Three species of phycocyanin mRNA of sizes 1.4, 1.5 and 3.5 kb and two species of allophycocyanin of 1.4 and 1.7 kb were found in low light grown cells. However in high light, the 3.5 kb phycocyanin transcript disappeared while the others remained with subtle changes. This observation indicates that the largest transcript encoding for phycocyanin subunits and linker polypeptides and the phycobilisome structure are modulated by light intensity.