Zeaxanthin and ß‐carotene in Synechococcus WH7803 respond differently to irradiance

Synechococcus WH7803 responded to growth irradiance between 30 and 2,000 µ Einst m ‒2 s ‒1 by adjusting the cellular concentration of β ‐carotene but not zeaxanthin, β ‐carotene decreased from 0.68 to 0.15 fg cell ‒1 with increasing light intensity and maintained a constant molar ratio with Chl a (0.24 β ‐carotene: 1 Chl a ). The zeaxanthin concentration remained at 1.8 fg cell ‒1 . The responses of the two carotenoids are consistent with previous evidence that zeaxanthin and β ‐carotene are embedded in different membranes of the cell and that the bulk of carotenoids in cyanobacteria do not serve as photosynthetic light‐harvesting pigments. In cells grown in high light, the concentration of zeaxanthin was significantly higher than either Chl a or phycoerythrin, indicating that a significant percentage of absorbed light was not photosynthetically active. The constant amount of zeaxanthin per cell may serve as a useful tool in quantifying Synechococcus abundance and growth in natural waters.