Growth rates of Synechococcus types with different phycoerythrin composition estimated by dual-laser flow cytometry in relationship to the light environment in the Uwa Sea

In the Uwa Sea, Synechococcus types with low- and high-phycourobilin (PUB) to phycoerythrobilin (PEB) ratio co-occurred throughout a year. To clarify the effects of light quality and quantity on the two types of Synechococcus, we measured the growth rates of two pigment types of Synechococcus cells with in situ incubation experiments. Incubations were conducted at 2, 10, and 20 m depth between May and October 2002. Synechococcus were divided into high- and low-PUB types using a dual-laser flow cytometer. Two indexes were used to evaluate the light environment: one was the relative light intensity (RLI) to that at the surface, and the other was the ratio of the light intensities of blue (490–500 nm) to green (540–550 nm). At 2 m depth, where the relative light intensities were above 20%, the growth rates of the low-PUB type were generally slightly higher than those of the high-PUB type. In contrast, at 10 and 20 m depth, the type that grew faster did not depend on the combination of light intensity and quality. Available light in the deeper layer of the Uwa Sea ranged from 490 to 550 nm. The range covers absorbance maxima of both PUB (ca. 495 nm) and PEB (545 nm). For this reason, light quality may not have caused one type to grow faster. These results explain the co-occurrence of two pigment types of Synechococcus in coastal waters.