Vertical distributions and photosynthetic action spectra of two oceanic picophytoplankters, Prochlorococcus marinus and Synechococcus sp.

Two picophytoplankters, Prochlorococcus marinus and Synechococcus sp., were isolated from the bottom of the euphotic zone (150 m depth) in the western Pacific Ocean, The concentration of P. marinus at this depth was more than 10(4) cells ml(-1) while that of Synechococcus sp, was less than 10(2) cells ml(-1). The P. marinus isolate has a high divinyl-chlorophyll a:b ratio similar to that of the Mediterranean strain, while the Synechococcus sp. isolate is of the phycourobilin-rich type. The growth rate of P. marinus was higher than Synechococcus sp. when both were cultured under weak blue-green to blue-violet light (ca. 2 mu E m(-2) s(-1)). While the chlorophyll-specific absorption spectra showed higher values in Synechococcus sp., the photosynthetic action spectra revealed that P. marinus was able to use blue-violet light, whereas Synechococcus sp. was able to use blue-green light, more efficiently for photosynthesis. The photosynthetic quantum yield of P. marinus was higher than that of Synechococcus sp. at any wavelength between 400 and 700 nm. The calculated in situ photosynthesis rates per cell volume for P. marinus were estimated to be higher than for Synechococcus sp. at 50 and 150 m depth. These results indicate that P. marinus photosynthetically surpasses Synechococcus sp. in the blue-light-rich environment of the oceanic euphotic zone. This may be why the former predominates at depths in temperate to tropical open ocean waters.