Low blue light enhances growth rate, light absorption, and photosynthetic characteristics of four marine phytoplankton species

Abstract

The effects of blue light on the growth rate, biochemical properties, light absorption, and photosynthetic characteristics based on chlorophyll a fluorescence were studied in the marine phytoplankton prymnesiophytes Isochrysis galbana, chlorophytes Dunaliella salina, diatoms Chaetoceros gracilis, and dinoflagellates Heterocapsa circularisquama to understand the variability in ecological roles of blue light among the four species. Monocultures were acclimated to blue and white light at the same irradiance of 60μmol photonsm−2s−1 for more than three dilutions in semi-continuous culture before the comparative experiment. Under our experimental conditions, growth rates under blue light condition were significantly higher (1.6–2.0 fold) than those rates under the white light condition (p<0.05), although the cell volume was not influenced. In addition, cellular contents of Chl a per cell volume significantly increased by more than 1.2-fold under blue light, excluding C. gracilis. Moreover, the cellular contents of both carbon and nitrogen per cell volume significantly increased by more than 1.8-fold under blue light in only H. circularisquama. The significant increase in the ratio of Chl a to carbon was associated with an enhanced growth rate in I. galbana and H. circularisquama, whereas the growth rate of the other two species had an inverse correlation with these conditions. Chlorophyll a-specific absorbance for blue light (400–550nm) for the cells that were exposed to blue light were significantly higher (1.3-fold) than those absorbance of PAR (400–700nm) for the cells that were exposed to PAR (p<0.05). The amount of light that was absorbed by chlorophyll a under the blue light condition was significantly higher (3-fold) than that under the white light condition (p<0.05). The potential quantum efficiency (Fv/Fm) was not influenced by blue light in all species, whereas the dependence of rETRmax or α on the Chl a/C ratios suggests the potential usefulness of the Chl a/C ratio as a photoacclimation index, even for the chlorophyll a fluorescence analysis. Among the species that were examined in the present study, D. salina was highly sensitive to the blue light and indicated a significant enhancement in the initial slope α, the maximum rETR, and the Ek, which were estimated from the rETR vs. E curve (p<0.05). The high utilization efficiency of blue light may suggest an ecological advantage for some species of phytoplankton that live in habitats where blue light prevails.