Photoacclimation strategies of different phytoplankton species revealed by physiological, biochemical, and...

Light utilization and tolerance responses vary greatly among phytoplankton species. The molecular mechanisms underlying these differences remain elusive. Here, we investigated the growth-irradiance responses of three phytoplankton species from different functional groups. The coccolithophore Gephyrocapsa huxleyi was found to prefer lower irradiance than the diatom Phaeodactylum tricornutum and the chlorophyte Chlorella sp., which had the highest preference of the three. All three species upregulated their light-harvesting complex proteins under low light to increase light absorption and increase their cellular carbon and nitrogen fixation under elevated irradiance. G. huxleyi and P. tricornutum enhanced nitrogen assimilation and promoted nitrogen use efficiency (NUE) for carbon fixation, whereas Chlorella sp. decreased NUE under high irradiance. P. tricornutum rapidly recycles carbon skeletons at the cost of a deficiency in lipid metabolism, whereas Chlorella sp. prioritizes lipid breakdown to maintain rapid growth rates at high irradiance. By investigating the global transcriptomic expression data of these phytoplankton species, we demonstrate that different light preferences and utilization strategies largely explain their distributions and niche partitioning, which may further impact carbon and nitrogen fixation in different light regimes.