BIO-02 Key changes in ocean variability and the effects of climate change
Effects of Light Intensity on Growth & Photosynthesis of Marine Phytoplankton
Le Huang* , The Hong Kong University of Science and Technology
Kedong Yin, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai)
Qiong Zhang, The Hong Kong University of Science and Technology; Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai)

Marine phytoplankton possess an essential role in food webs and carbon sequestration as primary producers. They generate half of the oxygen in the atmosphere by doing photosynthesis, which requires light as the main energy source. Therefore, the availability of light, which varies between latitudes and altered by perturbations of stratification associated with oceanic warming, may be a key control for the growth and photosynthesis of phytoplankton. Owing to variations in light harvesting proteins and pigment compositions, different phytoplankton groups may have difference in their preferred range of light intensities. Here we investigated the impacts of a wide range of light intensities (30, 50, 130, 200, 400, 600 µE/m2/s) on the growth, photosynthesis, nutrient utilisation, and carbon fixation of three ubiquitous phytoplankton species, i.e., a diatom Phaeodactylum tricornutum, a coccolithorid Emiliania huxleyi, and a chlorophyte Chlorella sp., to understand their biogeographic distribution and potential contribution to the nutrient cycle and carbon export in response to different light conditions in the modern ocean. We monitored the growth rates and a variety of photosynthetic parameters, including the maximum photosynthetic efficiency FV/FM, effective photosynthetic efficiency FV’/FM’, maximum electron transport rate ETRmax, and half saturation constant Ik, over the course of the experiments. We found that the diatom P. tricornutum could acclimate to a wide range of light intensities, with a peak of growth rates found under middle light intensities (130, 200 µE/m2/s), whereas maximum ETRmax and Ik appeared under relatively high light intensity (400 µE/m2/s). E. huxleyi was likely to face challenges under high light exposure, with the highest photosynthetic efficiency (FV/FM, FV’/FM’), ETRmax and Ik found under low light intensity (50 µE/m2/s). Chlorella sp. instead preserved good growth status and possessed an adaptive ability under the highest light intensity investigated (600 µE/m2/s), despite that it showed a decrease in photosynthetic efficiency (FV/FM, FV’/FM’) and ETRmax. Our data revealed the preferences and adaptative potentials of the diatom, coccolithophorid, and chlorophyte investigated towards light availability, and such preferences may be a key drive to their biogeographic distribution in the modern ocean. Their responses to different light availabilities can also affect their nutrient utilisation and carbon fixation in different oceanic regions, which can be revealed in their elemental stoichiometries and proteomic responses.