Calcifying coccolithophores are ubiquitous marine phytoplankton that produce both organic carbon and inorganic carbon (CaCO₃), and thus play a crucial role in primary production and the global carbon cycle. The growth and calcification process of Emiliania huxleyi, a widespread coccolithophore, is influenced by various environmental factors, such as temperature, light, and nutrient concentration. As the ocean's temperature increases due to global warming, the water column becomes more stratified, exposing phytoplankton to higher light intensity. However, the response of calcified and naked coccolithophores to elevated light intensity remains unclear. In this study, two coccolithophores, calcified E. huxleyi (RCC1216) and naked E. huxleyi (RCC1217), were selected to investigate their growth rate, photosynthesis, and elemental composition under elevated light intensity. Isochrysis galbana (RCC1353), a microalga in the class Prymnesiophyceae with a close evolutionary relationship to E. huxleyi, was also chosen to examine its differential response compared to E. huxleyi.
Results demonstrated that the growth rate of all three microalgae exhibited a similar increase and subsequent decrease trend with elevated light intensity, with naked E. huxleyi consistently showing a higher growth rate than calcified E. huxleyi. The maximum yield of photochemistry of photosystem II (F_v/F_m) of naked E. huxleyi was higher than that of calcified E. huxleyi and they both remained steady under high light, whereas Isochrysis galbana displayed a sharper decrease in F_v/F_m than both E. huxleyi strains. The declining trend in half-saturated light intensity (I_k) and maximum electron transport rate (ETR_max) indicated that calcified E. huxleyi can withstand higher light intensity than naked E. huxleyi. The elevated particulate organic carbon production rate and higher δC¹³ values observed in calcified E. huxleyi compared to naked E. huxleyi suggest that calcification in E. huxleyi may enhance the photosynthetic process. Furthermore, proteomics will be analyzed to elucidate the metabolic mechanism of calcified and naked E. huxleyi under varying light intensities. This study provides an initial insight into the distinct responses of calcified and naked E. huxleyi under a global light enhancement environment and its impact on the global carbon cycle.
 

coccolithophore, calcification, light intensity, photosynthesis