Comment on egusphere-2025-5131

Abstract. The calcifying coccolithophores Gephyrocapsa oceanica and Emiliania huxleyi can grow preferentially in deep waters (150–200 m), however, their physiological and biochemical strategies for acclimating to the combined constraints of low temperature and low irradiance remain unclear. In this study, we subjected three coccolithophore strains (G. oceanica NIES–1318, E. huxleyi PML B92/11 and RCC1266) to low temperature (9 °C) and low light intensity (15 μmol photons m–2 s–1), and compared their growth rates, particulate inorganic carbon (PIC), particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP) contents, as well as carbohydrate and lipid levels, with those under standard cultivation (21 °C, 150 μmol photons m–2 s–1). The results revealed that low temperature and low light intensity acted synergistically to decrease growth rate, POC contents and the POC : PON and POC : POP ratios, whereas did not significantly affect POP content in any of the strains. While increased light intensity enhanced PIC and PON contents at high temperature, it reduced them at low temperature. Low light intensity was identified as the primary factor leading to reduced carbohydrate and lipid level. Collectively, these findings indicate that to acclimate to low–temperature and low–light conditions, coccolithophores prioritized reducing the metabolic cost of carbohydrate and lipid biosynthesis, thereby allocating more resources to phosphorus metabolism–a physiological adjustment that can significantly influence biogeochemical cycles in the deep ocean.