LAUSR

Abstract. Climate-driven changes in environmental conditions have significant and complex effects on marine ecosystems. Variability in phytoplankton elements and biochemicals can be important for global ocean biogeochemistry and ecological functions, while there is currently limited understanding on how elements and biochemicals respond to the changing environments in key coccolithophore species such as Emiliania huxleyi. We investigated responses of elemental stoichiometry and fatty acids (FAs) in a strain of E. huxleyi under three temperatures (12, 18 and 24  ∘ C), three N : P supply ratios (molar ratios 10:1 , 24:1 and 63:1 ) and two p CO 2 levels (560 and 2400  µ atm). Overall, C : N : P stoichiometry showed the most pronounced response to N : P supply ratios, with high ratios of particulate organic carbon vs. particulate organic nitrogen (POC : PON) and low ratios of PON vs. particulate organic phosphorus (PON : POP) in low-N media, and high POC : POP and PON : POP in low-P media. The ratio of particulate inorganic carbon vs. POC (PIC : POC) and polyunsaturated fatty acid proportions strongly responded to temperature and p CO 2 , both being lower under high p CO 2 and higher with warming. We observed synergistic interactions between warming and nutrient deficiency (and high p CO 2) on elemental cellular contents and docosahexaenoic acid (DHA) proportion in most cases, indicating the enhanced effect of warming under nutrient deficiency (and high p CO 2) . Our results suggest differential sensitivity of elements and FAs to the changes in temperature, nutrient availability and p CO 2 in E. huxleyi, which is to some extent unique compared to non-calcifying algal classes. Thus, simultaneous changes of elements and FAs should be considered when predicting future roles of E. huxleyi in the biotic-mediated connection between biogeochemical cycles, ecological functions and climate change.