A meta-analysis on environmental drivers of marine phytoplankton C:N:P

The elemental stoichiometry of marine phytoplankton plays a critical role in global carbon cycle through carbon export. Although extensive laboratory experiments have been carried out over the years to assess the influence of different environmental factors on elemental composition of phytoplankton, a comprehensive quantitative assessment of the processes is still lacking. Here, we synthesised the responses of P:C and N:C ratios of marine phytoplankton to 5 major factors (phosphate and nitrate, temperature, irradiance, and iron) by meta-analysis of laboratory experimental data available in the literature. Our results show that the response of the ratios to changes in macronutrients is consistent across all the studies, where the nutrient availability is positively related to changes in P:C and N:C ratios. We found that diatoms are more sensitive to the changes in macronutrients compared to other eukaryotes and cyanobacteria, possibly due to their larger cell size and their abilities to quickly regulate their gene expression patterns required for nutrient uptake. The response to temperature changes was mixed by species, except warming consistently decreased P:C ratio in cyanobacteria. This may explain why P:C is consistently low in the cyanobacteria-dominated subtropical oceans. The effect of irradiance on P:C was mixed and not significant, but the same effect on N:C was significant and constant across all studies where increase in irradiance decreased N:C. The effect of iron on P:C and N:C was neither statistically significant nor consistent across studies, but the small sample size precludes drawing firm conclusions. Overall, our findings highlight the high stoichiometric plasticity of diatoms and importance of macronutrients in determining P:C and N:C ratios, which both provide us insights on how to understand and model plankton diversity and productivity.