Plasticity in algal stoichiometry: Experimental evidence of a temperature-induced shift in optimal supply N:P ratio

There is growing empirical and theoretical evidence for a positive relationship between the nitrogen (N)-to-phosphorus (P) ratio of phytoplankton and temperature. However, few have tested how the optimal supply N:P ratio; the dissolved N:P ratio at which nutrient limitation switches from one element to the other, responds to temperature. In this study, we conducted a factorial experiment crossing 12 temperature levels with 8 supply N:P ratios to determine the effect of temperature acclimation on the optimal supply N:P ratio of the microalgae Chlamydomonas reinhardtii. We found that the optimal supply N:P increased in a sigmoidal manner from 26.5 to 36.5 (atomic ratio) over a temperature gradient spanning from similar to 10 to 18 degrees C, with the steepest change around 15 degrees C. This result is in accordance with trends observed for cellular and seston N:P ratios, and indicates that phytoplankton populations may be shifted toward N-limitation in a scenario of warmer waters.