The impact of irradiance on optimal and cellular nitrogen to phosphorus ratios in phytoplankton.

ABSTRACT

Phytoplankton acclimates to irradiance by regulating the cellular content of light-harvesting complexes, which are nitrogen (N) rich and phosphorus (P) poor. Irradiance is thus hypothesised to influence the cellular N  P ratio and the N  P defining the threshold between N and P limitation (the 'optimal' N  P). We tested this hypothesis by first addressing the response of the optimal N  P to irradiance in a controlled experiment with Chlamydomonas reinhardtii. Then, we did a meta-analysis of experimental data on optimal and cellular N  P ratios across light gradients to test the generality of an N  P to light response within species. In both the experiment and the meta-analysis, N  P ratios decreased with irradiance, indicating that factors affecting underwater irradiance, like depth and the composition of the water, may influence the relative N  P requirement. The effect of irradiance did not differ between optimal and cellular N  P ratios, but observations of optimal N  P were on average 2.8 times higher than observations of cellular N  P.