Goldman revisited: Faster-growing phytoplankton has lower N : P and lower stoichiometric flexibility

In their seminal paper, Goldman et al. suggested that phytoplankton close to maximum growth rate attains a restricted optimal N:P ratio close to the Redfield ratio of molar N:P = 16. Recently, the presence of such a global attractor for optimal phytoplankton stoichiometry has been questioned in models and empirical analyses. As the chemical composition of phytoplankton is of major importance for our understanding of global elemental cycles and biogeochemical transformations, we assembled 55 data sets of phytoplankton growth rate and biomass N: P ratios in a meta-analysis testing (1) whether phytoplankton N: P converges at high growth rates, (2) whether N: P ratios scale with growth rate, and (3) whether the optimal N: P ratios achieved at highest growth rates reflect organism traits or environmental conditions. Across systems and species, phytoplankton N: P decreased with increasing growth rate and at the same time showed decreasing variance, i.e., fast-growing phytoplankton is more P rich and has a more confined elemental composition. Optimal N: P increased with increasing N: P of available nutrients, i.e., with increasing P limitation. Other differences were rare, except cyanobacteria showed higher optimal N: P than diatoms. Understanding the role of phytoplankton in biogeochemical transformation requires modeling approaches that are stoichiometrically flexible to reflect the dynamics of growth and nutrient supply in primary producers.