Temperature‐dependent evolution of cell morphology and carbon and nutrient content in a marine diatom

Abstract

AbstractPhytoplankton are key players in global biogeochemical cycles, and the effects of ocean warming on their carbon–nitrogen–phosphorus (CNP) stoichiometry, photosynthesis, size, morphology, growth rates, and other traits are of great ecological consequence. The physiological mechanisms of adaptation to temperature in phytoplankton are poorly understood, as are the consequences of the evolution of these processes (e.g., nutrient uptake, photosynthesis) for global biogeochemistry. In general, high temperatures favor smaller cells with higher surface area‐to‐volume ratios, but repeatable patterns in cellular CNP stoichiometry across temperature remain elusive. Here, we compared thermal reaction norms for cellular C, N, P, and chlorophyll a (Chl a) content and for carbon assimilation rate in replicate populations of the marine diatom Thalassiosira pseudonana evolved for 500 generations at 16°C and 31°C. We also examined the thermal reaction norms for cell volume and morphological traits. T. pseudonana has a cylindrical frustule and likely primarily exchanges materials with the environment through the round valve faces. We found that the 31°C‐selected T. pseudonana populations had smaller cells and higher per‐biovolume densities of nutrients and Chl a than the 16°C‐selected populations across assay temperatures but there were no detectable patterns in CNP stoichiometry. The 31°C‐selected populations also had higher valve surface area‐to‐cell volume ratio that increased more with temperature, suggesting better nutrient uptake capabilities than in the 16°C‐selected populations. Our study demonstrates that temperature‐dependent physiological plasticity may evolve differently at different temperatures and suggests that future phytoplankton communities will consist of smaller, more nutrient‐dense cells.