Data_Sheet_1_The Role of Sustained Photoprotective Non-photochemical Quenching in Low Temperature and High Light Acclimation in the Bloom-Forming Arctic Diatom Thalassiosira gravida.docx

Thalassiosira gravida is a major Arctic diatom responsible for the under-ice spring bloom. We investigated T. gravida physiological plasticity growing it at two temperatures (0 and 5°C) and under different light intensities typically found in its natural environment. T. gravida showed remarkable thermal- and photo-acclimatory plasticity including: low light saturation parameter for growth (K_E) and photosynthesis (E_K), low μ_max but relatively high Chl a/C, low C/N, and decreasing light-saturated carbon fixation rate (PmC) with increasing growth irradiance. T. gravida also showed remarkable photoprotective features, namely a strong sustained non-photochemical quenching (NPQs, hour kinetics relaxation) supported by a high amount of xanthophyll cycle pigments. T. gravida growth remained possible under a wide range of irradiances but photosynthetic plasticity was higher at moderately low light (up to ~50 μmol photons m⁻² s⁻¹), nevertheless corresponding to the mean in situ conditions under which it predominates, i.e., underneath the spring thin-ice punctuated with melting ponds. The potential role of NPQs in the photophysiological plasticity of T. gravida is discussed.