Growth response of Thalassiosira pseudonana under combined light and temperature changes

The rise of atmospheric CO2 concentrations due to human activities is warming the upper levels of the ocean leading to changes in mixing and thus affecting the upward nutrient supply and the level of light available for phytoplankton. Researchers have focused on investigating how these fast-approaching changes are affecting phytoplankton; however, it is still difficult to obtain a consistent understanding of the expected response because of interactive effects to simultaneous changes in multiple stressors. This study aims to present an alternative way to determine the growth rate of phytoplankton through the measure of their optical properties (1). In addition, this study investigates the interactive effects of light and temperature changes on the growth rate (µ) of diatom Thalassiosira pseudonana CCMP 1335 (2). We monitored the growth rate of T. pseudonana at four temperatures (13.4, 18.5, 22.5 and 24.4°C) and eight light regimes (35, 50, 65, 80, 95, 110, 125 and 140µmol photons m-2s-1) by measuring the cultures chlorophyll fluorescence F0, optical density (OD680 and OD720) and cells number. Our results show an overall statistically highly significant correlation between µF0 and µOD and µ based on cell counts (n: 25 p < 0.001). We assume this relationship to be light and temperature dependent and F0 and OD to be valuable proxies for cell counts under our experimental conditions. We highlight the importance of testing the correlations for each species individually. Our multi-drivers experiments show light limitation for all our cultures at 35 and 50µmol photons m-2s-1 and temperature limitation at 13.4 and 18.5°C. Furthermore, we found the optimal growth conditions of our T. pseudonana cultures at 22.5°C and 110µmol photons m-2s-1. Interesting interactive effects between light and temperature were found at 18.5°C and 24°C and at 110µmol photons m-2s-1. These results show that simultaneous changes in temperature and light exposure lead to interactive effects.