Temperature fluctuation alleviates the negative effects of warming on marine diatoms: comparison between <i>Thalassiosira</i> sp. and <i>Nitzschia closterium</i> f. <i>minutissima</i>

Abstract. Marine phytoplankton are subjected to a wide range of environmental heterogeneity, from mean climate change to natural fluctuations under the climate change scenario. These changes include changes in the frequency of temperature fluctuations in the sea surface. Here we conducted semi-continuous incubation experiments on two ecologically significant marine diatom species, Thalassiosira sp. and Nitzschia closterium f. minutissima, to examine their physiological responses to ocean warming and temperature fluctuation (±4 °C) under low (20 °C) and high (25 °C) average temperatures. Our results demonstrate that temperature fluctuation alleviated the negative effects of elevated temperatures on the growth of both species. For Thalassiosira sp., warming under constant temperature significantly reduced the growth rate but significantly increased the cellular elemental contents and sinking rate. However, warming significantly reduced cellular particulate organic carbon (POC) content, biogenic silica (BSi) content, and the sinking rate, while increasing the protein content to cope with thermal stress under temperature fluctuation. The effects of temperature fluctuation were dependent on the average temperature: at low average temperatures, temperature fluctuation increased cellular POC, BSi, POC productivity, and sinking rates, whereas at high average temperatures, these parameters were decreased significantly. For Nitzschia closterium f. minutissima, warming under both constant and fluctuated temperatures significantly increased the POC, particulate organic nitrogen (PON), and POP quotas. The interaction between warming and temperature fluctuation had antagonistic effects on most parameters examined for Thalassiosira sp., whereas it had synergistic effects on the physiological parameters of Nitzschia closterium f. minutissima. Overall, Nitzschia closterium f. minutissima exhibited stronger tolerance to warming and temperature fluctuation, suggesting species-specific responses of diatoms to warming and temperature fluctuations. These findings highlight the important yet often underestimated influence of temperature fluctuation on the physiology of marine diatoms in the context of global warming, thus having implications for a further understanding of biogeochemical feedbacks.