Nutrient availability influences the thermal response of marine diatoms

Understanding how phytoplankton growth responds to temperature is critical for forecasting marine productivity in a warming ocean. While previous laboratory studies have shown that phytoplankton thermal traits such as optimal temperature (T_opt) can be affected by nutrient availability, it is unclear whether this can be extrapolated to natural communities. To address this, we tested the impacts of nutrient availability on the thermal responses of two cosmopolitan diatom genera, Pseudo-nitzschia and Leptocylindrus, through a series of in situ manipulation experiments on natural phytoplankton communities. Analysis of the thermal performance curves revealed that nutrient limitation during summer not only limited the growth of these two genera but also reduced their T_opt and the maximum growth rates (μ_max). T_opt was close to or lower than in situ temperature under ambient nutrient conditions, suggesting that further warming may have a detrimental effect on their growth. However, increasing nutrient supply could counteract this by enhancing T_opt and μ_max. To further confirm the interactive effects of nutrients and temperature on diatoms, we analyzed a 20-yr monitoring dataset on Pseudo-nitzschia, Leptocylindrus, and the whole diatom assembly in Hong Kong coastal waters. We found that the abundances of marine diatoms were significantly higher at high temperatures under nutrient-rich environments while relatively low under low nutrient concentrations. Findings on natural diatom cell density align with the growth performance derived from in situ manipulation experiments, suggesting that abundant nutrients bolster marine diatoms in coping with warming. Our results highlight the importance of considering the influence of nutrient availability on thermal response of phytoplankton growth, which sheds light on how marine primary production may change under climate warming.