Responses of a harmful algal bloom-causing dinoflagellate Gymnodinium catenatum to higher temperature and elevated N:P ratio.

Abstract

Harmful algal blooms (HABs) dominated by toxic dinoflagellates are expanding under concurrent ocean warming and coastal eutrophication. Yet, their interactive effects and underlying mechanisms remain poorly understood. This 40-day multifactorial experiment investigated physiological, biochemical, elemental stoichiometric, and transcriptomic responses of Gymnodinium catenatum to higher temperature (26 °C vs. 22 °C control) and an elevated N:P ratio (180:1 vs. the baseline 40:1), alone and in combination. Higher temperature was the dominant driver stimulating cell growth and chain formation by enhancing photosystem repair and energy supply. Elevated N:P ratio alone moderately promoted growth via metabolic reorganization. Notably, the two stressors interacted antagonistically under combined exposure: growth promotion was significantly attenuated, accompanied by suppressed biosynthesis and decoupling between photosynthetic electron transport and carbon fixation. Elevated N:P ratio thus antagonized the growth stimulation by the higher temperature. Responses under multifactorial global change cannot be predicted from single-factor experiments. Our findings highlight the importance of integrating temperature-nutrient stoichiometry interactions into HAB predictive models and support coordinated nitrogen and phosphorus reduction strategies for mitigating HAB risks in future warming coastal ecosystems.

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