Thermal response of single species and interaction between Gymnodinium impudicum and Heterocapsa pygmaea:...

Driven by climate change, rising seawater temperatures are expected to alter the physiology and competitive dynamics of harmful algal bloom (HAB) species. However, how warming differentially affects coexisting dinoflagellates and whether such species-specific responses translate into altered interspecific interactions remain unclear. In this study, we examined the effects of rising seawater temperatures (20, 24, and 28°C) on the physiological responses and interspecific interactions of the harmful dinoflagellates Gymnodinium impudicum and Heterocapsa pygmaea. We analyzed the maximum specific growth rates, photosynthetic efficiency (F_v/F_m), cell-size plasticity, and harmful traits, including extracellular polymeric substance (EPS) production and brine shrimp lethality of each species. Warming accelerated the initial growth, whereas the maximum carrying capacity declined at higher temperatures. G. impudicum exhibited high thermal tolerance, maintaining stable F_v/F_m values even at 28°C, whereas H. pygmaea displayed higher photosynthetic sensitivity to temperature. G. impudicum showed significant cell volume enlargement at 28°C at the end of incubation. Considering species-specific bioactivities, G. impudicum significantly increased EPS secretion per cell at higher temperatures, whereas H. pygmaea showed enhanced mortality effects on Artemia salina. Under co-incubation conditions, the cell density of each species was consistently lower than that in monoculture across all temperatures, and the reduction in maximum cell density became more pronounced with warming. Growth suppression was consistently stronger in H. pygmaea, likely owing to allelopathic effects from G. impudicum. These findings indicate that under future climate change scenarios, HABs may be accelerated by temperature-dependent changes in growth, physiology, harmful traits, and competitive interactions.