Effects of marine heatwaves and marine cold spells on toxin production in Alexandrium catenella

Marine heatwaves (MHWs) and marine cold spells (MCSs) are increasing in frequency and intensity, emerging as major climate-driven stressors in coastal ecosystems. In northern Patagonia, these thermal extremes have been associated with shifts in plankton communities and an increasing incidence of harmful algal blooms (HABs), particularly those dominated by Alexandrium catenella, the main producer of paralytic shellfish toxins (PSTs) in Chile. However, the physiological responses of HAB species to thermal extremes remain poorly understood, limiting their use as ecological indicators of HAB risk under climate variability. We experimentally evaluated the effects of simulated MHWs and MCSs on cell density, pigment concentration and toxin production in a monoclonal A. catenella strain isolated from northern Patagonia. Extreme thermal treatments were imposed using progressive temperature changes of +1 °C day⁻¹ to 16 °C for the MHW treatment and −1°C day⁻¹ to 8 °C for MCSs treatment, followed by two exposure–recovery cycles, followed by two exposure–recovery cycles. Responses were assessed by quantifying cell density, pigment content, and PST composition (neoSTX, STX, GTX2/3, GTX4/1). MHW exposure promoted transient increases in cell density and significantly enhanced cellular toxin quotas, particularly GTX2/3. In contrast, MCSs produced contraction–recovery dynamics in population size with moderate effects on toxin profiles. Multivariate analyses (PERMANOVA) revealed significant temporal variation, across exposure–recovery cycles. Overall, our results demonstrate that simulated thermal extremes can modify cell density and toxin production in A. catenella, with MHW-like conditions intensifying toxin production and altering toxic profiles.