Award: OCE-2120619

Intellectual merit. Highly toxic Pseudo-nitzschia harmful algal blooms along the U.S. West Coast have been associated with two contrasting regional events: seasonal upwelling and marine heatwaves. While upwelling delivers cool water rich in CO₂ and an abundance of nutrients to the upper water column, marine heatwaves instead lead to warmer surface waters, low CO₂, and reduced nutrient availability. Understanding Pseudo-nitzschia dynamics under these two conditions is important for bloom forecasting and coastal management. To gain a better understanding of what drives Pseudo-nitzschia growth and toxicity during these complex events, multiple-driver scenario or ‘cluster’ experiments were conducted using temperature, CO₂, and nutrient levels reflecting conditions during upwelling (13◦C, 900 ppm CO₂, replete nutrients) and two intensities of marine heatwaves (19◦C or 20.5◦C, 250 ppm CO₂, reduced nutrients). While Pseudo-nitzschia grew equally well under both heatwave and upwelling conditions, similar to observations in the natural environment, cells were only toxic in the upwelling treatment. Single-driver experiments were used to gain a mechanistic understanding of which drivers most impact Pseudo-nitzschia growth and toxicity. These experiments indicated that nitrogen concentration and N:P ratio were likely the drivers that most influenced domoic acid production, while the impacts of temperature or CO₂ concentration were less pronounced. However, a further investigation of the global distributions of the toxic diatoms Pseudo-nitzschia spp and long-term adaption experiment with different species of Pseudo-nitzschia provide compelling evidence that sea surface temperature is also an important factor in determiningtheir distribution, toxicity, and metabolism, raising troubling questions about temperature-driven increases in toxic blooms in the future ocean. 

Broader impacts. This project supported three Ph.D students Kyla Kelly, Bradley MacKett and Ran Duan. It includes the second chapter of Kyla’s thesis, as well as her first author Harmful Algae paper published in 2023. Ran Duan was trained to use molecular tools to investigate the mechanisms of the domoic acid production under warming and ocean acidification. She is a co-author in the paper published in Marine Environmental Research in 2025. Bradley MacKett successfully isolated multiple strains and species of Pseudo-nitzschia during the bloom events. In addition to the Ph.D. students, two USC undergraduates (Amjad Mansour and Joseph Chang) were actively involved in the multiple Pseudo-nitzschia experiments. They have been working on this project for more than two years. They progressed from having only limited knowledge about harmful algal bloom species to being able to understand how environmental changes could affect the distribution and growth of the toxic diatoms Pseudo-nitzschia.

Last Modified: 04/05/2025
Modified by: Feixue Fu