How a Fast Food Feast Can Create a Toxic Beast: Investigating the Anthropogenic Impact of Toxic Algae Pseudo-nitzschia

Most of my childhood memories are rooted in the Florida Panhandle—white sand beaches, crystal-clear emerald waters, and vibrant marine life. It felt like paradise. But even amid those idyllic scenes, I remember moments of devastation: the 2010 Deepwater Horizon oil spill, the lingering effects of Hurricane Katrina, and most vividly, the toxic algal blooms.

Those blooms were the hardest to understand. One day the beach was pristine; the next, it was a tainted wasteland. The water turned a rusty red, and you couldn’t step outside without coughing up a lung. As a child, I struggled to grasp how something so beautiful could turn toxic overnight—but more importantly, I wished there was something I could do about it.

A Southern gentleman turned SoCal beach regular, I began my PhD hoping to answer the questions I once couldn’t. I now study harmful algal blooms (HABs) under the guidance of Dr. David Hutchins and Dr. Feixue Fu at the University of Southern California, where our research focuses on Pseudo-nitzschia—a native California alga known for producing the potent neurotoxin domoic acid.

Pseudo-nitzschia is no stranger to California’s coast; it appears annually to some degree. Our region’s productive marine ecosystem is powered by wind-driven upwelling, which brings nutrient-rich deep waters to the surface. Unfortunately, these same conditions allow Pseudo-nitzschia to thrive—often outcompeting other phytoplankton and producing harmful levels of domoic acid. This toxin accumulates in filter feeders, like mussels and anchovies, which can pass it along the food chain. Intoxicated animals have made headlines for biting surfers or stopping coastal roads during rush hour traffic. In the most severe cases, Pseudo-nitzschia blooms have led to mass die-offs of sea lions and dolphins, and have disrupted fisheries, sometimes costing millions in lost revenue.

While marine biologists have identified broad patterns in bloom dynamics, California’s diverse coastal landscapes and extensive human activity create many unknowns. One key driver of toxicity is nutrient imbalance—essentially, a “bad diet” for the algae.

Climate change and anthropogenic impacts are shifting the availability and composition of nutrients in coastal waters. My job is to collect Pseudo-nitzschia isolates during toxic events and run multivariate incubation experiments to understand how different stressors—especially those tied to human activity—affect bloom toxicity. This work can help us predict how blooms might change in the future and inform more effective coastal management strategies.

Originally, I planned to spend this summer comparing how blooms differ between California’s spring and fall seasons—and how climate change might influence those patterns. But nature had other plans. This year’s bloom arrived months earlier than expected and became one of the most toxic events we’ve seen in recent years. Hundreds of sea lions and dolphins washed ashore, and several coastal regions closed to fishing. Luckily, I was aboard a research cruise near Catalina Island just days before the first public reports of the bloom—and was able to collect seawater samples at just the right time. Since then, our lab—along with many collaborators—has been working to understand what drove this early, intense bloom.

One of my leading hypotheses is that runoff from recent wildfires in Los Angeles may have played a role. During our cruise, we didn’t detect signs of significant upwelling, but the bloom followed shortly after a major rainstorm that likely flushed fire debris into the ocean. This has sparked an exciting line of inquiry: we’ve been “feeding” Pseudo-nitzschia different types of fire-derived materials and monitoring their responses. So far, the algae appear to benefit from certain types of debris, though we’re still working to pinpoint which components are responsible.

We’re also testing materials used in wildfire suppression to see if they contribute to Pseudo-nitzschia‘s rapid growth or competitive edge, given that wildfires are becoming more frequent and intense in California. While it’s clear they can alter nutrient inputs to the ocean, we still don’t fully understand how this affects harmful algal blooms. If we can identify specific runoff or fire-related compounds that trigger domoic acid production, we could help guide more sustainable fire management practices and improve coastal response strategies.

There are many working groups that are helping maintain the health of our coastal waters and marine life during this recent bloom. I hope my research helps shape policies that protect our marine ecosystems in a changing climate—and inspires a broader awareness of how deeply our actions on land are connected to life in the ocean.

Bradley Mackett is supported by the USC Dornsife Wrigley Graduate Fellowship.