A new UC San Diego study finds that petroleum-based microplastics can help drive harmful algae blooms by wiping out tiny animals that normally keep algae in check. Biodegradable plastics tested in the same experiment had a much smaller impact.
Toxic algae outbreaks that close beaches, kill marine life and threaten drinking water may be getting a hidden boost from plastic pollution, according to new research from the University of California San Diego.
In a three-month experiment using outdoor test ponds, UC San Diego scientists found that microplastics made from conventional, fossil fuel-based polyurethane triggered algae blooms by rapidly wiping out zooplankton, the tiny animals that normally graze on algae. Biologically based, biodegradable plastics tested side by side had a much smaller effect on these key organisms and on the broader pond community.
The study, published in the journal Communications Sustainability, adds a new twist to the story of harmful algal blooms, sometimes called red tides, which have been appearing more often and lasting longer in coastal waters and lakes around the world. These events can stretch for thousands of miles, as in a recent nine-month toxic bloom off Southern Australia that killed large numbers of marine animals and disrupted fisheries and tourism.
For years, scientists have largely blamed excess nutrients from farm runoff, sewage and other land-based pollution for fueling these outbreaks. That nutrient-driven process is often described as a bottom up problem: extra nitrogen and phosphorus pour into rivers, lakes and oceans, supercharging algae growth. When the algae die and decompose, they strip oxygen from the water and create aquatic dead zones.
The UC San Diego team does not dispute that role. But their new results suggest plastic pollution can add a second, very different push toward hazardous water conditions.
Senior author Jonathan Shurin, a professor in UC San Diego’s Department of Ecology, Behavior and Evolution, noted that researchers are only beginning to understand how the flood of plastic into the environment reshapes living communities.
“We see all this plastic out there but how is it changing populations of algae, bacteria, seabirds or fish? We really don’t know,” Shurin said in a news release. “We know algae blooms are partly due to nutrient pollution, but this study is showing that some of the algae blooms that we see around the world may also be due in part to the effects of plastic on the animals that normally control algae.”
Instead of adding more nutrients, microplastics in this study acted from the top down, by harming consumers at the upper levels of the microscopic food web.
The research team, from UC San Diego’s Department of Ecology, Behavior and Evolution and Department of Chemistry and Biochemistry, set up 30 experimental pond ecosystems in outdoor tanks. They added either conventional petroleum-based polyurethane microplastics or newer biodegradable plastics, including a bio-based material developed in UC San Diego labs and commercialized by campus spinoff Algenesis. Other tanks served as controls.
Over three months, they tracked how algae, bacteria, zooplankton and other members of the pond communities responded.
Inside tanks with fossil fuel plastics, zooplankton numbers dropped sharply and quickly. Without these grazers, algae populations surged, clouding the water in blooms. In contrast, tanks with bioplastics showed much smaller changes in zooplankton and algae.
“The petroleum plastic seemed to have a strong negative effect on the zooplankton populations,” added first author Scott Morton, a graduate student in the Department of Ecology, Behavior and Evolution. “They seemed to either die off or reduce their reproduction very quickly. Bioplastic didn’t have the same effect. That cascades down to the algae. In the petroleum tanks, fewer zooplankton consuming all that algae means you have more in the system and that leads to the algal blooms that we saw.”
The researchers also saw distinct communities of bacteria emerge around the different plastics, though they do not yet know whether those microbes were directly breaking down the materials, feeding on other organisms, or both.
In their paper, the authors write that “Our results indicate that microplastics may tip the balance of conditions in favor of algal blooms,” and that “These results collectively illustrate that microplastics, particularly petroleum-derived plastics, may destabilize microbial community structure and function.”
The findings build on a growing body of work showing that microplastics — tiny fragments that form as larger items like bottles, bags and foam break apart — can alter aquatic ecosystems in subtle but important ways. Microplastics have now been found everywhere from deep-sea sediments to Arctic ice, and even in human blood, lungs and brain tissue.
Most previous studies have focused on direct toxicity to individual animals or on the physical damage caused when organisms ingest or become entangled in plastic. The UC San Diego experiment highlights a different kind of risk: that plastics can quietly shift the balance of entire food webs, making waters more prone to dangerous blooms even if nutrient levels stay the same.
The work also offers some cautious optimism about alternatives. For the past decade, co-author Michael Burkart, a professor in the Department of Chemistry and Biochemistry, and his group have been designing and commercializing bio-based plastics meant to break down in the natural environment. These materials are already being used in products such as surfboards, flip-flops and phone cases.
“It is critical for us to understand how these new materials compare to traditional petroleum plastics when discarded in the environment,” Burkart said in the news release. “While all man-made objects have an impact on the planet, our goal is to minimize the ecological and health hazards of these now ubiquitous materials.”
In this study, the biodegradable plastics still altered pond communities, but far less dramatically than the petroleum-based plastics. That suggests that shifting away from fossil fuel plastics could help reduce some of the ecological damage in lakes, rivers and coastal waters, the authors noted.
The team is now expanding its work to test a wider range of biodegradable materials, including what they describe as living plastic filled with bacterial spores. Those spores are designed to wake up and break down the material at the end of its life, potentially preventing long-lived fragments from accumulating in the first place.
Biodegradable plastics are not a license to litter. Any material produced and discarded at large scale will leave a mark on the environment. But by revealing how conventional plastics can push aquatic ecosystems toward toxic blooms, the researchers hope to give policymakers, companies and consumers better information to guide choices.
As harmful algal blooms continue to threaten coasts and freshwater supplies worldwide, the study underscores that tackling nutrient pollution alone may not be enough. Reducing fossil fuel-based plastic waste — and designing safer materials from the start — could be an important part of keeping waters clear, oxygen-rich and safe for both wildlife and people.