USF study identifies viruses in red tide blooms for the first time

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• The study is the first to identify viruses associated with Karenia brevis, the single-celled organism that causes red tide
• The findings are an initial step toward exploring viruses as biocontrol agents for red tide

TAMPA, Fla. (March 18, 2025) – A new study led by researchers at the University of South Florida shines light on the environmental drivers of red tide blooms.

Published in the American Society for Microbiology’s journal mSphere, the study is the first to identify viruses associated with Karenia brevis, the single-celled organism that causes red tide. By testing water samples collected from red tide blooms off southwest Florida, the researchers found several viruses — including one new viral species — present in K. brevis blooms.

Identifying viruses associated with red tide can help researchers forecast the development of blooms and better understand environmental factors that can cause blooms to terminate. The study marks an initial step toward exploring viruses as biocontrol agents for red tide.

“We know that viruses play an important role in the dynamics of harmful algal blooms, but we haven’t known what viruses might be associated with Karenia brevis blooms,” said Jean Lim, the study’s lead author and a postdoctoral researcher at the USF College of Marine Science (CMS). “Now that we’ve identified several viruses in red tide blooms, we can work to determine which viruses might have an influence on these events.”

To conduct the study, Lim and her team partnered with researchers from the harmful algal bloom monitoring and research program at the Florida Fish and Wildlife Conservation Commission’s (FWC) Fish and Wildlife Research Institute (FWRI), who collect samples during red tide events.

In the lab, Lim used a method known as viral metagenomics to identify the viruses present within bloom-containing water samples. Viral metagenomics was pioneered more than 20 years ago by Mya Breitbart, a Distinguished University Professor at CMS and the senior author of the recent study.

“Given the severe consequences of red tide events, it is surprising that no viruses infecting K. brevis have been described,” Breitbart said. “Viral metagenomics is a great tool for exploring viruses associated with these harmful algal blooms.”

Red tide blooms are complex problems. They’re naturally occurring events that are driven in part by environmental factors such as ocean circulation, nutrient concentration, and climate change. The neurotoxins emitted by K. brevis can kill marine life, cause respiratory issues for beachgoers, and impact coastal economies based around tourism and fishing.

Current monitoring efforts rely on satellite images of chlorophyll concentrations and field samples such as those taken by FWC-FWRI. Ocean circulation models operated by researchers at CMS can help forecast the movement of red tide blooms. A better understanding of viruses that influence red tide could improve long-term monitoring and forecasting efforts by signaling that a bloom will develop or terminate.

“There may be a correlation between viral abundances and bloom dynamics,” Lim said. “For example, an increase in the number of viruses found in a sample might suggest that a red tide bloom is about to begin, or that it is going to end. Researchers could use information about viral abundances to help predict bloom cycles.”

Since viruses target specific organisms, they may even provide an environmentally-friendly way to manage blooms.

“There could be specific viruses that may only infect Karenia brevis,” Lim said. “If we can identify and isolate those viruses, they may be used as a biocontrol agent that won’t have a broader negative impact on marine ecosystems.”

Moving forward, Lim and her colleagues will attempt to determine whether viruses identified in the recent study have an influence on K. brevis or other species that co-occur with red tide blooms.

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