What viruses make up a red tide bloom?

TAMPA – A study led by researchers at the University of South Florida has for the first time identified viruses associated with the organism Karenia brevis, which often leads to bouts of red tide.

In the study recently published in the American Society for Microbiology’s journal mSphere, researchers said an examination of water samples from off of Southwest Florida found several viruses percolating in red tide blooms.

The viruses are largely not harmful to humans in the traditional sense, but they can help researchers gain insights into the development of blooms that can plague coastlines for weeks, months, or even years at a time.

“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,” Jean Lim, the study’s lead author and researcher at the USF College of Marine Science, said in a statement. “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.”

The study’s findings were heavily based on water samples collected off Southwest Florida during what was considered a severe bloom in 2021. 

The red tide event was the worst in recent memory along Florida’s Gulf Coast, with widespread reports of fish die-offs and respiratory irritations, from Sarasota to Naples. 

The extensive bloom even impacted the manatee population, resulting in hundreds of deaths of the giant sea cows.

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Using a technique called viral metagenomics, researchers identified multiple viral species, including one previously unknown virus, among the algae. 

Most of the viruses belonged to the order Picornavirales and the family Marnaviridae, which are considered important in regulating marine ecosystems.

By better understanding the viruses, researchers may hold a potential key to controlling the growth of blooms and triggering their decline.

“There may be a correlation between viral abundances and bloom dynamics,” Lim stated. “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 is near its end. Researchers could use information about viral abundances to help predict bloom cycles.”

Additionally, by understanding what makes a bloom grow, there theoretically could be advanced alerts before an event impacts a coastal region.

Traditionally, marine experts have relied on satellite images and ocean circulation models to track the movement of blooms, but by understanding the viral dynamics, researchers may be able to identify certain patterns and predict the onset or decline of a red tide event.

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In 2024, satellites detected significant levels of chlorophyll in the Gulf, which can sometimes signal the onset of a red tide event. 

The bloom gradually began to affect coastal regions across Southwest Florida in early 2025, prompting health alerts for several communities.

Rather unexpectedly, during the spring, samples started to show fewer traces of Karenia brevis, suggesting that the bloom might be in its waning stages.

The recent event is one that university researchers may be able to develop a timeline for once viruses and their impacts on microscopic algae are better understood. 

According to the Florida Fish and Wildlife Conservation Commission, blooms are most commonly found in the Sunshine State during late summer and fall, but they can occur year-round and be detrimental to marine life and the tourism industry.