New research highlights Vibrio risks in coastal seaweed habitats
Opportunistic macroalgae like sea lettuce (Ulva) are increasingly common in coastal ecosystems. New research in Narragansett Bay found that these fast-growing species may harbor higher concentrations of potentially pathogenic Vibrio bacteria.
As seaweed farming grows in popularity across New England, new research supported by Rhode Island Sea Grant has found that common seaweeds in Narragansett Bay can harbor naturally occurring bacteria that under certain conditions may pose health risks.
Researchers from Boston University and Roger Williams University examined seven types of macroalgae, or seaweed, including familiar species like sea lettuce (Ulva) and Gracilaria, which are common in coastal waters and widely used in food, aquaculture, and other marine industries.
The researchers found that these seaweeds can host two types of Vibrio bacteria—Vibrio parahaemolyticus and Vibrio vulnificus. Both occur naturally in marine waters but can cause gastrointestinal illness or, in rare cases, serious infections, especially in people with open wounds or weakened immune systems.
Published in Frontiers in Marine Science, the study is one of the first to measure potentially harmful Vibrio bacteria living directly on seaweed surfaces in a temperate estuary.
“This isn’t necessarily a new issue, it’s just one we haven’t been looking for until recently. The conversation is starting to emerge, but that doesn’t mean it hasn’t been happening all along,” says co-author Dr. Robinson Fulweiler from Boston University. “Without consistent monitoring, it’s hard to say how long it’s been present.”
Alex Geisser, a Ph.D. candidate in the Fulweiler Lab at Boston University and lead author, along with Fulweiler and others from the lab collected seaweed samples from three locations in Narragansett Bay during summer and early fall of 2022, a time when water conditions typically favor both seaweed and Vibrio growth. Laboratory tests confirmed the presence of these bacteria, and in many cases, also detected genes associated with their ability to cause disease.
“We focused sampling on three different sites within Narragansett Bay, all easily accessible from docks or rocks. The idea was to sample in places where people would likely be interacting with the environment,” says Geisser, explaining that they used the Food and Drug Administration’s (FDA) method for enumerating Vibrio in oysters and applied it to macroalgae. “This work combined traditional microbiological methods, such as manual colony counting, with molecular techniques like quantitative PCR (qPCR) to confirm species identity. We further our analysis by screening for specific pathogenic genes.“
Macroalgae provide a structurally complex habitat that may serve as a refuge for Vibrio bacteria in Narragansett Bay. As seaweed habitats expand due to climate and nutrient shifts, scientists are calling for increased monitoring to understand the public health implications.
Notably, the highest concentrations of both pathogens were found on opportunistic, fast-growing seaweed species that are increasingly dominant in coastal waters influenced by warming temperatures and nutrient input. In Rhode Island, nutrients primarily come from sources like lawn fertilizers, septic systems, and wastewater discharge—all of which can fuel excessive seaweed growth. It’s a trend that Geisser notes is happening worldwide.
We need to be aware of this potential pathway for exposure.
“Globally, shifts in ocean conditions are driving a widespread replacement of perennial, slower-growing macroalgae with more opportunistic species that grow and decay much more rapidly,” she says.
Interestingly, the study found that environmental factors such as temperature and salinity were not seen as direct predictors of Vibrio abundance. Instead, the structure and growth strategy of the seaweed themselves may offer these pathogens a protective refuge.
Although finding Vibrio on seaweed doesn’t necessarily mean it will make people sick, the study suggests seaweed may be an overlooked route for bacterial exposure, especially as its uses continue to grow. Seaweeds are not only harvested for food but are also used to wrap and transport shellfish or to improve water quality in aquaculture systems.
“We need to be aware of this potential pathway for exposure,” explains Fulweiler. “This study provides a call to action for monitoring seaweed for pathogens, and more research on the environmental conditions that help pathogenic Vibrio thrive on a variety of surfaces.”
As seaweed farming expands and coastal ecosystems continue to change, the findings raise important questions for seafood safety, aquaculture practices, and public health planning.
“We’re wrapping up a manuscript that looks at how Vibrio presence varies across oysters, macroalgae, and sediment collected from the same sites,” says Geisser. “We’re aiming to submit it by the end of the summer, so stay tuned!”
Frontiers in Marine Science
Macroalgae host pathogenic Vibrio spp. in a temperate estuary
–Meredith Haas, RISG Research Communications & Digital Manager