The perfect pathogen storm: Sargassum and plastic debris

A new study led by the Florida Atlantic University (FAU) has examined how the highly pathogenic Vibrio bacteria, the macroalgae Sargassum spp., and plastic marine debris can interact to create a “perfect pathogen storm” threatening both marine life and public health.

Vibrio bacteria are abundant in ocean waters all over the world and represent the main cause of death in humans from marine environments. For instance, Vibrio vulnificus – known as “flesh-eating bacteria” – can lead to life-threatening foodborne diseases from seafood consumption, as well as deaths from open wound infections. 

However, the possible interactions of these pathogenic bacteria with the currently expanding Sargassum populations and the rising levels of plastic marine debris polluting the world’s oceans have not yet been explored.

Since 2011, Sargassum populations have been rapidly expanding in the Sargasso Sea and other parts of the open ocean, frequently leading to unprecedented seaweed accumulation events on beaches. 

Meanwhile, plastic debris, which is known to persist decades longer than natural substances in the marine environment, increases at an unprecedented pace in the Sargasso Sea and beyond. Still yet, genomic and metagenomic evidence as to whether vibrios colonizing Sargassum and plastic debris could potentially infect humans is currently lacking.

To address this issue, the researchers fully sequenced and analyzed the genomes of 16 Vibrio cultivars isolated from eel larvae, Sargassum, plastic marine debris, and seawater samples from the Sargasso and Caribbean Seas. The investigation revealed that Vibrio pathogens have a unique ability to adapt and “stick” to both Sargassum and microplastics, thus considerably expanding their range.

“Plastic is a new element that’s been introduced into marine environments and has only been around for about 50 years,” said study lead author Tracy Mincer, an assistant professor of Biology at FAU. “Our lab work showed that these Vibrio are extremely aggressive and can seek out and stick to plastic within minutes. We also found that there are attachment factors that microbes use to stick to plastics, and it is the same kind of mechanism that pathogens use.”

According to the experts, open ocean vibrios posses a blend of pathogenic and low nutrient acquisition genes, reflecting their pelagic habitat and the substrates and hosts they can colonize. For instance, a secreted toxin that increases intestinal permeability (known as “zonula occludens toxin” or “zot” genes and first described in Vibrio cholerae) was one of the most highly retained and selected genes in the vibrios examined. 

“Another interesting thing we discovered is a set of genes called ‘zot’ genes, which causes leaky gut syndrome,” Mincer explained. “For instance, if a fish eats a piece of plastic and gets infected by this Vibrio, which then results in a leaky gut and diarrhea, it’s going to release waste nutrients such as nitrogen and phosphate that could stimulate Sargassum growth and other surrounding organisms.”

Since many vibrios were found to have an “omnivorous” lifestyle targeting both plant and animal hosts and being able to persist in oligotrophic conditions, with the expansion of Sargassum and plastic debris, ways of infecting humans increase exponentially. 

“I don’t think at this point, anyone has really considered these microbes and their capability to cause infections. We really want to make the public aware of these associated risks. In particular, caution should be exercised regarding the harvest and processing of Sargassum biomass until the risks are explored more thoroughly,” Mincer concluded. The study is published in the journal Water Research. 

Sargassum blooms and impact

Sargassum is a genus of brown macroalgae (seaweed) that’s widely distributed throughout temperate and tropical oceans worldwide. There are around 150 to 350 species in the Sargassum genus. Sargassum is unique among seaweeds because some species are free-floating, meaning they don’t anchor themselves to the sea floor or any other solid surface.

The most famous area where Sargassum accumulates is the Sargasso Sea, an area of the Atlantic Ocean bounded by four currents forming an ocean gyre. Unlike all other regions called seas, it has no land boundaries. 

It’s distinctive for its deep blue color and exceptional clarity of water, caused by minimal amounts of nutrients and suspended particles. The Sargasso Sea is named after the Sargassum seaweed that floats en masse on the surface there.

Sargassum plays a vital ecological role by providing habitats and food for a diverse range of marine species. For instance, young sea turtles often shelter and feed within these floating mats, and many fish and invertebrates live in Sargassum beds.

However, in recent years, large blooms of Sargassum have caused issues when they wash ashore in huge quantities on the beaches of the Caribbean and the Gulf of Mexico. This phenomenon, called a Sargassum tide, is believed to be driven by a combination of human impacts such as nutrient pollution and climate change. 

These mats can smother coastal ecosystems, harm tourism, and emit foul odors as they decompose. Much research is ongoing to understand the causes of these Sargassum blooms and how they might be managed or mitigated.

More about marine plastic debris 

Marine plastic pollution is a serious environmental issue that has garnered increased attention over the past several years. This pollution comprises a wide range of products, from plastic bags and straws to microplastics, which are tiny plastic particles less than five millimeters in length.

Plastic enters the marine environment through numerous pathways. It can be washed into rivers and seas from landfills, blown by the wind, or directly dumped into the ocean. It can also originate from ships and fishing vessels in the form of discarded equipment or nets, which is known as “ghost gear”.

Once in the ocean, plastic can cause significant harm to marine life. Animals can become entangled in larger pieces of plastic waste, leading to injury or death. Many marine creatures, including birds, fish, and mammals, can also mistake plastic for food, resulting in blockages in their digestive systems when they ingest it, or they can starve because plastic makes them feel full.

Microplastics are particularly concerning because they can be ingested by a wide range of organisms and can accumulate up the food chain. They can also absorb and concentrate other pollutants, like pesticides and heavy metals, increasing their potential harm.

Moreover, the durability of plastics means they can persist in the environment for hundreds to thousands of years. They gradually break down into smaller and smaller pieces, but they do not biodegrade completely.

Combating marine plastic pollution requires a multi-faceted approach. This includes efforts to reduce plastic use and waste, improve waste management systems, develop biodegradable alternatives to conventional plastics, and implement clean-up initiatives in marine environments. Legislation and international agreements can also play a crucial role in regulating the production, disposal, and clean-up of plastic waste.

In addition, public education is important for raising awareness about the issue and promoting behavior changes that can reduce plastic waste. Many researchers are also studying the impacts of marine plastic pollution on ecosystems and human health, as well as exploring innovative solutions to this problem.

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By Andrei Ionescu, Earth.com Staff Writer

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