Allison Plante
Along the Southeastern coast of Florida down to the Brazilian coastline, the annual accumulation of floating seaweed in the shallow tropical waters and along the shoreline is a familiar sight for the beaches of this region. This mass of brown seaweed that washes up on the tropical shores is largely composed of a macroalgae called Sargassum. A majority of the Sargassum bloom originates from a region of the Atlantic Ocean between South America and Africa called “The Great Atlantic Sargassum Belt”. Within this oceanic belt, warm sea surface temperatures and calm conditions provide an ideal environment for macroalgae growth. During the summer months from July through September, these conditions are amplified, resulting in a denser and more abundant algal bloom. Over time, with climate change increasing overall ocean temperatures and nutrient loads, the Sargassum bloom will continue to grow larger and more frequent in the region.
With larger amounts of Sargassum forming and more frequent oceanic storm currents causing the algae to wash up along shorelines, there is a growing concern for the health and safety of not only the beaches but also the nearby human populations. The microbial composition of stranded and pelagic (growing in open ocean) Sargassum is a major factor in this concern as more pathogenic Vibrio species, including Vibrio parahaemolyticus, have been found within macroalgae samples. This bacterium can cause gastrointestinal illnesses and is most commonly transmitted through direct ingestion or open wound exposure. Illness due to contact with this bacterium poses a high risk for younger children who may be inclined to play with stranded Sargassum on beaches. Multiple studies conducted at The University of Miami (Shibata et al., 2012) & (Lutes et al., 2024), produced a Quantitative Microbial Risk Analysis (QMRA) to evaluate Vibrio vulnerabilities within human and ecosystem health. The study emphasizes young children (ages 3-7) with pica eating disorders, where pica behaviors include consuming non-nutritional items such as sand, dirt, seaweed, etc. These children, playing in stranded Sargassum on the beaches, are at the highest risk for bacterial illness. According to these studies and the U.S. EPA (2011), children (ages 3-7) with pica disorder have an estimated ingestion rate up to 5 grams of Sargassum per event of consumption, compared to about 0.06 grams for children of the same age group without these behaviors. The overall risk of illness for the pica study group is about 22 times greater than the compared non-pica group for short-time stranded Sargassum contact (62% and 2.8% risk respectively). This presents a significant concern for families regularly visiting beaches which are experiencing an abundance of onshore macro-algae (Lutes et al., 2024). Any parents with children experiencing a pica disorder should be diligent in avoiding freshly stranded Sargassum.
Regarding the overall environmental health effects related to the increase of pathogenic microbial communities within Sargassum, current conditions will enhance the microbial growth rate and negatively interact with natural growth processes. As the water temperatures continue to rise and the discharge of nutrients and bacteria into ocean systems persists, the probability of illness also increases. Climate change effects such as warming waters, melting of ice caps, and the lack of oceanic mixing, result in an imbalance between dissolved organic carbon (DOC) levels and the demand of DOC from aquatic vegetation. Vibrio can consume 20-70% of DOC released by brown algae and outcompete other bacteria for DOC due to its fast-doubling rate and its lows sensitivity to iron and solar radiation (Michotey et al., 2020). This creates overall preferable conditions for V. parahaemolyticus to thrive within macro-algal blooms.
In conclusion, maintaining pelagic Sargassum species under culture conditions has proven to be a challenge, especially with a changing climate. There are known risks, especially for young children, for gastrointestinal illness due to Vibrio species in Sargassum, including but not limited to Vibrio parahaemolyticus. In terms of environmental engineering remediation, in-situ methods may be the best alternative. Sargassum microbial treatment for intended biofertilizer reuse is also applicable as an agricultural and environmental advancement method. Further research and QMRA is needed in this topic as conditions may fluctuate.
Citations
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