Harvesting the Golden Tide

What are the possible applications of the seasonal influx of Sargassum on our island shores? Dr Anjani Ganase considers the research and looks at some of the ways that Sargassum is already used.

While sargassum, brown fleshy macroalgae, is known and readily used in many Asian countries, in the Caribbean, it is mostly regarded as a nuisance. While some species of sargassum grow attached to the sea bottom, the oceanic species that we’re exposed to seasonally grows on the surface of the open ocean forming great seaweed rafts. These rafts create shelter and a source of food for marine creatures out at sea, but when they enter shallow coastal areas, they smother and entangle marine life, and decay on our beaches and wetland habitats.

Since 2011, these golden tides – rafts of sargassum –- are regularly washing ashore on Caribbean and American coasts in large quantities. The two common oceanic sargassum species identified are Sargassum natans and Sargassum fluitans and originate from the southern Atlantic Ocean gyre. The rafts drift towards the Caribbean and West African coasts. It is thought that the combination of higher water temperatures and richer nutrient conditions from the discharges of the Amazon and rivers on the West African coast promotes such blooms of sargassum. Similar golden tides have impacted coasts around the world, including Japan, Korea, Taiwan, Algeria and even along the Great Barrier Reef in Australia.  As this becomes the new norm, strategies for safely disposing or repurposing it in a way that creates opportunity are being explored but research shows that the solutions are not so simple and require considerably more investigation for sustainable and safe solutions.

 

Sargassum in Speyside in 2020. Photo by Anjani Ganase

 

Many scientists have been researching macroalgae, including many sargassum species that occur naturally in the marine environment. Many Asian countries already use sargassum in their daily lives in food and medicinal supplements. Researchers note that macroalgae are like sponge, great absorbers of the surrounding environmental conditions and may be used as bioindicators of water quality in marine environments. As one example, sargassum is known to absorb organic micropollutants, such as PAHs (polycyclic aromatic hydrocarbon - think about the sticky residue left behind from the combustion of fossil fuels). Scientists found that there were significant elevations of these pollutants in beached sargassum after the deep-water horizon oil spill in the Gulf of Mexico compared to before. Similar findings also occur in Trinidad and Tobago, where there was higher PAHs in sargassum that grew in areas of higher human activity (Trinidad’s west coast) versus more remote areas in Tobago. Another study highlighted high concentrations of pesticides, another organic pollutant, in oceanic sargassum collected in bays adjacent to banana estates in Martinique.

TESTING FOR HEAVY METALS

Sargassum is also known to have high absorption of heavy metals, such as cadmium, iodine and lead. It is particularly good at taking in arsenic, and high levels of inorganic arsenic can be very toxic. The mechanism for this uptake seems to be an accident. Researchers found that the phosphate transporters present in the algae would mistake arsenic for phosphate owing to similar chemical structure. This results in the build-up of arsenic in the tissues of sargassum species over time, even in the most pristine ocean environments. In its raw state, the concentrations of cadmium, lead and iodine are above the regulatory levels for safe consumption or for feeding livestock and even for wholesale disposal owing to the risks of heavy metal pollution. Therefore, many of the potential uses of sargassum require treatment to reduce its toxicity.

While many Asian countries have been consuming brown algae in their foods and in traditional medicines for centuries, the use of sargassum in food in other regions is a novelty with uncertain benefits. It must be noted that much of the sargassum used in Asian countries are grown and harvested on coastal areas. The use of beached sargassum brings additional challenges, such as unknown content and environmental quality, along with the degree of decay that has occurred. Such uncertainties limit the use of drifting and beached sargassum directly in food; analysis of the nutrition content indicates that sargassum is a poor choice for human consumption.

With processing, the use of sargassum expands considerably.  For example, a process such as anaerobic digestion in which microorganisms convert the carbohydrates into methane for fuel allows sargassum to be used in biofuels generation. Certain compounds may be extracted for pharmaceuticals and cosmetics. As sargassum is rich in carbohydrates, mixing sargassum into animals feed has shown some benefit to overall heath and growth rates. Adding sargassum to the diet of fish and shrimp also provided some improved resistance to diseases. In other animals, such as poultry and ruminants, there were mixed results that require further investigation. Of concern is the regulation of the salt and heavy metal intake by the animals; so treatment methods must ensure that it becomes suitable for consumption. The simplest preparation used in many Asian countries, is boiling for long periods. This seems to remove up to 90 % of the heavy metals depending on the temperature and pH.

 

Sargassum washed up at Lambeau in 2021. Photo by Anjani Ganase

COMPOSTING

Unfortunately, a major challenge continues to the high variability in the composition of the sargassum species where percentages of the carbohydrates, proteins, metals vary from place to place, making standard production difficult to regulate. Strategies include harvesting the sargassum before it reaches land, but even the prediction of sargassum influx poses a challenge. Here in the Caribbean, sargassum collected on beaches has been used by coastal farms as a form of fertilizer, especially for organic farms that are in demand. Several studies show that the mixing of sargassum into compost of wood chip and local food waste significantly boosts the nutrient quality. However, there are growing concerns about the transfer of the heavy metals to the vegetable produce. A recent study showed this to be the case and speculated that further investigations on treatments before use or the use of reduced amounts may yield safer produce. The other major concern is soil salinization, when the sargassum is added directly to the soil - instead of composting - and causes the soil to become too salty and ultimately less suitable for crops. Research into the uses of sargassum continues across the region with significant head way. Some scientists also suggest studies to observe the natural ecological changes when sargassum is left on coastal areas: what are the solutions provided by nature.

  

REFERENCE:

Damien Devault, Ronan Pierre, Hélène Marfaing, Franck Dolique, Pascal Jean Lopez. Sargassum con- tamination and consequences for downstream uses: a review. Journal of Applied Phycology, Springer Verlag, 2020, 10.1007/s10811-020-02250-w . hal-03032938