Written by Jasmine Haskell and edited by Danielle Moloney
Fig. 1 Sargassum piles up in the coast of Cancun, Mexico. Taken from: https://thecoastalstar.com/profiles/blogs/along-the-coast-sargassum-surge-viewed-as-disaster-in-caribbean.
If you live in or have travelled to the Caribbean in recent years, you may have had the chance to catch a whiff of Sargassum, a brown algae that has plagued many picturesque coastlines with seemingly endless thickets of seaweed. That characteristic rotten egg smell from these seaweed thickets is caused by the emission of hydrogen sulfide gas, which at high concentrations can be deleterious to human health following continued exposure. Not only do these swaths of Sargassum pose a health risk to humans, but they have negatively impacted the tourism industry in many small island-nations, cost millions of dollars to remove, and have had detrimental impacts on fisheries and the ecosystem at large by smothering seagrass beds and coral reefs. However, there is potential to turn this Sargassum biomass into a valuable resource for biofuel production, offering an environmentally friendly solution while addressing the challenges posed by its accumulation on coastlines.
While blooms of Sargassum are naturally occuring, they have historically been contained around the aptly named Sargasso Sea and the Gulf of Mexico with occasional on-shore landings. However, starting around 2011, massive mats of Sargassum started smothering the entire Atlantic basin and washed ashore en masse. These massive Sargassum blooms in the Atlantic are the largest in the world and stretch across a region now regarded as the “Great Sargassum Belt”. Blooms in recent years have been recorded at 6600 km long, stretching from West Africa to the Caribbean and can be seen from satellites. Record amounts of Sargassum were recorded in March and April of this year by a research group at the University of South Florida with peak biomass expected to reach record highs next month.
Fig. 2 Satellite imagery of Sargassum blooms in recent years with June of this year expected to have record biomass. CS (Caribbean Sea) GoM (Gulf of Mexico). Image provided by University of South Florida Optical Oceanography Laboratory.
The underlying cause of these massive blooms is still up to debate, but several theories suggest an increased influx of nutrients from both the Amazon and Congo basins, changes in upwelling patterns off the coast of Northwest Africa and in open oceans, changes in depositional patterns of African dust and increases in sea surface temperature could individually or synergistically be responsible. While in the last decade these blooms have wreaked havoc, these floating Sargassum beds are actually vital to the regular functioning of the marine ecosystem as they serve as nurseries and protection for sea turtles and fishes. The problem now lies in the sheer volume and density of these blooms. While there are several innovative exploitative strategies currently in place, it is important to carefully consider the environmental and economic implications of these approaches such as to not disturb the ecosystem services that Sargassum beds do provide in the pelagic zone.
With this phenomena having such a large sphere of influence, many teams from around the world have been searching for novel ways to dispose of or repurpose the Sargassum beyond dumping it in landfills. One such team from New Zealand has come up with a new approach to turn the Sargassum into biofuel and fertiliser. As Sargassum does not degrade easily, Terrell Thompson and their team have combined the technologies of hydrothermal pretreatment and anaerobic digestion (digestion in the absence of oxygen) to aid in the conversion and breakdown of Sargassum into a viable biofuel.
Hydrothermal pretreatment is a green technology that involves using high pressure and high temperature to break down resistant biostructures. Treating Sargassum for 30 minutes with this method aids in breaking down the tough cellulose and lignin structures. Not only does this process yield more energy than non-treated Sargassum, but it also reduces the amount of hydrogen sulfide gas produced. Once pre-treated, it moves to the anaerobic digestion stage where the Sargassum is mixed with food waste. The biogas produced can then be used for electricity while the remaining solids can be used as a nutrient-rich fertiliser.
Fig. 3 An overview of the biorefinery process. HTP (hydrothermal pretreatment); AD (anaerobic digestion); AS (ammonia stripping); DSF (desulfurisation); CHP (combined heat and power). Dashed lines represent recycled components. (Thompson et al., 2021)
Thompson et al. (2021) propose to use Barbados as a case study for the biorefinery suggesting that it would keep 15,000 tonnes of Sargassum out of landfills while also providing electricity to the national grid and producing biofertilizer for local and international markets. While the authors realise that this is not a mitigation technique tackling the blooms from occurring in the first place, it is an upcycling technology with a lot of potential for tourism-reliant countries.
To tackle the greater issue of diminishing these blooms requires more research into the specific causes of these blooms. More preventative solutions could involve measures to decrease nutrient runoff from agricultural and urban areas and should primarily aim to reduce greenhouse gas emissions that are causing these increased sea surface temperatures and increased CO2 concentrations.
References
Dabor Resiere, Hossein Mehdaoui, Jonathan Florentin, Papa Gueye, Thierry Lebrun, Alain Blateau, Jerome Viguier, Ruddy Valentino, Yannick Brouste, Hatem Kallel, Bruno Megarbane, André Cabie, Rishika Banydeen & Remi Neviere (2021) Sargassum seaweed health menace in the Caribbean: clinical characteristics of a population exposed to hydrogen sulfide during the 2018 massive stranding, Clinical Toxicology, 59:3, 215-223, DOI: 10.1080/15563650.2020.1789162
Elizabeth M. Johns, Rick Lumpkin, Nathan F. Putman, Ryan H. Smith, Frank E. Muller-Karger, Digna T. Rueda-Roa, Chuanmin Hu, Mengqiu Wang, Maureen T. Brooks, Lewis J. Gramer, Francisco E. Werner (2020) The establishment of a pelagic Sargassum population in the tropical Atlantic: Biological consequences of a basin-scale long distance dispersal event, Progress in Oceanography, Volume 182, 102269, https://doi.org/10.1016/j.pocean.2020.102269.
Terrell M. Thompson, Pedram Ramin, Isuru Udugama, Brent R. Young, Krist V. Gernaey, Saeid Baroutian (2021) Techno-economic and environmental impact assessment of biogas production and fertiliser recovery from pelagic Sargassum: A biorefinery concept for Barbados, Energy Conversion and Management, Volume 245, 114605,
https://doi.org/10.1016/j.enconman.2021.114605.
Ute C. Marx, John Roles, Ben Hankamer (2021) Sargassum blooms in the Atlantic Ocean – From a burden to an asset, Algal Research, Volume 54,102188, https://doi.org/10.1016/j.algal.2021.102188.
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