Weeding out the secrets of Red Sea macroalgae

Marine and coastal plant-based ecosystems, including mangroves, seagrass meadows and macroalgae (seaweeds), play a significant role in capturing and storing atmospheric carbon. Understanding these blue carbon resources is increasingly important in tackling climate change, so KAUST researchers are finding out more about the understudied realm of the Red Sea’s macroalgae species^[1].

“Not many people will think of seaweed as a versatile tool. But alongside sequestering carbon, certain macroalgae species hold potential for bioremediation, helping restore polluted coastal areas and improve overall ecosystem health,” says Chunzhi Cai, former Ph.D. student at KAUST. “Some species are excellent at filtering out metal contaminants, for example, while others can help prevent eutrophication and algal blooms.”

For Saudi Arabia, where oil refineries and industrial activity contribute significantly to emissions, understanding the health and the role of macroalgae is particularly relevant. Alongside its carbon-sequestration capabilities, macroalgae are often used as ingredients in food, pharmaceutical and beauty products. Hence, a comprehensive analysis of nutrients and pollutants is critical to safeguard human health. 

Cai collaborated with colleagues, under the supervision of KAUST faculty Susana Agusti and Carlos M. Duarte, to conduct a comprehensive analysis of 161 macroalgae samples collected from 45 sites along the Saudi Arabian Red Sea coast. They determined the concentrations of 22 chemical elements, including nutrients and heavy metals, in the 19 different species of macroalgae sampled.

Their results revealed high levels of potassium, sodium and sulphur in many species, which can be attributed to the Red Sea’s unique high salinity, low rainfall rates and high evaporation levels.

There were significant differences in nutrient and trace metal levels depending on the location and habitats the macroalgae came from. For example, sediments can trap pollutants such as metals, resulting in macroalgae harvested from coastal seagrass meadows exhibiting higher metal accumulation compared to those from coral reefs, where sediment is limited or absent. Macroalgae in the southern Red Sea showed higher levels of total organic carbon, nitrogen, phosphorus and cadmium compared to those in northern locations.

“This trend is influenced by nutrient inflows from the Indian Ocean and the unique semi-enclosed nature of the Red Sea,” says Cai. “Crucially, we also showed that as macroalgae absorb more heavy metals, their ability to store carbon is potentially reduced.”

The macroalgae Amphiroa fragilissima, Padina sp. and Udotea flabellum had the highest trace metal contents of all samples taken. A. fragilissima shows considerable potential as a bio-remediator, given its ability to absorb large amounts of metals like chromium.

The team also identified the Red Sea’s Halymenia species as particularly nutritious, providing a vital food source for marine creatures. Halymenia may also be suitable for human consumption. Another macroalgae species with high potassium content may be useful for agricultural fertilizers.

However, the researchers also uncovered worrying contamination trends in certain locations. Samples collected near several Saudi coastal towns exhibited chromium and nickel levels that exceeded toxicity thresholds.

“Given that the Red Sea is a shared environment, regional collaboration is crucial to safeguard marine ecosystems,” concludes Agusti. “All Red Sea nations need to establish coordinated pollution management strategies. Regional initiatives could include joint monitoring programs, data sharing and the development of common regulations for protecting and utilizing these valuable ecosystems.”

Reference

1. Cai, C., Anton, A., Duarte, C.M. & Agusti, S. Spatial variations of nutrient and trace metal concentrations in macroalgae across blue carbon habitats of the Saudi Arabian Red Sea. Science of the Total Environment 956: 177197 (2024). ).| article