Multifaceted assessment of the content of potentially toxic elements and microplastics in the sargassum habitats of Caribbean beaches, Mexico.

The present study aimed to evaluate potentially toxic elements (PTEs) (Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, Li, Mg, Mn, Ni, Pb, S, Se, Sr, Tl, Zn) in sargassum habitats at two distinct sites: Puerto Morelos (PM) and Sian Ka'an Biosphere Reserve (SK) along the Mexican Caribbean coast. Median dissolved concentrations of S (1937.8 mg/L), Mg (1172.5 mg/L), and Ca (408.7 mg/L) were relatively higher than those of other elements studied. Similarly, elemental concentrations in sediment followed an order of Ca > Mg > Sr > S > Fe > Al > B > Ba > Mn > Cr > Se > Cu > As > Zn > Pb > Tl > Li > Ni > Co > Cd. The distribution pattern of the studied elements revealed significant spatial variability, with Puerto Morelos, a tourist destination, exhibiting high enrichment, and the calculated contamination indices reflected intense anthropogenic influences, including wastewater, plastic waste, and high levels of beach tourism. At the same time, Sian Ka'an showed relatively lower but still notable pollutant levels. In the case of sargassum, the mean total elemental concentrations were 4549.8 mg/kg (SK) and 3770.8 mg/kg (PM), indicating substantial bioaccumulation of certain PTEs in sargassum tissues. The calculated bioconcentration factors, namely bioconcentration and biota-sediment accumulation factors, indicate potential long-term ecological risks. Microplastics in sargassum tissues were prevalent in all the samples, with 193 particles in PM and 3578 particles in SK, respectively, and were mainly polyethylene, polypropylene, and polystyrene. The PCA biplot revealed that PM samples are primarily clustered along the positive axis of PC1, indicating a significant positive correlation with potentially toxic elements and suggesting common anthropogenic sources. Conversely, SK displayed a unique geochemical profile, characterized by a high BSAF for alkaline-earth and lithogenic elements. Calculated geochemical and ecotoxicological indices revealed high contamination in PM. To summarize, the quantitative results of this study highlight spatial differences and indicate that sargassum is a potential bioindicator for biomonitoring studies. Additionally, the findings of this study underscore the vulnerability of pelagic sargassum and coastal zones to metal and microplastic contamination, addressing the need for targeted management strategies to mitigate associated ecological and human health risks in sensitive coastal regions.