Sargassum spp. extract elicits toxic responses and malformations in zebrafish (Danio rerio) embryos

In the last few years, the amount of pelagic Sargassum spp. arriving on the coasts of the Mexican Caribbean have increased dramatically. This poses a severe threat to the stability of coastal ecosystems and the health of marine animals. Still, few studies have examined the direct toxicological effect of sargasso exposure on fish fauna. In this study, we used a 96-hour static bioassay to determine the toxicological effects of increasing concentrations of Sargassum spp. ethanolic extracts on zebrafish (Danio rerio) embryos (ZFE). The estimated median lethal concentration of the extract was 251 mg/L, and 400 mg/L caused 100% death. Exposed embryos exhibited extensive morphological changes, including edema in the yolk sac, scoliosis, and loss of pigmentation, as well as malformations of the head, eyes, and tail. These abnormalities were integrated using the General Morphological Score (GMS) and Integrated Biological Response (IBRv2) indices, through which we observed a greater biological response in ZFE exposed to 200 mg/L. Comparative transcriptomic analysis of ZFE exposed to 200 mg/L of Sargassum spp. ethanolic extract uncovered 66 up-regulated and 246 down-regulated genes. Several gene ontology (GO) enriched terms, including cellular response to chemical stimulus, transcriptional regulation, pigment metabolic process, erythrocyte differentiation, embryonic hematopoiesis, extracellular matrix organization, and chondrocyte differentiation involved in endochondral bone morphogenesis were identified among the up-regulated genes, while repressed genes included those associated with nervous system processes, sensory and visual perception, response to abiotic stimuli, and nucleoside phosphate biosynthetic processes. Our findings indicate that exposure to Sargassum spp. has a wide range of harmful effects on zebrafish embryos, implying that its massive arrival along the coasts may also have a negative impact on the early stages of wild species.