Mercury and selenium stable isotopes across contrasting food webs: marine insights, amazon priorities

Stable isotope approaches are rapidly transforming how we investigate trace-element cycling in living systems, offering information that goes far beyond concentration measurements. Mercury (Hg) stable isotopes, in particular, have proven highly informative across a broad range of environments, and marine studies have highlighted their value for disentangling sources and in vivo processing. Studies on apex marine predators (e.g., seabirds) show that Hg isotopes can track internal processing and trophic transfer. In contrast, selenium (Se) isotopic characterization in biota, more specifically in animals, is still limited and technically challenging, but it opens promising perspectives, especially given Se’s recognized antagonistic role in Hg toxicity.Key gaps persist in the Brazilian Amazon, where complex Hg (and Se) exposure scenarios call for higher-resolution tracers. Translating Hg and Se isotope approaches to Amazonian freshwater systems, from fish to riverside populations, may clarify bioaccumulation pathways and fate. Recent progress achieved in marine organisms, including compound-specific strategies, will be presented, together with the main analytical challenges and opportunities for extending these approaches to the Amazon.