Decoding the marine biogeochemical cycling of mercury by stable mercury isotopes

The oceans, serving as some of the most extensive reservoirs of mercury (Hg) on Earth, play an essential role in the global Hg biogeochemical circulation, as a sink for atmospheric deposits and surface runoff while simultaneously emitting Hg to the atmosphere. Clarifying the mechanisms during cycling of Hg in marine ecosystems is a crucial segment in elucidating global Hg behavior and assessing its risks to wildlife and humans. The stable Hg isotope technique has emerged as a powerful tool for identifying potential sources and certain processes of Hg. Herein, the research advances in marine Hg biogeochemical cycling were reviewed from an isotopic tracing perspective, including sources of marine Hg, isotopic fingerprints across various marine media, and applications of Hg isotopes in pathways, transport, and transformation mechanisms associated with marine environments. The knowledge gaps and challenges were further discussed in the development of ultratrace level and species-specific isotopic analytical methods, the isotope fractionation mechanisms in reactions and processes, and the integration of multiple approaches and cross disciplines. These future studies would advance our understanding of Hg behavior in oceanic environments, its authentic function, and interactions with other ecosystems in the global Hg cycle, and evaluate its response to environmental changes.