Chromium transformations and biological impacts in aquatic systems: From sediment-water interfaces to food web complexity.

Chromium (Cr), as a dual-state pollutant that is both essential (Cr(III)) and highly toxic (Cr(VI)), has attracted considerable attention for its environmental behaviour and biological effects in aquatic ecosystems. This review systematically analyses its speciation transformation, bioaccumulation and toxicity mechanisms based on the latest advances in multi-interface processes. Natural and industrial activities annually introduce approximately 1.3 × 10 tons of Cr into aquatic systems. The dynamic equilibrium of Cr at the water-sediment interface is regulated by factors such as pH, Eh, Mn oxides and organic matter, forming a dynamic cycle that influences Cr bioavailability. Different organisms have distinct Cr uptake pathways (e.g., passive adsorption by phytoplankton, dual uptake via gills and intestines in fish), and Cr(VI) exhibits significantly higher membrane permeability than Cr(III). Cr(VI) is reduced intracellularly to Cr(V/IV) and reactive oxygen species (ROS), triggering gill epithelial detachment, hepatocyte vacuolisation and immune dysfunction. Cr(III) participates in metabolic regulation at low concentrations but exhibits neurotoxicity at high concentrations; however, its essentiality threshold overlaps with the toxic dose of Cr(VI), complicating risk assessment. The transmission of Cr through aquatic food webs exhibits a contradictory pattern of 'freshwater dilution and marine local amplification', with the underlying mechanisms yet to be fully elucidated. This review summarises the latest understanding of the environmental behaviour and biological fate of Cr in aquatic ecosystems, highlights knowledge gaps and identifies future research priorities to better understand Cr bioaccumulation processes and mechanisms.