Biotic Transformation of Abiotically Stable Nanoscale UiO-66 Metal–Organic Framework by <i>Daphnia magna</i> Results in Chronic Reproductive Toxicity

Metal-organic frameworks (MOFs) are entering water technologies on the premise that abiotic stability predicts ecological safety. We overturn this assumption by showing that UiO-66 - often regarded as chemically and structurally robust - remains intact after 7-day aging in natural borehole water yet undergoes rapid <i>in vivo</i> transformation in <i>Daphnia magna</i>. Synchrotron Microfocus X-ray absorption spectroscopy (XAS) revealed collapse of the ordered Zr-carboxylate coordination into disordered Zr-O environments within the gut; Extended X-ray Absorption Fine Structure (EXAFS) showed loss of second-shell features, and Transmission Electron Microscopy (TEM) confirmed loss of crystallinity with nanoscale aggregates appearing within 24 h of ingestion. Although acute immobilization was limited (48 h EC₅₀ ≈ 26.5 μg mL^-1), a sublethal, environmentally relevant exposure (10 μg mL^-1) caused pronounced chronic effects: brood initiation was delayed by 3-5 days and cumulative reproduction decreased by ∼74% without mortality. We attribute these outcomes to gut-level transformation and associated energetic/physiological burdens, not captured by standard acute tests. These results show that abiotic stability does not necessarily imply biological inertness and highlight the need to integrate <i>in vivo</i> transformation pathways with chronic end points in environmental risk assessment for water-sector materials. This perspective provides a mechanistic basis to inform Safe-and-Sustainable-by-Design (SSbD) MOFs before widespread deployment in water treatment.