Biotic transformation of abiotically stable nanoscale UiO-66 metal–organic frameworks by the waterflea Daphnia magna 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 ageing in natural borehole water yet undergoes rapid in-vivo transformation in Daphnia magna. Microfocus X-ray absorption spectroscopy revealed collapse of the ordered Zr–carboxylate coordination into disordered Zr–O environments within the gut; EXAFS showed loss of second-shell features, and TEM confirmed loss of crystallinity with nanoscale aggregates appearing within 24 h of ingestion. Although acute immobilisation 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. Our results establish a general principle—abiotic stability ≠ biological inertness—and argue that environmental risk assessment for water-sector materials must integrate in-vivo transformation pathways with chronic endpoints. This provides a mechanistic basis for Safe-and-Sustainable-by-Design of MOFs before widespread deployment in water treatment.