“BIOLOGICAL EFFECTS are observable for high doses of D2O. While bacteria or yeasts can function in practically pure D2O, albeit with somewhat hindered growth rate, for higher organisms damaging effects on cell division and general metabolism occur at around 25% deuteration, with lethal conditions for plants and animals typically occurring at ~40–50% deuteration of the body water.”
https://www.nature.com/articles/s42003-021-01964-y