Why can high concentrations of heavy water harm plants and animals?

High concentrations of heavy water (deuterium oxide, or ${\text{D}}_2\text{O}$) are harmful to plants and animals because they disrupt the delicate balance of biological processes.1

While hydrogen isotopes are often used as isotopic tracers because they behave similarly to ordinary hydrogen, the chemical properties of deuterium and tritium differ from those of ordinary hydrogen in both thermodynamic and kinetic terms. Specifically, chemical reactions involving these heavier isotopes proceed at measurably different rates—a phenomenon known as the kinetic-isotope effect. Because biological systems rely on precise reaction rates and hydrogen-bonding networks to maintain the structure and function of proteins and enzymes, the substitution of deuterium for hydrogen can impair these essential life-sustaining mechanisms. Consequently, neither plants nor animals can thrive in environments where heavy water is present in high concentrations.12