Tungsten is biologically essential in some primitive micro-organisms that thrive in very hot environments, as around deep sea thermal vents. However, it is not known to be essential for animal or plant life. Molybdenum-based enzymes, however, are essential for some metabolic processes in humans and animals and for the incorporation of nitrogen into plants. Because of the similar chemistries of tungsten and molybdenum, tungsten may compete with molybdenum and block some biological processes, for example uptake of nitrate from the soil. This Biology Section of the Database includes papers that describe tungsten enzymes, tungsten’s competitive inhibition of molybdenum-dependent enzymes and other interactions with biological systems.