In biology, diatoms are single-celled algae with cell walls made of silica. Found in nearly every water environment on Earth, they are a main source of food for zooplankton (such as fish larvae and marine microanimals) and an important source of atmospheric oxygen. Diatoms contribute to the development of harmful algae blooms (HABs), which in large part are caused by warming oceans. One species of diatom produces a dangerous neurotoxin that accumulates in algae blooms and spreads through fish populations.