A reference to minute plankton varying in size from 0.2 to 2.0 micrometers: At the smallest scale, life on Earth is made possible by humble photosynthetic organisms called phytoplankton, which lie at the base of the ocean food chain and produce most of Earth’s oxygen.

The smallest among them, called picoplankton, make up in number what they lack in size. Picoplankton, including bacteria, photosynthetic, and nonphotosynthetic cells, are the most abundant organisms on Earth. Studying them is a challenge, however, since they are too small to see, like dark matter making up most of the ocean universe.