Protists are a diverse group of eukaryotic microorganisms that encompass mostly unicellular and some multicellular organisms that do not fit phylogenetically into the well-defined kingdoms: Animalia, Plantae, and Fungi. Historically, protists were treated as the kingdom Protista, but this group has been contested in modern taxonomy. Protists do not have much in common besides a relatively simple organization, either unicellular or multicellular, with no specialized tissues. The term protista was first used by Ernst Haeckel in 1866. Protists live in almost any environment that contains liquid water. Most protists, such as algae, are photosynthetic and are fundamental primary producers in many ecosystems, making up a large part of the plankton of the oceans.