Protista

Protista is a historically defined group of mostly single-celled eukaryotes that do not fit neatly into the kingdoms Plantae, Animalia, or Fungi. In many modern classifications it is treated as a heterogeneous assemblage rather than a natural, monophyletic group. As a result, protists are distributed across several eukaryotic lineages rather than forming one clade.

Most protists are unicellular, though some form colonies or simple multicellular structures. They include photosynthetic algae and heterotrophic protozoa, with nutrition ranging from autotrophy to heterotrophy and mixotrophy. Cells have a nucleus and organelles; many possess plastids via endosymbiosis. Motility includes cilia, flagella, and amoeboid movement. Reproduction is usually asexual, by fission or budding, while some lineages have sexual stages.

Protists inhabit diverse environments, especially aquatic and moist habitats, such as freshwater, oceans, soils, and hosts. They occupy key ecological roles as producers, consumers, and decomposers and influence nutrient cycling. Some protists are parasites or pathogens; others are model organisms.

Examples include Amoeba, Paramecium, dinoflagellates, diatoms, and the malaria parasite Plasmodium. Because protists are polyphyletic, Protista is not a formal kingdom in many classifications; protists span major eukaryotic groups such as Excavata, SAR, Archaeplastida, and Amoebozoa.