Protist

Protists are a diverse group of eukaryotic organisms that are not animals, plants, or fungi. Historically the term referred to a catch-all kingdom of mostly microscopic organisms, but modern biology treats Protista as a paraphyletic assemblage rather than a single coherent lineage. Today, protists include many unicellular species as well as some colonial and a few multicellular forms, and they occupy a wide range of habitats, especially damp or aquatic environments, including freshwater, marine ecosystems, and soils. They can live free-living or as symbionts or parasites.

Most protists are eukaryotes, possessing a nucleus and organelles. They exhibit a broad spectrum of nutritional strategies: photoautotrophs such as algae that perform photosynthesis; heterotrophs such as protozoans that ingest food; and mixotrophs that combine both methods. Their motility structures vary: flagella, cilia, or amoeboid pseudopodia.

Reproduction is equally varied, with asexual methods such as binary fission or budding, and sexual reproduction via meiosis and fertilization in many lineages. Some species have complex life cycles with multiple hosts or encystment stages.

Ecologically, protists are essential in many ecosystems. Photosynthetic protists contribute substantially to global primary production and carbon cycling, while heterotrophic protists help regulate bacterial populations and participate in nutrient turnover. Some protists are important pathogens of humans, animals, and crops (for example, Plasmodium causing malaria, Giardia, and Entamoeba), and others form symbiotic relationships with animals or corals.

Taxonomically, Protista is not a single clade; molecular phylogenetics place its constituents in several major eukaryotic supergroups, such as Excavata, SAR (stramenopiles, alveolates, rhizarians), Archaeplastida, and Amoebozoa. The term remains useful as a historical and practical label for diverse unicellular eukaryotes and simple multicellular organisms that are not animals, plants, or fungi.

Examples include Amoeba, Paramecium, Euglena, diatoms, dinoflagellates, and the malaria parasite Plasmodium.