protozoans

Protozoans are a diverse group of mostly unicellular eukaryotic organisms traditionally regarded as animal-like microorganisms within the broad category of protists. They are characterized by heterotrophic nutrition, motility, and a lack of photosynthetic capabilities in most species; however, some are photosynthetic or mixotrophic through endosymbionts. The term protozoan is informal; modern classifications place these organisms in several distinct lineages within the eukaryotes, and protozoa is not a single taxonomic group.

Most protozoans are unicellular, although some form colonial or aggregated structures. They move using pseudopodia (ameboid), cilia, or flagella, and occasionally glide or drift. They feed by phagocytosis or osmotrophy, ingesting bacteria, algae, smaller protists, or organic particles. Some have cysts for survival in harsh conditions.

Protozoans inhabit a range of environments, including aquatic settings—freshwater and seawater—as well as moist soil and intracellular habitats in hosts. They occupy ecological roles as predators of bacteria and as parasites, contributing to nutrient recycling and food webs.

Reproduction is primarily asexual, by binary fission or budding; some lineages undergo multiple fission, schizogony, or cyst formation. Sexual processes occur in several groups, such as conjugation in ciliates and various forms of meiosis in others.

Protozoans include species of medical significance. Examples causing human disease include Plasmodium (malaria), Giardia lamblia (giardiasis), Entamoeba histolytica (amoebiasis), Trypanosoma species (sleeping sickness and Chagas disease), and Toxoplasma gondii. Transmission routes vary, including vectors, ingestion of cysts, or contact with infected hosts. Identification relies on morphology, life cycle stage, and molecular methods due to their diversity.