protozoer

Protozoer are a diverse group of single-celled, eukaryotic organisms historically regarded as animals because of their heterotrophic lifestyle and lack of photosynthetic pigments in many groups. They occur in nearly every habitat that supports life, including freshwater and marine environments, soils, and as parasites inside other organisms. Most protozoer feed by ingesting bacteria, archaea, or other small particles, while some are predatory or form symbiotic relationships. A subset is photosynthetic or mixotrophic due to the presence of photosynthetic organelles acquired through endosymbiosis.

Movement and form vary widely among protozoer. Many move by pseudopods (amoeboid motion), others by cilia, or by flagella; some are non-motile at certain life stages or form resistant cysts to survive unfavorable conditions. Reproduction is primarily asexual, through binary fission, budding, or multiple fission. In several lineages, sexual processes occur as well, such as conjugation in ciliates or other forms of genetic exchange, contributing to diversity and adaptation.

Ecology and importance are broad: protozoer play key roles in aquatic and soil ecosystems as grazers of bacteria, recyclers of nutrients, and as prey for larger organisms. Some species are parasitic or pathogenic, causing diseases in humans and animals (for example, Plasmodium in malaria, Giardia in giardiasis, Entamoeba in amoebic infections). Others form symbiotic relationships within hosts or environments.

Taxonomically, protozoer are not a single valid clade but a historical grouping of numerous unicellular eukaryotes. Modern classifications place them across multiple supergroups within the eukaryotes, reflecting shared traits rather than a single common ancestor. The term protozoer remains common in education and certain languages, even as molecular data refine their relationships.