Protozoat

Protozoa, also known as protozoans, are a broad and diverse group of mostly unicellular eukaryotic organisms traditionally described as “animal-like” protists. They are not a single taxonomic group but a convenience term for several distinct lineages that inhabit a wide range of environments, including freshwater and marine systems, soil, and as parasites in animals and humans.

Protozoa exhibit a variety of body plans and lifestyles. Many are motile at some life stage, using pseudopodia for amoeboid movement, flagella, or cilia; others are non-motile or glide. Their sizes range from a few micrometers to a few hundred micrometers. They feed primarily by ingesting bacteria, algae, or small particulate matter through phagocytosis; some are mixotrophic, harboring photosynthetic endosymbionts or combining photosynthesis with heterotrophy.

Reproduction is predominantly asexual, via binary fission, budding, or schizogony, but several groups also exhibit sexual processes such as conjugation in ciliates or other forms of genetic exchange. Many protozoa form cysts or other resistant stages to endure unfavorable conditions.

Ecologically, protozoa play essential roles as microbial consumers and as prey in aquatic ecosystems, contributing to nutrient cycling and energy flow. Some protozoa are parasites of humans and other animals, causing diseases such as malaria (Plasmodium), sleeping sickness (Trypanosoma), amoebic dysentery (Entamoeba histolytica), and giardiasis (Giardia). Others have complex life cycles involving multiple hosts.

Taxonomically, protozoa do not form a single clade; they are distributed across several major eukaryotic lineages, including Amoebozoa, Excavata, Rhizaria, and SAR. Consequently, they are treated as a historical grouping rather than a formal, monophyletic taxon. In research, protozoa have served as model organisms for cell biology, development, and host–pathogen interactions.