protozoos

Protozoa are a diverse group of single-celled eukaryotic organisms traditionally viewed as "animal-like" protists. They are not a single clade but a historical grouping that includes many lineages of unicellular organisms that feed by heterotrophy. They occur in aquatic environments, soils, and as parasites or symbionts. Size ranges from a few micrometers to a few tenths of a millimeter.

Most protozoa move using pseudopods, flagella, or cilia, though some glide using internal membranes. They exhibit a trophozoite stage in which they feed and grow, and many form cysts or other resting stages as dispersal forms in adverse conditions. They can be free-living or associated with hosts.

Reproduction is mainly asexual through binary fission or budding, while some lineages undergo genetic exchange or sexual processes, such as conjugation in ciliates or syngamy in other protists. Life cycles can be simple or complex with multiple stages.

Ecologically, protozoa are important in nutrient cycling and as microbial predators. Medically and economically significant protozoa cause diseases such as malaria (Plasmodium), sleeping sickness (Trypanosoma), amoebic dysentery (Entamoeba histolytica), and giardiasis (Giardia lamblia). Others are beneficial in gut ecosystems or used in research as model organisms (e.g., Paramecium, Tetrahymena).

In modern biology, "protozoa" is an informal term; contemporary classifications place these organisms within various protist clades (for example, Amoebozoa, SAR, Excavata, and Archaeplastida). They are diverse in genetics and metabolism and do not form a single taxonomic group.