Abstract
Protozoa may be considered as (mostly) microscopic, essentially single-celled, eukaryotes that ingest other organisms or their products (i.e. as heterotrophic protists). Protozoa are no longer recognised as a taxonomic group because they do not constitute a monophylum and can be found in all main eukaryotic lineages, informally called supergroups.
They play important roles in the ecology of aquatic habitats and soils, for example, by controlling bacterial populations and releasing nutrients for use by other organisms. As parasites, protozoa have very profound effects on humans, both directly and through their effects on domesticated animals.
Some protozoa have shells or skeletal structures that can fossilise and certain groups, particularly the foraminifera, are used in biostratigraphy and in locating oil deposits.