Protozoans are single-celled animals (microbes). In the ocean environment they have the role of primary consumers in the food chain (they are zooplankton).
Protozoa are a diverse group of unicellular eukaryotic organisms. Historically, protozoa have been defined as single-celled organisms with animal-like behaviors, including motility (able to move) and predation. Protozoan that are major contributors to the food chain and to marine sediments include foraminifera and radiolarians.