Amoeba. Polarised light micrograph of an Amoeba sp. protozoan with symbiotic green algae. Amoebae are freshwater single-celled organisms which feed on bacteria and smaller protozoa. The ingested algal cells photosynthesise inside the amoeba, providing it with sugars and oxygen, while deriving protection from their host. Magnification: x400 when printed at 10 centimetres wide.