Oocystis

Oocystis is a genus of unicellular green algae that belongs to the family Oocystaceae within the order Oocystales (chlorophytes). Members of the genus are typically spherical or ovoid cells ranging from 5 to 20 µm in diameter. Each cell contains a single, centrally positioned chloroplast that can be either lobed or plate‑like, with a surrounding pyrenoid in some species. The chloroplast may be surrounded by a single layer of chlorophyll a, and the cell wall is often composed of cellulose microfibrils.

These algae are commonly found in freshwater environments, such as ponds, lakes, and slow‑moving streams, where they form part of the phytoplankton community. Oocystis favours well‑oxygenated, nutrient‑rich waters and is often associated with other green algal genera such as Chlamydomonas and Oocystis. Individuals reproduce asexually by cell division, and some species have been reported to undergo sexual reproduction under stressful conditions, producing oogonium and subsequent zygotes that develop into dormant cysts.

Ecologically, Oocystis plays a role in primary production and serves as a food source for small zooplankton. In addition, the genus is occasionally used as an indicator of water quality, with certain species proliferating in eutrophic conditions. Some species of Oocystis have been studied for their potential applications in biotechnology, leveraging their high chlorophyll content for biofuel production or their ability to accumulate nutrients for bio-remediation purposes. Despite its relatively modest size, Oocystis contributes to the biodiversity and functioning of freshwater ecosystems.