Closterium costatum is a single-celled desmid, characterized by its elongated shape resembling a flattened cigar or slipper, with pointed ends. The cell wall is smooth and made of cellulose. Inside, there is a single large chloroplast that is ribbon-shaped and spirally twisted, containing pyrenoids for starch synthesis. In freshwater habitats like ponds and lakes, Closterium costatum is prevalent, contributing to the ecosystem through oxygen production via photosynthesis and serving as a food source for aquatic organisms.