Cosmarium portianum is a desmid characterized by semicircular or crescent-shaped cells with a central constriction dividing each cell into two halves. Cell size ranges from 30 to 80 micrometers in length, with a cellulose cell wall providing structural support. It contains lobed chloroplasts for photosynthesis and a centrally located nucleus. Reproduction occurs through both asexual (binary fission) and sexual means. Cosmarium portianum inhabits freshwater environments like ponds and lakes, thriving in nutrient-rich waters where it attaches to surfaces or floats.