Polarised light micrograph of Nitzshia sigmoidea diatoms on Spirogyra sp. algae. Nitzshia is a common marine and freshwater diatom. They are usually long, straight, and narrow but may also be ovoid. They typically occur singly but may form colonies. Spirogyra is a genus of filamentous green algae is named for the spiral arrangement of the chloroplasts. There are more than 400 Spirogyra species commonly found in freshwater habitats. Magnification: x186 when printed at 10 centimetres wide.