Differential interference contrast (DIC) light micrograph of Anabaena sp. algae (shape of W) and Aphanizomenon flos-aquae (bottom part) . Anabaena is a genus of filamentous cyanobacteria and are known for nitrogen-fixing abilities. Anabaena is capable to differentiate specialized cells, the heterocysts (round green cells in the photograph) to survive under different stress conditions. Under nitrogen limited condition, heterocysts provide the filament with nitrogen by fixing nitrogen. Both Anabaena and Aphanizomenon live in most freshwater lakes an ponds as well as in brackish water environments around the world. Magnification: x325 when printed at 10 centimetres wide at its longest edge.