Light micrograph of filaments of Anabaena sp., a blue-green algae or cyanobacterium. Filaments of Anabaena are found on grass and in freshwater pools and sediments. In this image, the Anabaena have formed thick-walled cells called heterocysts that specialize in fixing nitrogen, an essential biological nutrient. Anabaena are used to increase soil fertility and crop yields. These algae also contain poisons that are dangerous to animals if eaten. 100x at 35mm.