Anabaena algae. Coloured scanning electron micrograph (SEM) of Anabaena sp. freshwater alga from a pond. Anabaena is a filamentous species of blue-green algae (cyanobacteria). It can form blue-green surface clumps or be distributed throughout the water column. Anabaena produces several types of toxins that can affect the nervous system and the liver. Magnification: x3000 when printed 10 centimetres wide.