Rivularia cyanobacteria on watermilfoil plant. Polarised light micrograph a colony of the cyanobacteria Rivularia sp. growing on a leaf of watermilfoil (Myriophyllum sp.), an aquatic plant. Cyanobacteria are bacteria that obtain their energy through photosynthesis. Small, spherical heterocysts are present at the base of the Rivularia filaments. These are specialized nitrogen-fixing cells. Magnification: x200 when printed at 10 centimetres wide.