Light micrograph of the freshwater blue-green alga (cyanobacterium) Nostoc spec. Nostoc forms large spherical or irregular gelatinous deposits in which the irregularly curved cell filaments are embedded. The cells are strung like pearls, as in all cyanobacteria without nucleus and plastids. There are some thick-walled, colourless heterocysts in the filaments. Culture material of the algae collection at the University of Cologne (CCAC). Microscopic contrast method : Differential interference contrast. Magnification 350x at a print width of 10 cm.