Transmission Electron Microscopy shows the longitudinal section of a single filament of Z. circumcarinatum SAG 698-1b. The algal cells are connected with very thin cross cell walls (cCW). The cell wall (CW) is surrounded by a thick extracellular matrix (ECM). Pyrenoids: Py, Nucleus: N. Scale bar: 5 μm. [Image Credit: Elke Woelken and Klaus von Schwartzenberg, University of Hamburg]