All cyanobacterial members have heterocyst.

Not all cyanobacterial members have heterocyst. Heterocyst are specialized cells found in some cyanobacteria, particularly in filamentous species, that are involved in nitrogen fixation. While heterocyst are a distinguishing feature of certain cyanobacterial species, not all cyanobacteria have them. Many cyanobacteria are unicellular or form simple colonies and do not produce heterocyst. The presence of heterocyst depends on the specific species and its environmental conditions.