Which of the following is incorrect about Cynobacteria?

The cyanobacteria or blue-green algae ($BGA$) have chlorophyll-$a$ similar to green plants and are photosynthetic autotrophs. The $BGA$ are unicellular, colonial or filamentous, freshwater, marine or terrestrial algae. The colonies are generally surrounded by a gelatinous sheath. They often form blooms in polluted water bodies. Some of these organisms can fix atmospheric nitrogen in a specialised cells called heterocysts.