Question

why cyanobacteria were placed in algae previously?​

Answer 1

Answer:

It’s probably easier to explain this by asking the question the other way round: Why are cyanobacteria also called blue-green algae? The reason is that cyanobacteria appeared to look a lot like green algae when they were first discovered. We now know that they really are bacteria (prokaryotes). One criterium to distinguish cyanobacteria from true algae is that prokaryotes lack a whole bunch of organelles present in algae, plants and animals (eukaryotes). For example: they don’t have a Golgi apparatus! >:-(