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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! >:-(
it's copied!