This is a review of the literature on the differences between cell contents of cyanobacteria and other eukaryotic algae. In comparison, cyanobacteria, as prokaryotes, are not only structurally simpler than eukaryotic algae, but also have significantly smaller genome sizes and fewer cellular contents. Cyanobacteria and eukaryotic algae bear witness to Earth’s ancient history. Cyanobacteria were the first to have the ability to photosynthesize and change the Earth’s atmosphere. Algae still play an important role in ecosystems today. However, blooms caused by algae blooms continue to occur all over the world, with negative ecological, social and economic impacts. A better understanding of the contents of algae cells may lead us to develop more effective ways of managing blooms.