Blue-green algae are not a type of algae but rather a type of bacteria called cyanobacteria. Cyanobacteria are single-celled prokaryotic organisms and are the simplest life forms. They produce their own food by photosynthesis like plants, but cyanobacteria cells are simpler and lack some of the structural characteristics of these eukaryotic organisms.
Size and Shape
Blue-green algae or cyanobacteria cells are much smaller than plant, animal, fungus or protist cells. They are typically 1/10 to 1/20 the size of eukaryotic cells. Their external shape is round, and they may live singly or in pairs, clusters or chains. Like plant cells, cell walls provide structure to cyanobacteria cells. Although cyanobacteria are much smaller and structurally simpler than eukaryotes, they are larger and have more internal structures than viruses.
The DNA in cyanobacteria cells (blue-green algae) is contained within a single, circular, looping chromosome. Cyanobacteria lack nuclei, so the chromosome is not bound within a membrane but is held in the nucleoid, the region of the cell where DNA is located. Cyanobacteria do not reproduce by mitosis. The cell elongates, and the DNA replicates. The chromosome pulls apart as one cell splits into two cells in a process called binary fission.
Cyanobacteria do not have organelles such as mitochondria, chloroplasts, endoplasmic reticulum or golgi apparatus that are found in eukaryotic cells. Ribosomes are the only organelles in the cytoplasm of cyanobacteria. These structures that contain RNA are responsible for protein synthesis. They are approximately one-third smaller than ribosomes in eukaryotic cells but perform similar functions.
Like green plants, cyanobacteria, or blue-green algae, contain chlorophyll and are producers. Unlike plant and cells, the chlorophyll in cyanobacteria is not contained within chloroplasts. Instead, chlorophyll is held within folds in the cell membrane called thykaloids. Through photosynthesis, cyanobacteria provide oxygen and food to freshwater and saltwater animals.