Diatoms form the primary photosynthetic portion of phytoplankton. This small plant species is unique because during replication it will form long chains, thus producing a colony in a zig-zag formation. They are also unicellular, meaning an individual diatom is often made up of multiple cells. Their cell wall is made primarily of silica, giving them a glass-like appearance. The color of their chloroplast is also light brown, separating this species from the usual bright green of the chloroplast in most plants.
These small organisms are mostly photosynthetic, with some being parasitic. They are responsible for the infamous "red tide," which releases neurotoxins into their surrounding environment. These neurotoxins often spread up the food chain in a similar fashion as heavy metals such as mercury. If a human consumes these neurotoxins it can lead to death or mental disease. This organism can swim using flagella, reproduce asexually and obtain sustenance from both photosynthesis or parasitism, depending on the availability of food.
Cyanobacteria are seen as the evolutionary link between plants and bacteria. Chloroplasts are actually cyanobacteria living within the cells of plants and photosynthetic microorganisms, providing the cellular machinery necessary for photosynthesis. Cyanobacteria are seen as responsible for changing the atmosphere of the earth to include oxygen, changing the way that life would survive. They are also responsible for nitrogen fixation, converting nitrogen into fertilizer, which allows plants to grow. Cyanobacteria are also responsible in evolutionary terms for the mitochondria within eukaryotic cells.