Spirulina cyanobacteria and a diatom. Scanning electron micrograph (SEM). Spirulina (green) is a gram negative, non-toxic species of cyanobacteria with a wide array of uses in the natural and commercial world. While many bacteria are known for their pathogenic effects, Spirulina is primarily known across the world for its potential nutritional value. It is one of the rare edible bacteria. Diatoms may be extremely abundant in both freshwater and marine ecosystems. Diatoms are a major food resource for marine and freshwater microorganisms and animal larvae, and are a major source of atmospheric oxygen. A unique feature of diatom cells is that they are enclosed within a cell wall made of silica (hydrated silicon dioxide) called a frustule. Magnification: x2000 when printed 10 centimetres wide.