Coloured scanning electron micrograph (SEM) of marine pennate diatom frustule (Cymbella sp.). The genus Cymbella is an asymmetrical biraphid, pennate diatom. This genus occurs in both fresh water and salt water and is a periphytic diatom. The body or frustule is composed of two halves which fit tightly together. The cell walls contain silica dioxide (glass) which are preserved in deposits known as diatomaceous earth. Diatoms represent about 25% of the plant biomass in the world. These microscopic unicellular plants are important biomass and oxygen producers. Through their photosynthesis, greenhouse gasses are converted to organic material and they therefore play a decisive role in the global carbon dioxide circulation. When individual cells die they remain as siliceous (silica dioxide) cell walls and sink to the bottom of the lake or ocean. Magnification: x700 when shortest axis printed at 25 millimetres.