Coloured scanning electron micrograph (SEM) of salt water pennate diatom frustule (Navicula sp.). The genus Navicula is a genus of boat-shaped, single cell, photosynthetic alga. It is a pennate diatom that has a silica skeleton (frustule). These microscopic unicellular plants are important biomass and oxygen producers. Diatoms represent about 25% of the plant biomass in the world. 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: x600 when shortest axis printed at 25 millimetres.