Diatoms are a major group of typically unicellular algae, and are one of the most common varieties of phytoplankton. Unique to diatoms, their cells are encased in a cell wall of silica (silicon dioxide), which is responsible for their crystalline appearance. Diatoms are autotrophic, meaning they derive their nutrition from their surroundings by means of photosynthesis or chemosynthesis. This places them in the role of producers in the food chain.
Eukaryota > Chromalveolata > Heterokontophyta > Bacillariophyceae