They can perform photosynthesis and mainly consist of unicellular algae. They can be divided into a number of systematic groups according to the shape of their cells and the type of photosynthetic pigments they use.
• Chrysophyta or golden algae: they live in both sea and freshwater; the most common ones are diatoms, which are equipped with a typical siliceous shell (SiO2), consisting of two parts joined with each other like a box and a lid. The shell is provided with many small holes through which the cell communicates with the external environment. Diatoms usually live near the seabed.
• Dinoflagellata: they generally live in the sea and are also equipped with a shell, consisting of many cellulose plates. They have two flagella (resembling cilia, only longer) for covering small distances. They are among the most important components of the marine phytoplankton.
• Euglenida: they live in ponds and lakes. These organisms are provided with chloroplasts (cellular organelles containing chlorophyll, the pigment involved in the photosynthesis) and can therefore perform the photosynthesis; however, without light, they become heterotrophic and start therefore feeding on the organic substances they find in the surrounding environment. Their cell is equipped with two flagella and an eyespot, i.e. a concentration of light-sensitive pigments that allow Protista to move towards light.
Some protozoa have attracted the interest of the biotechnological research world...