Diatom species Epithemia. Diatoms are a large group of microscopic algae that grow as single cells or small colonies. This sample was taken as part of a Mongolian-American international partnership to survey the diatom flora of Hovsgol National Park in north-central Mongolia. Diatoms are a large group of microscopic algae that grow as single cells or small colonies. They are an important part of the 'primary producer' community in most aquatic habitats. They often live within narrow environmental conditions and can act as bioindicators for changes in pollution, water temperature, nutrient levels and salinity. Lake Hovsgol and the surrounding region is one of the most pristine large lakes on Earth and therefore globally significant as a natural laboratory for the study of ecology and evolution. The lake is estimated to be over 1.6 million years old.