Which class of algae is primarily unicellular and includes species like diatoms?

The class of algae that is primarily unicellular and includes species such as diatoms is Bacillariophyta. Diatoms are characterized by their unique silica cell walls, which form intricate and beautiful patterns. This class of algae plays a vital ecological role, particularly in aquatic environments, as they are major contributors to phytoplankton, serve as a food source for various marine organisms, and play a significant part in the global carbon cycle.

Bacillariophyta is distinct from other classes of algae, which might contain species that are multicellular or have different structural characteristics. For example, Phaeophyta, or brown algae, typically include larger and multicellular forms such as kelp. Similarly, Chlorophyta, or green algae, while they do have unicellular representations, also include many multicellular structures, leading to diversity in their forms and habitats. Rhodophyta, the red algae, are also mostly multicellular and have cell structures different from those of diatoms. Therefore, Bacillariophyta is uniquely identified for its primarily unicellular nature, with diatoms being a defining group within this class.