Steps of Silicic Acid Transformation to Siliceous Frustules: Main Hypotheses and Discoveries

Diatoms are unicellular photosynthetic organisms that live inside a silica-based glass exoskeleton (frustule). This exoskeleton consists of two valves, similar to parts of a petri dish, and girdle bands between the valves. Diatom valves are species-specific structures with a large number of submicrometric and nano-level elements: pores, spines, rimoportules, etc. Cellular mitosis takes place inside the frustule, after which a new valve is built in each daughter cell in addition to the parent valve. The formation of new valves takes several hours and requires the assimilation of large amounts of silicon from the environment in the form of silicic acid. Silicic acid is transported through the cytoplasm to the silica deposition vesicle where a new valve is synthesized. The mechanisms of these key stages of frustule morphogenesis are not sufficiently clear. We have tried to summarize the main hypotheses in this area and discuss them in the light of the available knowledge of diatom physiology, biochemistry, and chemistry of the substances involved.