Approaches for functional characterization of diatom silicic acid transporters

A major component of the diatom cell wall is silica, derived from silicon taken up from the environment. Due to limiting environmental concentrations of silicon, and a substantial requirement during cell wall synthesis, diatoms must transport silicon into the cell against a steep concentration gradient. This is accomplished through the silicic acid transporters (SITs). The SITs were first identified in the marine pennate diatom Cylindrotheca fusiformis. Five distinct SITs were found and have been classified as a novel family of transporters. This review covers our current understanding of silicon transport in diatoms with a focus on the SITs. Approaches for in-depth functional characterization of the SITs are discussed, including (1) isolating SITs from evolutionarily distant diatom species to identify conserved amino acids that may be important for function, (2) developing expression systems to assay the function of selected SITs, and (3) determining the cellular location and membrane topology of the C. fusiformis SITs to further clarify their roles in diatom silicon metabolism. Because of the specificity of interaction between the SITs and silicon, and the ability of the SITs to transport silicic acid across lipid bilayers, the SITs may have applications in nanotechnology.