Diatoms, a major group of primary producers in the ocean, are estimated to be responsible for over 40% of oceanic carbon fixation. As one of the predominant biosilicifying organisms in the world, diatoms are a model system for studying biological interactions with silicon. This research has focused on characterizing a novel family of transporters, called silicon transporters (SITs), that are specific for the uptake and efflux of silicon in diatoms. SIT sequences were isolated from evolutionarily diverse diatom species. Multi-gene copies were identified in most species, and phylogenetic analysis showed SITs grouped according to species. Structural analysis suggested SITs evolved through an internal gene duplication. Comparative sequence analysis revealed repeats of a conserved sequence motif, GXQ. A model of silicon transport consistent with known aspects of uptake was developed based on this motif. Analysis of SIT protein and mRNA expression, as well as measurements of uptake activity, was done on synchronously growing cultures of the diatom, Thalassiosira pseudonana. Immunoblot analysis using a newly developed SIT-specific antibody showed peaks in SIT protein levels correlated with active periods of silica incorporation. Quantitative PCR showed each T. pseudonana SIT (TpSIT1-3) peaked prior to cell wall synthesis. However, a disconnect between protein and mRNA levels suggested SITs were primarily regulated at the translational or post-translational level. In addition, rates of surge uptake suggested SIT activity was internally controlled by the rate of silica incorporation. Silicon uptake kinetics in diatoms were measured to determine the extent of nonsaturable uptake and the role of SITs. In all diatom species examined, a time-dependent transition from nonsaturable to saturable uptake kinetics was observed. In addition, both forms of uptake were affected by the SIT-specific antibody suggesting SITs were the predominant means of silicon uptake into the cell. Under some conditions, SITs had enormous flexibility in their rate of transport and appeared to act as selectivity gates rather than controlling agents in uptake. A model of diatom silicon uptake, consistent with this and previously published data, was developed based on the interplay between SITs, intracellular soluble silicon pools, and cell wall silica incorporatio
