The utilization of
silicon by diatoms has both global and small-scale implications, from oceanic primary productivity to nanotechnological applications of their
silica cell walls. The sensing and transport of
silicic acid are key aspects of understanding diatom
silicon utilization. At low
silicic acid concentrations (<30 μM), transport mainly occurs through
silicic acid transport proteins (
SITs), and at higher concentrations it occurs through diffusion. Previous analyses of the
SITs were done either in heterologous systems or without a distinction between individual
SITs. In the present study, we examined individual
SITs in Thalassiosira pseudonana in terms of transcript and
protein abundance in response to different
silicic acid regimes and examined knockdown lines to evaluate the role of the
SITs in transport,
silica incorporation, and
lipid accumulation resulting from
silicon starvation. SIT1 and SIT2 were localized in the plasma membrane, and
protein levels were generally inversely correlated with cellular
silicon needs, with a distinct response being found when the two
SITs were compared. We developed highly effective approaches for RNA interference and antisense knockdowns, the first such approaches developed for a centric diatom. SIT knockdown differentially affected the uptake of
silicon and the incorporation of
silicic acid and resulted in the induction of
lipid accumulation under
silicon starvation conditions far earlier than in the wild-type cells, suggesting that the cells were artificially sensing
silicon limitation. The data suggest that the transport role of the
SITs is relatively minor under conditions with sufficient
silicic acid. Their primary role is to sense
silicic acid levels to evaluate whether the cell can proceed with its cell wall formation and division processes.