Patterns and Regulation of Silicon Accumulation in Synechococcus spp.

Culture experiments confirm the accumulation of significant amounts of Si by six clones of marine Synechococcus representing four clades from a variety of marine habitats. Both cellular Si levels and the rate of Si accumulation in Synechococcus respond proportionately to changes in the concentration of silicic acid in the external medium. Si quota is inversely related to growth rate implying growth rate dilution of cellular Si quotas. The rate of Si accumulation as a function of silicic acid concentration is bi-linear between 1 and 500 μM Si(OH)₄ with an abrupt increase in the slope at 100 μM. The linear response at environmentally-relevant [Si(OH)₄] implies that in the surface ocean Synechococcus acquire Si through diffusion rather than through active transport. However, some evidence was obtained for Si uptake through a phosphate transporter under low-nutrient conditions. If all water-soluble Si associated with Synechococcus cells is internal, internal cellular concentrations would be above seawater silicic acid saturation (i.e. ~2 millimolar) implying the existence of a large pool of organically bound silicic acid, as has been hypothesized for diatoms. Organic complexation of intracellular Si would both prevent spontaneous precipitation of amorphous silica and maintain low internal free silicic acid concentrations to drive a diffusive flux of silicic acid into the cell. Our experiments were not designed to address the specific evolutionary or physiological roles played by Si, if any, in Synechococcus. A partial answer will involve determining the chemical environment of Si within Synechococcus and whether the organism has evolved specific molecules that target and bind Si intracellularly or whether the internal complexation of Si is gratuitous.