Bioavailability of iron complexed with organic colloids to the cyanobacteria Synechococcus and Trichodesmium

Iron (Fe) is an important limiting micronutrient to phytoplankton productivity in several major oceans due to its very low dissolved concentration and dominance by organic complexation. The biological uptake of Fe complexed with natural colloidal nanoparticles (measuring between 1 kDa and 0.2 μm), which are abundant in natural seawater, is important for the understanding of Fe limitation in the sea. In this study, we quantified the bioavailability of Fe complexed with colloids of various geochemical properties to 2 important marine cyanobacteria, Synechococcus and Trichodesmium. We demonstrated that the uptake of colloidal Fe was greatly dependent on the origin, size, aging, and Fe concentration of the colloids. The uptake of colloidal Fe was at least 1.7 x lower than the uptake of Fe complexed with low molecular weight compounds (<1 kDa). Fe bound with colloids isolated from estuarine waters was bioavailable to 2 cyanobacteria, whereas Fe bound with coastal colloids was little bioavailable to the cyanobacteria. Furthermore, colloidal Fe resulting from Synechococcus decomposition was also accumulated by Synechococcus, whereas colloidal Fe from diatom Thalassiosira pseudonana decomposition was unavailable to them. In general, smaller sized colloids (1 to 10 kDa) and shorter aging of colloids enhanced Fe uptake by the 2 cyanobacteria. These results indicate that dissociation of Fe from colloidal particles may be important in controlling Fe bioaccumulation by the cyanobacteria. At typical natural colloidal Fe concentrations, the calculated uptake rate-constant of Fe was relatively independent of the Fe concentration, whereas at a much higher colloidal Fe concentration (>549 nM), the uptake rate constant decreased significantly. Overall, our data demonstrated that natural colloidal Fe is bioavailable to marine cyanobacteria, but the bioavailability is clearly dependent on the colloidal geochemical characteristics.