Accelerated uptake by phytoplankton of iron bound to humic acids

Most of the dissolved iron (Fe) in seawater is complexed by organic ligands. One key issue in understanding Fe limitation in the sea is the bioavailability of different forms of Fe to marine phytoplankton. We measured Fe uptake by the coastal diatom Thalassiosira pseudonana and the cyanobacterium Synechococcus sp., using model ligands desferrioxamine B (DFB) and ferrichrome (FC), humic acids (HA), and natural organic matter of high molecular weight (HMW). We also determined the Fe binding capacity of organic matter in different size fractions. The order of the initial assimilation rates of different Fe species was: inorganic Fe ≥ HA-Fe(III) > DFB-Fe(III) ≈ FC-Fe(III). The high bioavailability of HA-bound Fe(III) implies that humic substances are important not only in controlling Fe geochemical behavior, but also in providing Fe for marine phytoplankton, especially in estuarine and coastal waters. Fe assimilation rates decreased with increasing Fe binding capacity of various types of organic matter, and with increasing molar ratio of organic matter to Fe(III), suggesting that the ultimate form of Fe(III) utilized by phytoplankton is the exchangeable inorganic Fe(III). By separating the uptake media from the phytoplankton, we demonstrated that the organic-bound Fe was first absorbed onto the cell surfaces and exchanged Fe into the specific surface sites before Fe internalization. Fe-depleted diatoms assimilated Fe much faster than Fe-replete diatoms, thus the Fe transport system is regulated by the nutritional status of the cells. Overall, our study showed the important role of iron-binding ligands in controlling Fe bioavailability to marine phytoplankton; it is necessary to evaluate the relative importance of various organic ligands for biological uptake of Fe.