Latitudinal variations of iron chemical speciation in the euphotic zone of the central Pacific Ocean

The concentrations of dissolved iron (DFe), iron-binding ligands (L_Fe), and electroactive humic-like substances (eHS) were revealed in the upper 200 m along the 170°W latitudinal transect of the central Pacific Ocean in summer, which was weakly influenced by terrestrial input. DFe was largely depleted throughout the transect, except in the Bering Sea, and below 100 m in the North Pacific Subarctic Gyre. The concentration of L_Fe was lowest within the subtropical gyres and was lower in the Southern Hemisphere, which is consistent with the results from the Atlantic Ocean. The vertical distribution of L_Fe was relatively constant in the subtropical regions, whereas in the subarctic regions the subsurface maximum appeared around or over the subsurface chlorophyll maximum at some stations. The higher concentration of L_Fe in the subarctic regions coincides with a lower stability constant, which suggests a higher contribution of weaker ligands, including humic and exopolymeric substances. The horizontal and vertical distribution patterns of eHS were largely similar to those of L_Fe, supporting their significant contribution to iron-binding capacity in the upper 200 m, particularly in the subarctic regions. However, the eHS concentration was only weakly correlated with that of the fluorescently determined humic-like substances, demonstrating the substantially different chemical properties of the two humic-like substances. The strong positive correlation between the concentrations of eHS and chlorophyll a and Synechococcus strongly suggests their biological origins. However, further research is required to examine whether eHS are directly produced by phytoplankton or released via relevant biological processes, such as grazing, bacterial composition, and viral lysis.