Fig. 4. The influence of 0.26 ± 0.20 pH increases/decreases in the phycosphere on the fraction of Fe species (i.e. inorganic Fe species, Fe bound to siderophores and dissolved organic matter DOM).

The green/red lines indicate the average changes in the phycosphere, while the shaded areas show their variations. The black lines represent for bulk seawater. The “coastal water” scenario has 1 nM total dissolved Fe, 5 pM siderophores and 229 µM DOM, while the “open ocean” scenario has 0.1 nM total dissolved Fe, 5 pM siderophores and 57 µM DOM. Here, the concentrations of total dissolved Fe, siderophores and DOM are assumed to be the same as those in bulk seawater. Changes in Fe speciation in the phycosphere produced by increased/decreased concentrations of H⁺, DOM and siderophores in the phycosphere relative to bulk seawater are shown in the supporting information.