Iron uptake kinetics by coastal micro- and macro-algae in relation to riverine and coastal organic matter

Iron (Fe) uptake kinetics by the coastal micro-algae Skeletonema marinoi-dohrnii and the macro-algae Eisenia bicyclis were investigated in the presence of Fe and dissolved organic matter (DOM) originating from terrestrial (mountainous rivers) and coastal bay water in northeast Japan. In addition, a synthetic chelator, ethylenediaminetetraacetic acid, was tested as a representative Fe-binding organic ligand with a high Fe affinity. 59Fe uptake assays showed a linear relationship between dissolved Fe concentration and Fe uptake rate by both algae at dissolved Fe concentrations ranging from 1.0 nM to 120 nM, indicating that the Fe uptake rate was limited by the concentration of Fe species. Between 8 and 69% of the Fe fraction remained as dissolved forms when river water samples were mixed with seawater. These dissolved forms were ascribed to fulvic-like components. The uptake rate of dissolved Fe originating from river water was slower than that from coastal water (i.e., ∼1.94 and ∼0.43 log for S. marinoi-dohrnii and E. bicyclis, respectively) when the same dissolved Fe concentration was compared. Thus, dissolved Fe in river water may be less bioavailable, and the Fe-binding ligand exchange from exogenous to autochthonous DOM may increase bioavailability in coastal seawater. Excitation-emission matrix-parallel factor analysis, DOC measurements, and Fe uptake tests under dark/light incubation with a Fe(II) scavenging agent indicated differences in dominant Fe uptake mechanisms between DOMs originating in river and bay samples, with photochemical reactions greatly contributing to uptake from riverine Fe-DOM and non-chromophoric DOM produced in marine environments playing an important role in coastal seawater. [Display omitted] •Eight to 69% of Fe remained as dissolved forms when river water was mixed with seawater.•Fe uptake by micro- and macro-algae was higher in seawater than river water.•Linear relation of 59Fe uptake with Fe concentration indicate Fe-limitation in seawater.•Fe uptake was light-sensitive in river water but the effect was less in seawater.