Combining voltammetric and mass spectrometric data to evaluate iron organic speciation in subsurface coastal seawater samples of the Ross sea (Antarctica)

Iron (Fe) is the most important trace element in the ocean, as it is required by phytoplankton for photosynthesis and nitrate assimilation. Iron speciation is important to better understand the biogeochemical cycle and availability of this micronutrient, in particular in the Southern Ocean. Dissolved Fe (dFe) concentration and speciation were determined in 24 coastal subsurface seawater samples collected in the western Ross sea (Antarctica) during the austral summer 2017 as part of the CELEBeR (CDW Effects on glacial mElting and on Bulk of Fe in the Western Ross sea) project. ICP-DRC-MS was used for dFe determination, whereas CLE-AdSV was used to obtain the concentration of complexed and free dFe, of the ligands, and the values of the stability constants of the complexes. Dissolved Fe values ranged from 0.4 to 2.5 nM and conditional stability constant (logK'Fe'L) from 13.0 to 15.0, highlighting the presence of Fe-binding organic complexes of different stabilities. Principal component analysis (PCA) allowed us to point out that Terra Nova Bay and the neighboring area of Aviator and Mariner Glaciers were different in terms of chemical, physical, and biological parameters. A qualitative investigation on the nature of the organic ligands was carried out by HPLC-ESI-MS/MS. Results showed that siderophores represented a heterogeneous class of organic ligands pool.