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 [1]. Fe speciation is important to better understand the biogeochemical cycle and availability of this micronutrient, in particular in the Southern Ocean [2]. 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 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. dFe values ranged from 0.30 to 2.50 nM and logKFeL from 23 to 25, highlighting the presence of Fe-binding organic complexes of different stabilities. Principal Component Analysis (PCA) allowed us to explore the correlations among dFe, speciation parameters and the biogeochemistry of the area. A deep investigation on the nature of the organic ligands was carried out by HPLC-ESI-MS/MS. The results obtained by this analytical technique evidenced the presence of a heterogeneous class of siderophores in organic ligands pool. [1] E. Ibisanmi, S. G. Sander, P. W. Boyd, A. R. Bowie, and K. A. Hunter, “Vertical distributions of iron-(III) complexing ligands in the Southern Ocean,” Deep. Res. Part II Top. Stud. Oceanogr., vol. 58, no. 21–22, pp. 2113–2125, 2011. [2] M. Gledhill and K. N. Buck, “The organic complexation of iron in the marine environment: A review,” Front. Microbiol., vol. 3, no. FEB, pp. 1–17, 2012.