Iron (Fe) is the most important trace element in the ocean ecosystem, being a micronutrient required for phytoplankton growth and thus involved in marine primary productivity and carbon export. The dissolved Fe (dFe) is the most bioavailable fraction and about 99.9% of the dFe is complexed with organic ligands (L), the nature of which is largely unknown. The concentrations of L can be determined by the electrochemical technique of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV), which also allow the binding affinity of Fe to be assessed through the values of the conditional stability constants (logK’Fe’L). However, CLE-AdCSV does not give details about the L nature. High performance liquid chromatography - electrospray ionization - mass spectrometry (HPLC-ESI-MS/MS) is a powerful new approach to characterize the Fe-binding ligands, thanks to the fragmentation patern of the MS/MS spectra. To date, the eastern sector is the most unexplored area of the Ross Sea, but its study has high scientific interest. In fact, it is the area where water masses from the western sector of Antarctica pass through and it is the gateway for the iron and organic ligands rich glacial meltwater coming from the nearby Amundsen Sea. In this work, the total concentration and speciation data of dFe determined in seawater samples collected during the austral summer of 2019-20 in the eastern Ross Sea (ESRS) and along the Ross Ice Shelf (RIS) in the framework of the ESTRO (Effect of the eaSTern inflow of water on the ROss Sea salinity field variability) project are presented. Principal component analysis (PCA) was used to explore correlations between dFe, speciation parameters, and the biogeochemistry of the area. This allowed us to assess the possible sources of dFe and L and to highlight the differences between the different water masses in this area. The subsurface samples were characterized by greater spatial dFe variability than the deep samples. Dissolved Fe average concentrations were higher in the ESRS (2.21 ± 1.20 nM) than those sampled along the RIS (0.62 ± 0.10 nM). In the ESRS, the distribution of Fe parameters was more controlled by the physical parameters, rather than the biological component. Particularly stable complexes were identified by CLE-AdSV between dFe and L (average logK’Fe’L = 13.4) and the HPLC-ESI-MS/MS analyses showed the presence of several organic ligands having different masses in the seawater samples.
Dissolved iron speciation and first characterization of organic ligands in the eastern Ross Sea
Paola Rivaro;Annalisa Salis;Enrico Millo;Davide Vivado
2023-01-01
Abstract
Iron (Fe) is the most important trace element in the ocean ecosystem, being a micronutrient required for phytoplankton growth and thus involved in marine primary productivity and carbon export. The dissolved Fe (dFe) is the most bioavailable fraction and about 99.9% of the dFe is complexed with organic ligands (L), the nature of which is largely unknown. The concentrations of L can be determined by the electrochemical technique of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV), which also allow the binding affinity of Fe to be assessed through the values of the conditional stability constants (logK’Fe’L). However, CLE-AdCSV does not give details about the L nature. High performance liquid chromatography - electrospray ionization - mass spectrometry (HPLC-ESI-MS/MS) is a powerful new approach to characterize the Fe-binding ligands, thanks to the fragmentation patern of the MS/MS spectra. To date, the eastern sector is the most unexplored area of the Ross Sea, but its study has high scientific interest. In fact, it is the area where water masses from the western sector of Antarctica pass through and it is the gateway for the iron and organic ligands rich glacial meltwater coming from the nearby Amundsen Sea. In this work, the total concentration and speciation data of dFe determined in seawater samples collected during the austral summer of 2019-20 in the eastern Ross Sea (ESRS) and along the Ross Ice Shelf (RIS) in the framework of the ESTRO (Effect of the eaSTern inflow of water on the ROss Sea salinity field variability) project are presented. Principal component analysis (PCA) was used to explore correlations between dFe, speciation parameters, and the biogeochemistry of the area. This allowed us to assess the possible sources of dFe and L and to highlight the differences between the different water masses in this area. The subsurface samples were characterized by greater spatial dFe variability than the deep samples. Dissolved Fe average concentrations were higher in the ESRS (2.21 ± 1.20 nM) than those sampled along the RIS (0.62 ± 0.10 nM). In the ESRS, the distribution of Fe parameters was more controlled by the physical parameters, rather than the biological component. Particularly stable complexes were identified by CLE-AdSV between dFe and L (average logK’Fe’L = 13.4) and the HPLC-ESI-MS/MS analyses showed the presence of several organic ligands having different masses in the seawater samples.
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