“…Knowledge about sources and chemical identity of detected ligands is scarce ( Vraspir and Butler, 2009 ) and its acquisition is hindered by very complex chemical composition of natural DOM ( Repeta, 2015 ). Useful qualitative information about the DOM pool in the aquatic environment (e.g., average aromaticity degree, molecular weight, and occurrence of humic-like, fulvic-like, and protein-like substances) can be gained indirectly via UV/Vis and fluorescence spectroscopic studies of its optically active fractions (colored and fluorescent dissolved organic matter, CDOM and FDOM) ( Yamashita et al, 2011 ; Osburn et al, 2012 ; Lee et al, 2018 ; Galletti et al, 2019 ; Marcinek et al, 2020 ). Including this information in speciation studies could improve our understanding of the sources and composition of metal-binding organic ligands ( Sanchez-Marin et al, 2010 ; Slagter et al, 2017 ; Watanabe et al, 2018 ; Wong et al, 2019 ; Whitby et al, 2020 ).…”