Characterization of freshwater natural dissolved organic matter (DOM): Mechanistic explanations for protective effects against metal toxicity and direct effects on organisms

Dissolved organic matter (DOM) exerts direct and indirect influences on aquatic organisms. In order to better understand how DOM causes these effects, potentiometric titration was carried out for a wide range of autochthonous and terrigenous freshwater DOM isolates. The isolates were previously characterized by absorbance and fluorescence spectroscopy. Proton binding constants (pK_a) were grouped into three classes: acidic (pK_a≤5), intermediate (5<pK_a≤8.5) and basic (pK_a>8.5). Generally, the proton site densities (L_T) showed maximum peaks at the acidic and basic ends around pK_a values of 3.5 and 10, respectively. More variably positioned peaks occurred in the intermediate pK_a range. The acid-base titrations revealed the dominance of carboxylic and phenolic ligands with a trend for more autochthonous sources to have higher total L_T. A summary parameter, referred to as the Proton Binding Index (PBI), was introduced to summarize chemical reactivity of DOMs based on the data of pK_a and L_T. Then, the already published spectroscopic data were explored and the specific absorbance coefficient at 340nm (i.e. SAC₃₄₀), an index of DOM aromaticity, was found to exhibit a strong correlation with PBI. Thus, the tendencies observed in the literature that darker organic matter is more protective against metal toxicity and more effective in altering physiological processes in aquatic organisms can now be rationalized on a basis of chemical reactivity to protons.