Fe(II) regeneration by Tryptophan in seawater at nanomolar levels

Iron (Fe) is an essential micronutrient for marine productivity and dissolved Fe can be found as Fe(III) and Fe(II), being the first one the most abundant and the second one the most bioavailable form. Natural organic ligands play a fundamental role in the Fe speciation and redox chemistry. However, there is a lack of information about the impact of individual organic compounds on the Fe(III) reduction to Fe(II) in seawater. Among the amount of organic ligands, the amino acid Tryptophan (Trp) is linked with marine microorganisms, contributing to the ligand pool in aquatic environments. In this current investigation, the Fe(II) production from Fe(III) reduction by Trp has been studied under different conditions of Trp concentration (50 – 500 nM), pH (7.0 – 8.0) and temperature (10 – 25 ºC) in seawater and NaCl solutions (0.7 M NaCl + 2 mM NaHCO₃). According to the results, the reaction was pH-dependent, not occurring above pH 7.8 in seawater. The Fe(III) reduction is also dependent of  Trp levels, with 6.49 % and 18.10 % reduction observed after 60 minutes at Trp concentration of 50 nM and 500 nM, respectively, at pH 7.8. A relationship between Trp levels and the reduction capacity at different pH values (7.0 and 7.8) was established, showing a more significant effect at lower pH, suggesting that Trp plays a more crucial role in Fe(III) reduction in lower pH environments. This effect can be understood by the competition with major ions in seawater. The impact of these ions (SO₄²⁻, K⁺, F⁻, Ca²⁺, Mg²⁺, and Sr²⁺) showed lower pseudo first-order Fe(III) reduction rate constant (k´_Fe(III)-red in s⁻¹) values than the reference solution (0.7 M NaCl + 2 mM NaHCO₃).

On the other hand, the Fe(III) reduction by Trp was a temperature-dependent process, leading to higher k´_Fe(III)-red values at higher temperatures (25ºC) with respect to lower temperatures (10ºC). The k´_Fe(III)-red increased from 0.62∙10^-5 to 1.99∙10^-5 s^-1 when temperatures changed from 10 ºC to 25 ºC, respectively, at pH 7.8. The Energies of Activation (E_a) were 77.80 kJ mol^-1 and 53.02 kJ mol^-1 at pH 7.0 and 7.8, respectively.

According to the current results, in a situation of global warming and ocean acidification, changes in the physico-chemical conditions enhance the Fe(III) reduction by organic ligands, such as amino acids, and influence the Fe marine biogeochemical cycles promoting the formation of bioavailable Fe(II) in seawater.