Phenolic compounds excreted by marine microalgae are part of the ligand pool in natural waters. The effect of the polyphenol gentisic acid (GA; 2,5 dihydroxybenzoic acid) on the Fe(III) reduction as a function of organic ligand concentration (100 nM – 1000 nM) and the pH (7.00–8.01) was investigated in seawater. Major seawater ions interactions with both GA and Fe(III) were also considered using 0.7 M NaCl +2 mM NaHCO₃ solutions with each major ions (Mg²⁺, Ca²⁺, SO₄²⁻, K⁺, F⁻, Sr²⁺) at the same ratio than that in a 35 salinity seawater. The Fe(III)-GA complex in solution was able to produce Fe(II) in a pH-dependent process that was faster in seawater than in NaCl-NaHCO₃ solution. The addition of each major ion affected the Fe(III) reduction process. At pH 7.5, the presence of Mg²⁺ and Ca²⁺ contributes to accelerate the reduction rate constant of Fe(III) to Fe(II). The K⁺, F⁻, Sr²⁺, and SO₄²⁻ ions slow down the Fe(III) reduction rate constant. At pH 7.8, the effect of the ions is counteracted. Increasing the pH to 8, the effect on the solubility of Fe(III) is more important, being greater in NaCl and no reduction of Fe(III) was detected, compared with seawater.

In this study, the formation and dissociation of the Fe(III)-GA complex in seawater was studied, resulting in k_f = 1.19(±0.18)•10⁴ (M⁻¹ s⁻¹), k_d = 1.86 (±0.53)•10⁻⁴ (s⁻¹), and conditional complexation constant of logK_Fe³⁺L^Cond = 17.81 ± 0.05 in seawater.

Accordingly, phenolic compounds like GA influence the Fe marine biogeochemical cycles promoting the formation of the bioavailable Fe(II) in solution. Taking this into account, the Fe(III) reactions with phenolic compounds have to be considered to improve our understanding of the global iron cycle.

海洋微藻分泌的酚类化合物是天然水中配体库的一部分。在海水中研究了多酚龙胆酸（GA；2,5 二羟基苯甲酸）对 Fe(III) 还原的影响，作为有机配体浓度 (100 nM – 1000 nM) 和 pH (7.00–8.01) 的函数。还考虑使用 0.7 M NaCl +2 mM NaHCO ₃溶液与每种主要离子（Mg ²⁺、Ca ²⁺、SO ₄ ²⁻、K ⁺、F ^-、Sr ²⁺) 与 35 盐度海水的比例相同。溶液中的 Fe(III)-GA 复合物能够在 pH 依赖的过程中产生 Fe(II)，该过程在海水中比在 NaCl-NaHCO ₃溶液中更快。每种主要离子的添加​​都会影响 Fe(III) 的还原过程。在 pH 7.5 时，Mg ²⁺和 Ca ²⁺的存在有助于加速 Fe(III) 向 Fe(II) 的还原速率常数。K ⁺、F ^-、Sr ²⁺和 SO ₄ ^2-离子减慢 Fe(III) 还原速率常数。在 pH 7.8 时，离子的作用被抵消。将 pH 值提高到 8，对 Fe(III) 溶解度的影响更为重要，与海水相比，它在 NaCl 中更大，并且没有检测到 Fe(III) 的减少。

在本研究中，研究了海水中 Fe(III)-GA 复合物的形成和解离，导致k _f  = 1.19(±0.18)•10 ⁴ (M ^-1  s ^-1 )，k _d  = 1.86 (± 0.53)•10 ^-4 (s ^-1 )，以及在海水中log K _{Fe ³⁺ L} ^Cond = 17.81 ± 0.05 的条件络合常数。

因此，像 GA 这样的酚类化合物影响 Fe 海洋生物地球化学循环，促进溶液中生物可利用的 Fe(II) 的形成。考虑到这一点，必须考虑 Fe(III) 与酚类化合物的反应，以提高我们对全球铁循环的理解。