Unraveling the toxicity response and metabolic compensation mechanism of tannic acid-Cr(III) complex on alga Raphidocelis subcapitata.

Due to their assembly properties and variable molecular weights, the potential biological toxicity effects of macromolecular organic ligand heavy metal complexes are more difficult to predict and their mechanisms are more complex. This study unraveled the toxicity response and metabolic compensation mechanism of tannic acid-Cr(III) (TA-Cr(III)) complex on alga Raphidocelis subcapitata using multi-omics approaches. Results showed TA-Cr(III) complex caused oxidative damage and photosystem disruption, destroying the cell morphology and inhibiting algal growth by >80 % at high exposure levels. TA-Cr(III) complex stress down-regulated proteins linked to proliferation, photosynthesis and antioxidation while upregulating carbon fixation, TCA cycle and amino acid metabolism. The increase of fumarate, citrate, isocitrate and semialdehyde succinate was validated by metabolomics analysis, which improved the TCA cycle, amino acid metabolism and carbon fixation. Activation of the above cellular processes somewhat compensated for the inhibition of algal photosynthesis by TA-Cr(III) complex exposure. In conclusion, physiological toxicity coupled with downstream metabolic compensation in response to Cr(III) complex of macromolecular was characterized in Raphidocelis subcapitata, unveiling the adaptive mechanism of algae under the stress of heavy metal complexes with macromolecular organic ligands.

Fenton reaction induced in-situ redox and re-complexation of polyphenol-Cr complex and their products.

In this study, in-situ Fenton oxidation was used for the de-complexation and degradation of tannin-Cr(III) complexes. Cr(III) can be oxidized into free Cr(VI) under the effect of ·OH and oxidation products of tannin can be used as reductant for Cr(VI) to establish a redox cycle of Cr(III)-Cr(VI)-Cr(III). Thus, it is crucial to investigate the interactions of Cr(III) with tannin derived oxidation products due to negligible accumulation of Cr(VI) during Fenton oxidation treatment. Here, sequential filtration/ultrafiltration was applied to reveal the distribution characteristics of TOC and Cr fractions during the oxidation of tannin-Cr(III). As the increase of colloidal size of tannic acid products, residual TOC and Cr mainly distribute in larger size range after the oxidation of tannin-Cr(III) which can be ascribed to re-complexation between oxidation products and Cr(III). Besides, analytical results indicate that carboxyl group and hydroxyl group in oxidation products may cause the re-complexation of Cr(III), resulting in the formation of highly conjugated materials containing Cr(III). It can be concluded that due attention should be paid to the efficient removal technology and mechanism for polymer-Cr complexes, as well as the oxidation intermediates in the role of conversion and removal of Cr species.