Elsevier

Aquatic Toxicology

Volume 245, April 2022, 106127
Aquatic Toxicology

Studies of the liposolubility and the ecotoxicity of MC-LR degradation by-products using computational molecular modeling and in-vivo tests with Chlorella vulgaris and Daphnia magna

https://doi.org/10.1016/j.aquatox.2022.106127Get rights and content

Highlights

  • The topological polar surface area (tPSA) of MC-LR has a hydrophilic character.

  • The topological polar surface area of by-product m/z 445 has a fat-soluble character.

  • The partition coefficient for n-octanol/water of m/z 445 (KOW = 2.02) is three times greater than that for MC-LR (KOW = 0.68).

  • The by-product m/z 445 has tPSA (133 Ų) three times smaller than MC-LR (341 Ų).

  • Toxicity tests with C. vulgaris and D. magna revealed that the m/z 445 by-product is more toxic than MC-LR.

Abstract

Computational molecular modelling, mass spectrometry and in-vivo tests with Chlorella vulgaris (C. vulgaris) and Daphnia magna (D. magna) were used to investigate the liposolubility and ecotoxicity of MC-LR degradation by-products generated after oxidation by OH radicals in Fenton process. Exposure of MC-LR (5 µg.L−1) to the most severe oxidation conditions (Fe2+ 20 mM and H2O2 60 mM) resulted in a reduction in the toxin concentration of 96% (0.16 µg.L−1), however, with the formation of many by-products. The by-product of m/z 445 was the most resistant to degradation and retained a toxic structure of diene bonds present in the Adda amino acid. Computational modeling revealed that m/z 445 (tPSA = 132.88 Ų; KOW = 2.02) is more fat-soluble than MC-LR (tPSA = 340.64 Ų; KOW = 0.68), evidencing an easier transport process of this by-product. Given this, toxicity tests using C. vulgaris and D. magna indicated greater toxicity of the by-product m/z 445 compared to MC-LR. When the conversion of MC-LR to by-products was 77%, the growth inhibition of C. vulgaris and the D. magna immobility were, respectively, 6.14 and 0%, with 96% conversion; growth inhibition and the immobility were both 100%  for both species

Section snippets

Introdution

The rapid growth of cyanobacteria in surface waters has attracted considerable attention due to the damage these organisms are causing to the ecological environment and human health (Wang et al., 2017; Su et al., 2020; Liang et al., 2020). These blooms may be associated with the formation of toxic metabolites in which cyanotoxins are commonly present (Liang et al., 2020). Among these toxins, Microcystine-Leucine Arginine (MC-LR) is one of the most common congeners and also one of the variants

Microcystin-LR and study water

The microcystin-LR used in this study was obtained from a strain of the cyanobacterium Microcystis aeruginosa (Word Data Center Microorganisms 835). The total MC-LR was considered: the sum of extracellular and intracellular fractions. For this, the cyanobacterial culture samples were subjected to three freeze/thaw cycles, with the objective of causing cell lysis and consequent release of the intracellular toxin to the extracellular medium (Brooke et al., 2006; Wang and Bai, 2017).

In the

Advanced oxidation process

Regarding the characterization of the study water, alkalinity of 53 mg.L−1 CaCO3 was obtained, hardness 40 mg.L−1 CaCO3 and pH around 7.6. Also observed were true color, turbidity and MC-LR in concentrations of, respectively, 105 Hz, 23 NTU and 5 µg.L−1. These physical-chemical characteristics are similar to those found in natural waters with high levels of contamination by cyanotoxins (Thees et al., 2018). Percent of color removal, turbidity and MC-LR concentration data at the end of the T1-T5

Conclusion

It was found that coagulation, decantation and filtration steps did not contribute to the removal of MC-LR, after oxidation by the action of OH radicals coming from the Fenton process. Toxin oxidation triggered the formation of oxidation by-products, of which the m/z 445 fragment was the most abundant and resistant to degradation, maintaining the molecular structure of diene bonds, which confer Adda toxicity. The surface charge intensity of the by-product molecule of m/z 445 (tPSA = 132.88 Ų)

CRediT authorship contribution statement

Maria Virgínia da Conceição Albuquerque: Methodology, Formal analysis, Investigation, Writing – original draft. Railson de Oliveira Ramos: Data curation, Software, Writing – review & editing. Valderi Duarte Leite: Conceptualization, Supervision, Resources. José Tavares de Sousa: Supervision, Resources, Writing – review & editing. Mário César Ugulino de Araújo: Software, Resources, Writing – review & editing. Beatriz Susana Ovruski de Ceballos: Methodology, Investigation, Supervision, Writing –

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors wish to thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Fundação de Apoio à Pesquisa do Estado da Paraíba (FAPESQ-PB). The English text of this paper has been revised by Sidney Pratt, Canadian, MAT (The Johns Hopkins University), RSAdip - TESL (Cambridge University).

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