Biodegradation pathways and mechanisms of 17α-ethynylestradiol via functional enzymes in the freshwater microalga Scenedesmus quadricauda

Abstract

17α-ethynylestradiol (EE2) is a potent synthetic hormone exhibiting very high estrogenic activity and low rates of biodegradation. The removal capabilities of EE2 by bacteria, fungi and algal-bacterial symbiotic systems have attracted considerable attention recently. Specifically, algal biodegradation has been explored recently; however, the pathway and mechanisms of EE2 degradation have remained largely unknown. Therefore, we investigated the pathways and mechanisms by which EE2 is degraded by the freshwater microalga Scenedesmus quadricauda. After exposure for 10.5 d, the algal species was able to metabolize 58 % of a 15 mg/L solution of EE2, with the highest removal rate of 13 % occurring at 1.5 d An Ultra Performance Liquid Chromatography-Q-Exactive Orbitrap Mass Spectrometry was used innovatively to identify the biodegradation products of EE2 through non-target screening, followed by the verification of standard compounds. Transcriptomic analysis and molecular docking analysis revealed several degradation pathways and mechanisms by this algal species. One pathway was the demethylation of EE2 to estradiol (E2) by short-chain dehydrogenase/reductase. Subsequently, we also observed interconversion of estrone (E1) and E2 by 17β-hydroxysteroid dehydrogenase through hydroxylation or ketonization, hydroxylation of E1 to 16α-hydroxyestrone (16-OH E1) by cytochrome P450 and flavin-containing monooxygenase. A second pathway was methoxylation of E2 to estradiol acetate by catechol O-methyltransferase. As a result, the ethynyl group was degraded to hydroxy, ketone and methoxyl groups, which promotes EE2 degradation. Considering that EE2 pollution could result in adverse effects for aquatic organisms, the results of this study provide insights and a comprehensive approach for practical and effective bioremediation of EE2 contamination in aquatic ecosystems.