Green microalgae in removal and biotransformation of estradiol and ethinylestradiol

Estrogens, a group of steroid hormones, are serious environmental pollutants due to their persistence, ubiquity, and endocrine-disrupting toxicity. Natural and synthetic estrogens, such as 17β-estradiol (E2) and 17α-ethinylestradiol (EE2), respectively, have been found in the influent of municipal wastewater treatment plants. The removal and biotransformation of mixed or single E2 and EE2 by six microalgal species, including three local isolates of Chlamydomonas sp. (WW), Chlorella sp. (2f5aia), and Chlorella sp. (1uoai) and three commercially available species, namely Scenedesmus quadricauda (= Desmodesmus communis) (SQ), Selenastrum capricornutum (SC), and Chlorella vulgaris (CV), were compared. The biological removal of estrogen was incubation time- and species-dependent. Among six species, SC had the highest removal ability for both E2 and EE2; their removal increased with incubation time and achieved 93.9 and 75.3 % removal, respectively, at the end of incubation. After 7-day incubation, the removal percentage of mixed EE2 by SC was significantly higher than that of single EE2 (P ≤ 0.05). Differences in the removal percentages of E2 and EE2 between commercial and local species were not significant, as reflected by the comparable removal efficiency among three Chlorella species, CV, 1uoai, and 2f5aia, with the highest removal of E2, either in mixed or in single treatment, at day 2. In all species, the percentages of E2 or EE2 taken up into cells were very low (0.2–9.6 %) when compared to the respective biological removal percentages. E2 was easily transformed into estrone (E1) and some unknown products by all microalgal species, while the products for EE2 transformation could not be identified due to the limited availability of the standards for metabolites.