Detoxification and removal of heavy metal by an acid-tolerant microalgae, Graesiella sp. MA1

Acidophilic/acid-resistant algae exhibit innate survival advantages in acid mine drainage (AMD), but their response remains poorly understood. In this study, the detoxification and removal of Graesiella sp. MA1 isolated from AMD to the heavy metals of Cu2+ and Mn2+ at pH 3.5 was evaluated. The physiological response mechanism of the algae to these two heavy metals was analyzed by measuring the changes of algal biomass, photosynthetic pigments, malondialdehyde (MDA), reduced glutathione (GSH) and sulfhydryl (-SH) contents, as well as the specific activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). The main functional groups bound to heavy metals in microalgae were analyzed using FITR and XPS spectra. Results indicate metal-induced growth and photosynthesis impacts, with MDA rising alongside metal concentrations. However, Graesiella sp. MA1 exhibited a certain degree of removal ability toward Cu2+ and Mn2+, and increased the pH of the medium from 3.5 to 5.12–5.73 and 9.07 (control group), indicating the potential ability to regulate the environmental pH. Additionally, this strain alleviated the toxic effects of heavy metals by regulating the level of SOD, APX, GSH, and -SH in the antioxidant system. FTIR and XPS analysis reveal that the functional groups, such as carboxyl, amide I and amide II, which related to proteins and polysaccharides on the microalgal surface, combined with Cu2+ and Mn2+ for extracellular removal. Clarifying the detoxification and removal process of heavy metals from acid-tolerant microalgae will help us to better use microalgae to repair AMD in situ.