Redox changes in microalga Chlorella sorokiniana exposed to high concentrations of Mn(II)

Abstract

Microalgae can be implemented in the remediation of mining and other metal-rich wastewaters as pioneer species. On the other hand, metals affect aquatic ecosystems through the negative impact on microalgae which are the primary producers of O2 and biomass. Redox processes represent an important component of the mechanisms of interaction of microalgae with transition metals. We analyzed the redox changes in Chlorella sorokiniana culture that are induced by high levels of Mn(II). Mn is the key metal pollutant, with five main oxidation forms that can bind to a variety of different ligands. Mn (1 mM) induced a significant increase in the intracellular production of reactive oxygen species. The boost appears to show two phases – the first is very fast (observed after 15 min), whereas the second starts after 1 h reaching a plateau at 24 h. The concentration of reduced thiols, which represent important targets of oxidation, appears to parallel this trend. Total glutathione concentration shows a drop at 1 h and recovery at 24 h. This implicates that either a glutathionylation of proteins or a synthesis of phytochelatins - sulfur-rich short-chain peptides that sequester metals, takes place early in the response to Mn. Further, FTIR analysis showed that Mn induced a decrease of C=C levels and CH2/CH3 ratio implicating increased lipid peroxidation. Finally, Mn ions that were accumulated in the cells were extracted with nitric oxide and analyzed by cyclic voltammetry. Two redox forms were detected - Mn(II) and Mn(IV). The latter appears to prevail at higher manganese concentrations and longer periods of incubation. These results demonstrate that redox response of C. sorokiniana to high Mn levels involves at least two phases. Initially, Mn(II) enters the cells and induces pro-oxidative changes that are mitigated by glutathione-based antioxidative defense. Later on, redox homeostasis is reestablished with concomitant inactivation of Mn in the more stable redox form.