Endogenous ascorbate level modulates ozone tolerance in Euglena gracilis cells

A wild type (WT) and a colourless mutant (CM) of Euglena gracilis were used to study the biochemical effects of ozone (O3) stress on the components of the ascorbate-glutathione cycle (AA-GSH cycle). The WT cells had higher initial ascorbate (AA) and ascorbate peroxidase (APX, EC 1.11.1.11) activity but lower glutathione (GSH), glutathione reductase (GR, EC 1.1.1.49) and glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activities than the CM cells. Ozone exposure (1.0 μmol O3·min−1) of both cell types resulted in a viability loss of 9 % and 15 % for the WT and CM cells, respectively. After O3 exposure, a similar reduction in GSH content was observed in both cell types, whereas the other components of the AA-GSH cycle showed no change. Our results indicate that the higher tolerance of the WT cells to O3 is associated with a higher initial AA content. To confirm this, the endogenous AA content in both cell types was modified by adding L-galactono-γ-lactone (L-GAL) or D-galactono-γ-lactone (D-GAL) to the growth medium. L-GAL treatment resulted in a 5.2-fold increase in the initial AA content and an increase of O3 tolerance in the CM cells. On the other hand, D-GAL treatment decreased the AA content as well as O3 tolerance of WT cells. Together, these results suggest that O3 tolerance is closely correlated with the AA content in Euglena cells.