High hydrogen peroxide production and antioxidative enzymes expression in the Chlamydomonas reinhardtii cia3 mutant with an increased tolerance to cadmium and anthracene

Chlamydomonas reinhardtii cia3 mutant is deficient in activity of CAH3, a carbonic anhydrase isoform necessary for proper functioning of photosystem II (PS II). We used cia3 to verify the hypothesis that despite high concentration of external CO2, deficiency in CAH3 activity results in slight disorder of photosynthesis, which leads to mild but permanent overproduction of reactive oxygen species (ROS). As a consequence, cia3 should possess high ability to rebalance ROS homeostasis to overcome the oxidative stress induced by environmental factors, including chemical contaminations. We have found that in control conditions cia3 had a larger fraction of active PS II reaction centers but their efficiency was lower as compared with the wild type (WT), while H2O2 production by cia3 was significantly higher than in WT. In regard to ROS-scavenging enzymes, the transcript level for superoxide dismutase (SOD) isoforms and the isoenzymes activity in cia3 were higher than in WT. There were no significant differences in the transcript level of ascorbate peroxidase (APX) and catalase (CAT) genes in both strains, and slightly elevated activity of the enzymes was observed in the cia3-cells. Comparing the sensitivity of WT and cia3 to chemically-induced stress we have found that the growth, photosynthesis and H2O2 production in WT were more strongly affected by cadmium and anthracene as compared with cia3. SODs expression and activity in the treated WT were higher than those in cia3, but APX and CAT activities were similar in both strains. Our results indicate that the CAH3-deficiency is associated with markedly elevated H2O2 production and with a relatively higher initial transcript level and the activity of ROS-scavenging enzymes. In consequence, cia3 revealed a higher tolerance to cadmium and anthracene compared with WT.