Inhibitory effects of hypo-osmotic stress on extracellular carbonic anhydrase and photosynthetic efficiency of green alga Dunaliella salina possibly through reactive oxygen species formation

In this study, Dunaliella salina (D. sauna) maintained in 30 parts per thousand salinity for more than two years was exposed to the salinities of 5 parts per thousand, 10 parts per thousand, 20 parts per thousand, 30 parts per thousand (control) in order to investigate oxidative burst and it's possible connection with extracellular carbonic anhydrase (CA) under hypo-osmotic stress (low salinity). The results indicated that intracellular ROS contents increased significantly when cells were exposed to salinity of 5 and 10 parts per thousand, and the increase also occurred at 20 parts per thousand. salinity. The activity of extracellular CA and its gene (P60) expression decreased significantly when cells were exposed to salinity of 5-20 parts per thousand. Data from H2O2 treatments hinted that ROS production was possibly one of the factors affecting CA, including enzyme activity and gene expression levels. Significant inhibition of effective quantum efficiency of PSII and photosynthetic oxygen evolution rate were observed with the increase of ROS production and decline of CA activities. Taken together, hypo-osmotic stresses could induce ROS production in D. salina, and CA enzyme activities and expression levels were consequently inhibited. As a result, algal photosynthesis and oxygen evolution were inhibited. (C) 2012 Elsevier Masson SAS. All rights reserved.