DCMU-resistance mutation confers resistance to high salt stress in the red microalga Porphyridium sp. (Rhodophyta)

The response of photosynthesis to salinity stress in wild-type and DCMU-resistant (strain DC-2) Porphyridium sp. was investigated. Both wild-type and DC-2 were able to acclimate and establish a new steady state of growth when exposed to salt stress, but growth rate of the DC-2 mutant strain was 1.25-fold higher than growth rate of wild-type cells. Following establishment of a new steady state under the salt stress conditions, the maximal irradiance-saturated photosynthetic capacity (P_max), light saturation (I_k) and the photosynthetic efficiency () were 1.5-, 1.8- and 1.1-fold, respectively, higher in the DC-2 mutant cells than wild-type. Furthermore, salt-stressed DC-2 mutant cells exhibited 1.6-fold higher maximal efficiency of PS II (Fv/Fm) after 200 min exposure and 3-fold higher level of D1 protein after 24h exposure to salt than wild-type cells. Exposing the salt-stressed cells to high photon flux density (3000mmol m^-2 s^-1) for 1h resulted in a higher resistance of mutant cells than wild-type to light stress, as reflected in 1.3-fold decline in the Fv/Fm. This finding corresponds well with the levels of the D1 protein measured in the cells. These observations indicate that the D1 protein of the PS II apparatus in the DC-2 mutant was more resistant to stress. However, under non-stressed (light and salt) conditions, no differences were observed between wild-type and the DC-2 mutant cells for all the parameters studied. The DC-2 mutant responded better than wild-type to environmental stresses, namely salinity and light due to the modification induced in PS II.