Effects Of Environmental Stresses (Salt-Stress, Heat-Stress, Osmotic-stress) On Two Marine Phytoplankton

The effects of salt-stress, heat-stress and osmotic-stress on diatom Chaetoceros gracilis and green alga Chlorella minutissima were investigated by analysis of chlorophyll (chl) fluorescence dynamics, superoxide dismutase (SOD) activity and proline (Pro) content in vivo. It was found that the optimal salinities for Cha. gracilis and Chl. minutissima were 10 ‰~15‰ and 20‰~35‰, respectively. Salt-stress can induce Fv/Fm, Fv'/Fm', qP and PS II to decrease but induce qN to increase. These results indicate that the PS II activities of the two algae were inhibited, but the algae can protect their PS II by increasing heat dissipation. Osmotic-stress decreased Fv/Fm of Cha. gracilis . However, the effect of osmotic-stress on Chl. minutissima was different from the effect on Cha. gracilis, there was no apparent changes in Fv/Fm and Fv'/Fm', but found an increase in qP and OPS II correlated with a decrease in qN. Meanwhile, salt-stress made the osmotic-stress effect more severe. Heat-stress can also inhibit the PS n activities of the two algae as showed in decrease of Fv/Fm and increase in Fo. There was a slightly increase of Fv/Fm in Cha. gracilis at 35C, which suggested there maybe an increase in the fluidity of the thylakoid membrane, which caused an acceleration of electron transport. Under heat-stress of salt-stressed algae, the thermostability of PS n was enhanced, so there was only a smaller decrease in Fv/Fm.When the salinities were lower than the optimal salinities of the two algae, the SOD activity decreased with salinity increasing. But when the salinities were higher than the optimal salinities, the SOD activity increased with the increase of salinity. Thus the SOD curve has a " V " style. Osmotic-stress, as well as mild heat-stress, also can lead to an increase of the SOD activity in both algae. However, severe heat-stress can induce the SOD activity to decrease. Under environmental stresses, the increase of SOD activity can protect the two algae from the damage of free radicals.Under the three type of stresses, both algae can accumulate Pro, and also can absorb exogenous Pro from outside. Pro must play an important role as a protectant under stresses in both algae.