Study Of Adaptive Features And Mechanism Of Photosynthetic System In Floating Ulva Prolifera Against Typical Environmental Changes

Green tides are a kind of harmful algal bloom. The green tides happened because the large-scale green algae abscised from the solid substratum and lived in a free-floating state, rapidly proliferated the biomass of the thalli. These years, it can be observed that a rising trend in the frequency of occurrence and the influence scale. From 2007 to 2014, the green tides happened in the Yellow Sea in 8 successive years. The science researchers called it the "Yellow Sea green tide". Local ecnomy, livelihood of the local inhabitantand local entironment was impacted by the green tides in these 8 years. It can be observed that the representative characteristic of the green tide breaking out for several continuous years:1. Ulva prolifera was the single dominant species of the yellow sea.2. The distance from the headstream to the disaster zone was very far. The headstream was nearshore of Jiangsu Province, while the green tide broke out alongshore the city Rizhao and Qingdao.3. When the green tide happened, the green algae abscised from the solid substratum and lived in a free-floating state. Therefore, the long time and long distance floating shift of the U. prolifera is the most representative characteristic of the "Yellow Sea green tide". Light, temperature and salinity of the surface of the ocean can be acutely influenced. There must be mechanism for U. prolifera to live in a free-floating state for a long time and long distance. However, few researchers studied the adaptive features of the free-floating U. prolifera. Photosynthesis is the motivity for the plants to stockpile substance and energy. Hence, we speculated that the photosystem of U. prolifera can adapt to the acute environmental change on the surface of the ocean. In view of this, this research studied the difference of photosynthesis photoreaction stage between the surface layer and lower layer floating U. prolifera. The main methods and results were that:1. Result of photosynthetic pigments concentration between the different states of thalli showed that the lower layer chlorophyll a concentration, chlorophyll b concentration and total chlorophyll concentration were obviously higher than in that of suface layer. However, the surface layer of chlorophyll a concentration/chlorophyll b concentrations were significantly higher than the lower layer2. Result of Chl fluorescence showed that the Fv/Fm of lower layer was significantly higher than that of surface layer. At the same tioe, it can be observed that similar result in the effective quantum yield of photosystems [Y(Ⅱ) and Y(Ⅰ)] and the rapid light response curves (RLCs), the related result of lower layer was higher than that of surface layer. Base on these results, the conclusion is that the photosynthetic activity in lower layer was higher than that of lower layer.3. Result of fluorescence quenching showed that nonphotochemical quantum yields of PSI and PSII [Y(NO), Y(NPQ), Y(ND) and Y(NA)] in surface layer was relatively higher than that of lower layer. This may mean that thalli of surface layer may suffer much more environmental disturbance, but it can protect itself effectively. The concussion is that thalli of surface layer can adapt to the environmental change very well.4. Result of energy distribution showed that thalli of surface layer can transmit the energy from PSII to PSI. When the inhibitor DCMU was added to the experiment system, it could be found that thalli of surface layer depended on the cyclic electron flow around PSI to share responsibility for the redundant energy from PSII.5. Result of genetic transcription showed that transcription level of gene LhcSR and PsbS in thalli of lower layer was much higher than that of surface layer (P<0.01, ANOVA). This result implied that NPQ in thalli of surface layer can work effectively to dissipate the excess energy to protect itself.Yellow Sea green tide had broken out for 8 successive years thanks to the characteristics of the thalli itself. Firstly, activity of photosynthesis in U. prolifera is very high even though the illumination intensity in lower layer was very low. To adapt to this kind of living environment. U. prolifera can enhanced chlorophyll a. chlorophyll b and total chlorophyll concentrations and reduced the ratio of Chla/Chlb to enhance the activity of utilizing luminous energy. Secondly, although activity of photosynthesis in surface layer was lower than that of lower layer, thalli of surface layer can protect itself with the help of NPQ to dissipate the excess energy and the energy distribution between PSII and PSI. At the same time, the location of surface layer and lower layer can exchange with each other. When the thalli of lower layer moved to the surface, even though the photosynthesis was inhibited, it wasn’t destroied in essence. When the thalli of surface layer went back to the lower, activity of photosynthesis can recover.In this present study, the knowledge of the photoprotection mechanism in U. prolifera. At the same time, it can explain to us the theoretical basis of governing the environment.