The Physiological And Bioche Mical Basis Of Ulva Prolifera Response To Stresses

Free-floating Ulva prolifera is one of the causative species of green tides. When green tides occur, massive mats of floating U. prolifera thalli accumulate rapidly in surface seawater within a short period of time. The upper thalli of the mats are usually exposed to the atmosphere for long periods of time, during which they are challenged by factors such as high salinity and dehydration. In addition, the attached thalli inhabit in the intertidal zone, where they are submerged in seawater at high tide and exposed to air at low tide. When at low tide, the thalli are severely challenged by environmental changes. Both the floating and attached thalli could be exposed to stresses. However, they demonstrate an extreme tolerance to these stresses. In this study, U. prolifera thalli were used as the material to study the physiological and biochemical responses to stresses. The main research contents include:(1) Study the correlation between photosystem(PS)I response and oxidative pentose phosphate pathway activity during salt stress in U. prolifera. In thalli that underwent salt treatments, the cyclic electron ?ow through PSI showed greater stress tolerance than the ?ow through PSII. Even though the linear electron ?ow was suppressed by DCMU, the cyclic electron ?ow still operated. The electron transport rate I(ETRI) increased as the salinity increased when the thalli recovered in seawater containing DCMU. These results suggested that PSI receives electrons from a source other than PSII. Furthermore, the starch content and RPK activity decreased, while the content of NADPH and total RNA, and the activity of G6 PDH increased under salt stress. Soluble sugar from starch degradation may enter the oxidative pentose phosphate pathway(OPPP) to produce NADPH and ribose 5-phosphate. Data analysis suggests that NADPH provides electrons for PSI in Ulva prolifera during salt stress, the OPPP participates in the stress response and total RNA is synthesized in excess to assist recovery.(2) Study the changes in the proteome of U. prolifera in response to salt stress. Stress-tolerance proteins appeared to have an importa nt role in response to salt stress, whereas the abundance of proteins related to metabolism showed no significant change under low salt treatments. The expression of the PSII complex protein decreased with the increase of salinity, whereas the PSI complex protein increased, which is consistent with the photosynthetic activity changes. The increasing activity and component of PSI is helpful to the distribution of light and reduce the damage to PSII.(3) The photosynthetic performances of PSI and PSII in U. prolifera thalli exposed to different atmospheric carbon dioxide(CO2) concentrations were measured. Changes in photosynthesis during salt treatment and dehydration under different CO 2 concentrations were also analyzed. The results showed that PSII activity was enhanced as CO2 increased, which suggested that CO2 assimilation was enhanced and that U. prolifera thalli can utilize CO2 in the atmosphere directly, even when under moderate stress. These findings could explain the extremely high growth rate of free- floating U. prolifera when green tides occur.The study analyzed the changes of physiological and biochemical of U. prolifera treated with stresses. It was found that the OPPP was enhanced during salt stress, and the produced NADPH plays an important role in providing electrons to PSI. In addition, there were no significant changes in the content of Calvin cycle-related enzymes in moderate stress, and in this time thalli can directly use CO2 in the atmosphere. These may be one of the main reasons for rapid accumulation of U. prolifera biomass during green tides outbreak.