| The depletion of stratospheric ozone resulted from human activities hasfacilitated more UV-B radiation reaching the Earth’s surface, which would furthercontribute to climate change. Therefore, photoautotrophic phytoplankton might beexposed to the enhanced UV-B radiation resulted from their obligatory requirement ofsolar radiation for growth and survival. Microalgae, a group of photosyntheticorganisms, are distributed extensively in the aquatic habitats, and play a basic role inenergy transfer and material metabolism in marine ecosystem, hence, UV-B radiation,especially its effect on microalgae, has heightened concerns over the past threedecades, and appeared to be a vital issue in the years to come. Furthermore, it couldprovide the theoretical basis for adaptation and develop a better understanding aboutthe evolution information of marine microalgae reponsed to changes of UV-Bradiation.In this study, marine model microorganism-Dunaliella salina was used as target,and enhanced ultraviolet-B radiation was performed as exposure stress. The responsesof growth, cell structure of D. salina as well as physiological and biochemical processto moderate UV-B radiation were discolsed. Moreover, the mechanism underlyingwas discussed based on ROS and Ca2+signaling pathway. The adaption of D.salina tolethal dose of UV-B radiation was discovered on the basis of the evolution model offluctuation analysis. The main goals are to enlarge the knowledge about the effect ofUV-B radiation on microalgae and supply the information with respect to adaptivestrategies of D. salina at the background of climate change.1. Responses of D.salina to moderate UV-B radiation based on different levels oforganization(1) Enhanced UV-B radiation significantly inhibited the population growth of D.salina, and provoked severe effect on the growth rate for D.salina according toDGRs. Whereas, dry biomass and soluble protein per cell was elevated by UV-Bradiation. The obvious dosage-effect relationship was observed between UV-Bradiation and dry biomass as well as soluble protein.(2) Inverted light microscopy and transmission electron microscope were used toreveal the known cell architechture of D.salina. Cell structure and microstructurewere found to change when exposed to UV-B radiation. The cell volume becamelarger and the shape turned into round. An amount of vacuoles around the nucleusincreased when UV-B radiation elevated. The nucleus appeared disintegrated in the1.00KJ m-2d-1group. In the chloroplast, grana lamellae became less clear, and net-likearrangement of the thylakoids was lost. Obviously, plasmalemma was fracturedinduced by enhanced UV-B radiation, and the previously compact starch sheathturned loose. Additionally, enhanced UV-B radiation also resulted in theaccumulation of lipid globule.(3) The process of physiology and biochemistry suffered stress from UV-Bradiation including photosynthesis, nutrient uptake and metabolism as well as osmoticregulation. The content of cell pigments including chlorophyll a, b and carotenoidswere shown diverse pattern of variance during96h when exposed to different levelsof UV-B radiation, and UV-B radiation had a marked effect on ratios of Chla/Chlb andChl/Car for microalgae. Based on the careful interpretation of PAM parameters,enhanced UV-B radiation was revealed to have a negative impact on the electrontransport rate, tolerance of light intensity, capacity of capturing energy and light utilityefficiency, and simultaneously alter the pathway of excess energy dissipation based onchlorophyll fluorescence via the combination of rapid light curves (RLCs) withinduction curve. In other words, light harvesting complex, electron transport chainand photochemical reaction may be the targets for UV-B radiation, but this conclusionstill need more details to be verified further. Moreover, UV-B radiations stimulatedthe uptake of NO3-notably apparently, of which the maximum was up to34.31×10-9μmol cell-1h-1in0.75KJ m-2d-1group. Unlike the uptake of NO3-, alltreated groups had a statistical decline in the uptake of PO43-for D.salina. Activities of acid phosphatase (APase) activity, alkaline phosphatase (AKP) and nitratereductase (NR) in D.salina were observed by different doses of UV-B radiation.Content of glycerol was enhanced in all treated groups compared with control group(P<0.05), that meant UV-B radiation disrupted the osmotic balance of microalgae.Above all, UV-B induced-damage is likely to be a dynamic balance between damageand adaptation.2. The mechanism underlying about the response of D.salina to moderate UV-Bradiation based on ROS and Ca2+signaling pathway.(1) UV-B had an oxidative stress on microalgae: the content of thiobarbituricacid reactive substances was enhanced remarkably; and the activity of antioxidantenzyme including SOD, GPx, GR, GST and CAT altered; and simultaneously, contentof glutathione and ascorbic acid was stimulated to increase. It seemed that ROSsignaling pathway was involved in the response of D.salina to moderate UV-Bradiation.(2) The activity of Ca2+-ATPase was induced to increase which meant that Ca2+signaling pathway was involved in the response of D.salina to moderate UV-Bradiation(3) The addition of CaCl2and LaCl3had no effect on the activity of antioxidantenzyme, but BAPTA, an inhibitor of intracellular Ca2+, significantly inhibited theenzyme activity. Additionally, the activity of Ca2+-ATPase decreased when NAC, akind of ROS inhibitor, was added into the medium. It was suggested that free calciumcame from internal calcium storage not external storage. Furthermore, there was crosstalk relationship between ROS and Ca2+signaling pathway, both of which mayberelated with the glutathione cycle.3. Responses of D.salina to lethal UV-B radiation based on fluctuation analysis(1) During the initial exposure phase, a large number of sensitive cells died.Whereas, the population remained to growth due to the occurrence of theUV-B-resistance cells.(2) The resistance cells rose from the pre-selective adaptation due to randommutation. It was proposed that the pre-selective adaptative mutations would ensure the survival of D.salina in the environment with constantly changing UV-B level. |
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