The Effects Of Low-dose UV-A Radiation On Photosynthetic Physiology And Metabolome In The Cyanobacterium Nostoc Sphaeroides

UV-A（315-400 nm）is the predominent component（up to 95%）of solar ultraviolet radiation（200-400 nm）,but in contrast to the widely reported biological effects of UV-B（280-315 nm）radiation,there are relatively few studies on the effects and mechanisms of UV-A radiation on photosynthetic organisms.UV-A radiation was reported to exert bi-lateral effects of photoautotrophs,and the biological effects are variable and usually influenced by other environment factors,such as visible light intensity and so on.Specifically,higher levels of UV-A under high light inhibit photosynthetic productivity of phytoplankton cells,but lower levels of UV-A under low light drive photosynthetic carbon fixation,especially for the algae with large size.What is the mechanism underlying low-dose UV-A radiation promoting photosynthetic productivity of algae?This is a topical current issue in the field of algal physiology and ecology.The present study investigated the effects of low-dose UV-A radiation on growth,photosynthetic physiology and metabolome on the rice-field nixtrogen-fixing cyanobacterium Nostoc sphaeroides,aiming to reveal the mechanism of UV-A radiation promoting algal productivity from the perspective of photosynthesis and metabolism,and to provide a theoretical basis for the application of UV-A irradiation in the high-yield cultivation of economic algae and microalgal green biomanufacturing.Effects of low-dose UV-A radiation on growth and photosynthetic physiology in Nostoc sphaeroides.Exposure to 5-15 W m^-2UV-A plus low visible light for 16 d significantly increased the growth rate and biomass production by 18-30%and15-56%respectively,compared with those under visible light alone.The UV-A acclimated cells showed a 24%increase in photosynthetic efficiency（α）,a 58%increase in photosynthetic capacity(P_max),a 73-81%and 47-60%increase in the actual photochemical efficiency Y（II）and the photochemically quench q_pin light(349-1197μmol photons m^-2s^-1)respectively,more rapid re-oxidation of the primary quinone-type acceptor,an 37%increase of the maximum photo-oxidizable P700along with 87%increased rate of cyclic electron flow around PSI.Western blot assay further showed that the abundance of PSI core proteins Psa A/B and Psa C of UV-A acclimated colonies significantly increased,its abundance of PSII core protein D1and CP47 showed no significant difference.We further observed that the UV-A acclimated cells exhibited a 30%rise of light-induced NADPH formation and a 26%rise of subsequent dark consumption by monitoring in vivo NADPH fluorescence kinetics relative to the non-acclimated cells.The results indicated that low dose UV-A radiation enhanced the photosynthetic performance and biomass of Nostoc sphaeroides.Effect of low dose UV-A radiation on the GC-MS metabolome of Nostoc sphaeroides.The results showed that 262 metabolites were detected,among which based on"P<0.05,fold changes greater than 1.5 and VIP>1",64 significantly changed metabolites were screened,consisting of 25 significantly up-regulated metabolites and39 significantly down-regulated metabolites.Among the significantly changed metabolites,the top three were organic oxygen compounds,Lipids,organic acids and their derivatives.Specifically,the significantly upregulated folds changes of sugars（lactose,mannose,fructose,etc.）were 1-5,the significantly upregulated folds changes of fatty acids（caproic acid,behenic acid,lignoceric acid,arachidic acid etc.）were 1-3,the significantly up-regulated folds changes of citric acid and methionine were 3.9and 1.0 respectively.The data indicated that low-dose UV-A radiation promoted the accumulation of carbon-related energy storage substances（sugars,fatty acids etc.）.