Investigations On The Production Of Mycosporine-like Amino Acids In Diverse Cyanobacteria From Different Habitats

Cyanobacteria are photoautotrophs prokaryotes appeared on the earth more than 2.6 billion years.They are widely distributed in terrestrial and aquatic ecosystems,and contributed significantly to global primary productivity and nitrogen fixation.However,the solar energy requirement for photosynthesis inevitably exposes cyanobacteria to the increasing UV radiation on the Earth surface.In the long life evolution history,cyanobacteria have developed tolerance and adaptation mechanisms against UV radiation,such as production of mycosporine-like amino acids(MAAs)as UV photoprotectants.MAAs represented a suite of low molecular weight,water-soluble and colorless UV-absorbing compounds.Various amino acids were conjugated with a cyclohexenone or cyclohexeniminone rings resulting in the formation of diverse MAAs.Different substituents on the central cyclohexenone or cyclohexeniminone rings conferred the variations of UV-absorbance and high molar absorptivity.MAAs absorbed the harmful UV radiation and converted it into heat in the environments.The formulation containing certain MAAs as active ingredients has even been developed in some cosmetic products.However,the new kinds of MAAs and MAA producers have being explored to develop next generation of sun care products.Recent work in the laboratory characterized a novel MAA of mycosporine-2-(4-deoxygadusolyl-ornithine)from Nostoc flagelliforme distributed in arid and semiarid areas around the world.Present work tested MAA production in cyanobacteria from soil exposed to strong solar radiation and desiccation,and also from various shaded environments.Besides,the widespread Nostoc cyanobacterial strains were chosen to further test MAA production.After all,the following results were gained for the current dissertation.One filamentous cyanobacterial strain was isolated from the topsoil in Chetimari,which was located in the southeast of Niger and characterized by desert climate with strong solar radiation and quite low precipitation all the year.The 16S rDNA phylogenetic analysis classified the isolate as Pseudanabaena sp.CCNU1.UV-B radiation significantly enhanced the production of active photoprotectants such as UV-absorbing compounds mycosporine-like amino acids(MAAs)and carotenoids in Pseudanabaena sp.CCNU1.One type of MAAs was induced by UV-B radiation in Pseudanabaena sp.CCNU1,and was further characterized as palythine-serine.The content of palythine-serine could reach up to 16 mg/g dry weight in Pseudanabaena sp.CCNU1 under UV-B treatments for 4 days.Five specific kinds of carotenoids have been characterized as myxoxanthophyll,zeaxanthin,canthaxanthin,echinenone,and ?-carotene in Pseudanabaena sp.CCNU1.UV-B radiation increased the content of myxoxanthophyll,zeaxanthin and canthaxanthin,but decreased the content of echinenone and ?-carotene.Pseudanabaena sp.CCNU1 was the first reported organism to only induce considerable palythine-serine production to protect against UV-B radiation.Present work provided an ideal producer of these unique UV photoprotectants and paved the way for genetics studies of their biosynthetic pathways from biotechnology perspective.MAAs played necessary roles in protecting cyanobacteria from UV radiation and were frequently reported in cyanobacteria exposed to high light radiation.Here,the MAA production was tried in the chlorophyll f-producing cyanobacterial strains isolated from the shaded environments.Four Chroococcidiopsis strains were finally induced to produce MAAs under 0.15 W·m-2 of UV-B exposure,and the MAA contents increased along with the prolonged UV-B treatments in these four Chroococcidiopsis strains.After separation by HPLC system,one MAA type was detected at the similar retention times in the methanol extracts of Chroococcidiopsis strains,and all the MAA compounds showed in-line absorption at 310 nm and mass spectra 246 m/z.The absorption spectra and mass spectra matched well the characteristics of the simplest MAA mycosporine-glycine.Chroococcidiopsis had the simplest MAA gene clusters for mycosporine-glycine.Furthermore,MAA gene clusters were also found in a few other chlorophyll f-producing cyanobacteria,and either contained the mysA,mysB and mysC1 or plus with nrps.Together,MAAs could also be produced in the cyanobacteria even distributed in the light-deficient niches.These results suggested other roles of MAAs in addition to UV-B protection in the special cyanobacteria from shaded environments.The Nostoc species were nitrogen fixing cyanobacteria and widely distributed in various terrestrial environments.Three Nostoc strains distributed in soil and freshwater were tested for MAA production under different nitrogen conditions.UV-B radiation significantly enhanced the production of MAAs in Nostoc linckia,Nostoc muscorum and Nostoc paludosum under nitrogen sufficient and deficient conditions.The contents of MAAs significantly increased in these three Nostoc strains with prolonged UV-B treatment under both nitrogen conditions.Both UV-B exposed N.muscorum and N.paludosum produced much less MAAs under nitrogen deficiency in comparison to culture under nitrogen sufficiency,respectively.There was just little difference of MAA contents in UV-B treated N.linckia when it was cultured under both nitrogen conditions.After separation by HPLC and characterization by in-line absorption spectra and mass spectra,one type of MAA porphyra was induced by UV-B in N.linckia under nitrogen sufficient and deficient conditions,and N.muscorum synthesized another kind of MAA shinorine under nitrogen sufficient and deficient conditions.However,UV-B induced N.paludosum to produce porphyra under nitrogen sufficient condition but shinorine under nitrogen deficient condition.All together these results indicated the nutrient nitrogen levels could regulate the types and amounts of MAAs in Nostoc species dependent manners.In summary,present studies provided useful sources for MAA products in various cyanobacteria.Palythine-serine was firstly characterized as the only one type of MAA production in the newly isolate of Pseudanabaena sp.CCNU1 from topsoil subjected to strong solar radiation and desiccation all the year.The simplest MAA mycosporine-glycine was UV-B inducible in the four Chroococcidiopsis strains from shaded environments of rock,soil,stream and pond.Porphyra and shinorine were biosynthesized in UV-B treated Nostoc strains from soils and freshwater,but were dependent on the specific strains and nitrogen levels.MAA production varied significantly in cyanobacteria from different environments.The shaded environments provided universal niches with depleted UV-visible light but enriched with far-red light for certain cyanobacteria to gain the abilities of production of single and simple MAAs.The extreme environments with strong solar radiation and desiccation could be explored for new cyanobacterial isolates in production of novel MAAs.The various terrestrial environments with fluctuations of light intensities and nutrient contents likely correlated with diverse MAA production in different cyanobacteria.Generally,light and nutrient seemed to play key roles in regulation of MAA production in cyanobacteria from natural niches.