| The soil salinization of crops is increasing,which seriously affects the growth,development,yield and quality of crops,and restricts the sustainable development of agriculture.Improving soil salinization and increasing plant resistance to salt stress are important measures to address the harms of salinization.There are many symbiotic bacteria in the phycosphere of photoautotrophic microalgae,some of which can promote the growth of algal cells or improve the resistance to stress.Whether the algal symbiosis bacteria of Dunaliella salina from high-salinity water can promote the growth of algal cells or improve the salinity tolerance of plants has not been reported.Therefore,the Ds-SXYC-2 strain of Dunaliella salina from high-salt water was used as the experimental material.The phycosphere microorganisms of Dunaliella salina were isolated.Among them,the beneficial strains that could promote the growth of algal and the enrichment of β-carotene were screened and identified.In addition,the algal-bacteria symbiosis system was constructed to establish an efficient Dunaliella salina culture system for the large-scale production of Dunaliella salina biomass and natural β-carotene.Further research was carried out to determine whether the isolated growth-promoting bacteria strain could promote the growth and salt stress resistance of pakchoi.The main research results were summarized as follows:(1)A microalga was isolated from water samples of Yuncheng salt lake and identified as Dunaliella salina,named Ds-SXYC-2 by evolutionary tree based on the 18 S sequence alignment and evolutionary tree analysis.The growth of Dunaliella salina was measured under different salt concentrations and the optimum growth salinity of Dunaliella salina was 1.0 mol/L.Under this salt concentration,Dunaliella salina had the highest biomass and could accumulate more pigments and oils.When cultured under the optimal salt concentration,the content of the biomass,β-carotene and lipid of Dunaliella salina were 1.93g/L,2.55 mg/L,and 15.25%,respectively.(2)Five symbiotic bacteria strains were isolated from the Dunaliella salina and identified by 16 S rDNA molecular,they belongs to three bacteria genera.Strains B1 and B2 were Nesterenkonia,strains B3 and B4 were Halomonas,and strains B5 were Marinobacter.The strain B3 could significantly promote the growth of Dunaliella salina and the accumulation of metabolites,which was the optimal growth-promoting strain.After 15 days of co-cultivation with strain B3 at 1:1,Dunaliella salina obtained the maximum biomass,reached to 2.3 g/L,which was 28.9% higher than that of the control group.A co-culture system with different inoculation ratio between Dunaliella salina and strain B3 was established to measure the growth and metabolite accumulation of Dunaliella salina.The results showed that when the algal bacteria ratio was 1:3,the maximum biomass of Dunaliella salina was 2.49 g/L,which was 35.2% higher than the control.At this time,the metabolite content in Dunaliella salina cells also reached a peak.Among them,the content of β-carotene and oleic acid(18: 1)were 65.3% and 18.8% higher than the control,respectively.Halomonas(strain B3)can be used as the dominant strain to promote the growth and metabolite accumulation of Dunaliella salina,and the further construction of co-culture system can be applied to the commercial production of Dunaliella salina.(3)The effects of probiotic-Halomonas(strain B3)on seed germination and seedling growth of pakchoi were detected under hydroponics salt stress in greenhouse.The results showed that strain B3 could effectively reduce the inhibition of pakchoi seed germination and seedling growth by salt stress,and the higher concentration of salt,the more effect.When the salt concentration was 200 mmol/L,adding strain B3 increased the germination rate of pakchoi seeds by one time and the fresh weight of seedlings increased by 83.5%,compared with the control group.Strain B3 also significantly reduced malondialdehyde(MDA)accumulation in pakchoi seedlings caused by salt stress,and increased antioxidant enzyme activity and proline content.In summary,this study was the first time to isolate and screen an excellent probiotic bacterium Halomonas(strain B3)from Dunaliella salina.The results showed that the strain could significantly promote the growth and the accumulation of metabolites of Dunaliella salina and improve the growth of pakchoi seedlings and their resistance to salt stress.These findings provide an excellent growth-promoting strain and theoretical basis for the establishment of a large-scale and efficient culture system of Dunaliella salina algae based on the algal-bacteria co-culture system,and also lay a foundation for the preparation of functional biological fertilizers based on microalgae and microorganisms in the phycosphere to improve the salt stress resistance and growth of crops. |
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