Screening,identification,fermentation condition optimization of an exopolysaccharide-producing strain and the influence on soil macro-aggregate formation

Abstract:

[Objective] To screen microbial strains with high production of exopolysaccharides (EPS) and provide strain resources for the development of soil improvement agents.[Methods] The string test with a LB-aniline blue plate was employed to qualitatively screen the strains of plant growth-promoting rhizobacteria (PGPR) with EPS production.After fermentation with each strain in four media,the EPS content in the fermentation liquid was determined by low temperature alcohol precipitation and the sulfate-anthranone colorimetric method,on the basis of which the PGPR strain with high EPS production was screened out.The fermentation conditions of the strain screened out were optimized by orthogonal test with EPS content in fermentation liquid as the indicator.The influence of the fermentation liquid of strain F1 on the content of macro-aggregates in sandy loam soil was analyzed by the petri dish culture experiment.[Results] Eight EPS-producing PGPR strains were primarily screened out,among which strain F1 had the highest EPS production.PDA was the best medium for F1 to produce EPS,with the EPS content of 867.54 μg/mL.Based on morphological,physiological and biochemical characteristics,phylogenetic analysis based on 16S rRNA gene sequence,and also average nucleotide identity analysis,F1 was identified as a strain of Bacillus megaterium.The optimal culture conditions for F1 to produce EPS were 28℃ and 180 r/min for 24 h,under which the EPS yield reached1123.39 μg/mL.After F1 was incubated in sandy loam soil for 40 days,the content of water-stable macro-aggregates with the grain diameter>0.25 mm in the soil increased by4.44 times compared with that of the control.[Conclusion] Strain F1 with high EPS production can promote the formation of water-stable macro-aggregates in sandy loam soil.The optimal conditions for F1 to produce of EPS was incubation in PDA at 28℃ and 180 r/min for 24 h.