The halophilic bacterium Halomonas elongata DSM 2581 T generally adapts well to high level of salinity by biosynthesizing ectoine, which functions as an important compatible solute protecting the cell against external salinity environment.Halophilic bacteria have specific metabolic activities under high-salt conditions and are gradually applied in various industries. The present study focuses on investigating the physiological and metabolic mechanism of H. elongata DSM 2581 T driven by the external salinity environment. The physiological metabolic dynamics under salt stress were investigated to evaluate the effect of NaCl stress on the metabolism of H. elongata. The obtained results demonstrated that ectoine biosynthesis transited from a nongrowth-related process to a growth-related process when the NaCl concentration varied from 1% to 13% (w/v). The maximum biomass (X m = 41.37 g/L), and highest ectoine production (P m = 12.91 g/L) were achieved under 8% NaCl. Moreover, the maximum biomass (X m ) and the maximum specific growth rates (μ m ) showed a first rising and then declining trend with the increased NaCl stress. Furthermore, the transcriptome analysis of H. elongata under different NaCl concentrations demonstrated that both 8% and 13% NaCl conditions resulted in increased expressions of genes involved in the pentose phosphate pathway, Entner-Doudoroff pathway, flagellar assembly pathway, and ectoine metabolism, but negatively affected the tricarboxylic acid cycle and fatty acid metabolism. At last, the proposed possible adaptation mechanism under the optimum NaCl concentration in H. elongata was described.

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