Enhanced salt tolerance in Synechocystis sp. PCC 6803 through adaptive evolution: Mechanisms and applications for environmental bioremediation.

As a significant environmental challenge, salt stress is common in saline-alkali soils and brackish water, where elevated salt levels hinder the growth of various organisms. Cyanobacteria are ideal models for studying adaptations to salt stress due to their wide distribution across aquatic and terrestrial ecosystems. In this study, we employed adaptive laboratory evolution to increase the salt (NaCl) tolerance of the model cyanobacterium Synechocystis sp. PCC 6803 from 4.0 % to 6.5 % (w/v). Through genome re-sequencing and mutant analysis, six key genes associated with salt tolerance were identified. Notably, overexpression of the slr1753 gene enhanced Na⁺ accumulation on the cell surface, enabling the engineered strain to effectively reduce Na⁺ concentration in seawater by 6.4 %. Additionally, the adapted strain showed promise in remediating saline-alkali soils, with observed increases in the germination rate (184.2 %) and average height (43.8 %) of Brassica rapa chinensis. Soil quality also improved, with a 25.3 % increase in total organic carbon content, a 1.8 % reduction in total salt content, and a 1.9 % decrease in pH. This study provides new insights into the mechanisms underlying salt tolerance and highlights the potential of engineered cyanobacteria for bioremediation in high-salinity environments.