Enhanced Salt Tolerance in Synechocystis sp. PCC 6803 Through Adaptive Evolution: Mechanisms and Applications for Environmental Bioremediation

Abstract

Salt stress is common in natural environments, where elevated salt levels in brackish water and saline soil can hinder the growth of organisms, thereby exacerbating environmental challenges. Developing salt-tolerant organisms not only uncovers novel mechanisms of salt tolerance but also lays the groundwork for managing and utilizing saline environments. Cyanobacteria, which are widely distributed in hydrosphere and soil, serve as ideal models for studying salt stress. In this study, the model cyanobacterium Synechocystis sp. PCC 6803 was selected, whose salt (NaCl) tolerance improved from 4.0% to 6.5% (m/v) through adaptive laboratory evolution. Genome re-sequencing and mutant analysis identified six key genes associated with salt tolerance. Notably, the deletion of slr1670, which encodes glycerol glucoside hydrolase, improved the strains salt tolerance. In addition, slr1753 encodes a membrane protein that may enhance salt tolerance by facilitating ion transport to the extracellular space. Further analysis revealed that overexpression of slr1753 significantly accumulates Na+ on the cell surface, enabling effective seawater treatment using the engineered strain, resulting in a 6.35% reduction of Na+ in the seawater. Moreover, the adapted bacteria can be used for the remediation of saline soil samples, leading to a 184.2% and 43.8% increase in the germination rate and average height of Brassica rapa chinensis, respectively, along with a 25.3% rise in total organic carbon content and reductions in both total salt content by 1.82% and pH by 1.91% in soil. This study provides novel insights into salt tolerance mechanisms and the bioremediation of high-salinity environments. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=105 SRC="FIGDIR/small/610226v1_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@cde16aorg.highwire.dtl.DTLVardef@8a27ccorg.highwire.dtl.DTLVardef@3af9bforg.highwire.dtl.DTLVardef@1e29cfb_HPS_FORMAT_FIGEXP M_FIG C_FIG