Identification of genes involved in cadmium-ion tolerance in evolutionary Synechocystis sp. PCC 6803 tolerant to both cadmium and high light

Abstract

Photosynthetic cyanobacteria have shown great potential as "autotrophic cell factories" for the synthesis of fuels and chemicals. However, poor tolerance to various environmental stressors like high light and heavy metals is an important factor limiting their economic viability. While numerous studies have focused on the tolerance mechanism of cyanobacteria to individual stressors, their response to simultaneous stresses remains to be recovered. To investigate the mechanism of cross tolerance to heavy metal Cd2+ and high light, the model cyanobacterium Synechocystis sp. PCC 6803 tolerant to both Cd2+ and high light was obtained via about 800 days’ cross-adaptive laboratory evolution. Three evolutionary strains capable of tolerating both 5.5 μM Cd2+ and 600 μmol photons m−2s−1 high light were successfully obtained, achieving about 83% enhancement of Cd2+ tolerance compared with the parent strain. The different response of parent and evolutionary strains to Cd2+ was elucidated via metabolomics. Further, a total of 15 genes that were mutated during evolution were identified by whole-genome re-sequencing. Finally, by single gene knockout and complementation analysis, four genes including ssl2615, sll1732, ssr1480 and sll1659 involved in the improvement of Cd2+ tolerance under high light condition were successfully identified. This work explored the tolerance mechanism of Synechocystis sp. PCC 6803 to cadmium under high light condition, and provided valuable reference for deciphering multi-tolerance mechanism of cyanobacteria in the future.