# Genetic engineering to improve resistance against heavy metal stress in Synechocystis sp. PCC 6803

## TL;DR

Scientists engineered cyanobacteria to better resist heavy metal stress, which could help in cleaning polluted water.

## Contribution

The study demonstrates that introducing specific exogenous genes significantly enhances cyanobacterial tolerance to heavy metals.

## Key findings

- Transgenic Synechocystis strains showed improved growth under heavy metal stress.
- Enhanced SOD and CAT activities reduced ROS levels in transgenic strains.
- Exogenous genes like mntH, HMP3, sodA, and sodC effectively improved stress resistance.

## Abstract

As urbanization accelerates, the severity of heavy metal pollution in soil and water bodies intensifies. Cyanobacteria possess significant water remediation capabilities. To promote their application in environmental bioremediation, the selection and genetic modification of superior cyanobacterial strains have become increasingly important. In this study, we successfully constructed transgenic Synechocystis sp. PCC 6803 strains expressing exogenous mntH (manganese transporter protein), HMP3 (human metallothionein), and sodA and sodC (superoxide dismutases) genes, respectively. We systematically evaluated the tolerance of these transgenic strains against stress induced by Cd2+, Pb2+, and Cr6+ heavy metal ions. Under normal conditions, no significant differences in growth were observed between wild-type and transgenic Synechocystis sp. PCC 6803 strains. When exposed to heavy metal stress, the growth of all strains was severely inhibited. However, compared to the wild type, the transgenic strains exhibited significantly improved growth performance, accompanied by marked increases in intracellular chlorophyll, carotenoids, phycobiliprotein, and total protein contents. Further analysis revealed that this improvement correlated with significantly enhanced activities of superoxide dismutase (SOD) and catalase (CAT), which effectively mitigated reactive oxygen species (ROS) levels generated by heavy metal ions, thereby enhancing the tolerance of the transgenic strains to heavy metal stress. By introducing effective exogenous genes, our study successfully yielded heavy metal-tolerant strains of Synechocystis sp. PCC 6803. This not only validated the efficacy of the four exogenous functional genes in enhancing cyanobacterial stress resistance but also provided theoretical and technical support for developing more resilient cyanobacterial chassis cells or applying them to bioremediation.

Cyanobacteria, which possess photoautotrophic capacity, demonstrate excellent capabilities in water remediation. As ideal species integrating both photosynthetic carbon fixation and bioremediation functions, cyanobacteria have garnered significant attention for environmental bioremediation applications. Consequently, the selection of superior cyanobacterial strains and genetic engineering for improvement have become increasingly critical to advance practical application in ecological remediation processes. Synechocystis sp. PCC 6803 possesses a well-defined genetic background and a natural DNA transformation system, making it an ideal platform for gene editing and metabolic engineering. In this study, we successfully constructed transgenic Synechocystis sp. PCC 6803 strains expressing exogenous genes encoding MntH, HMP3, SodA, and SodC, respectively. The heavy metal resistance of transgenic strains was significantly improved. This study underscores the efficacy of exogenous functional genes in improving cyanobacterial stress resistance and offers both theoretical and technical foundations for the development of more robust cyanobacterial chassis cells.