Surface display of CadR protein on Rhodopseudomonas palustris CGA009 for enhanced heavy metal bioremediation

Abstract

Heavy metal pollution poses significant ecological and public health risks, and surface display engineering shows promise for bioremediation in this area. Although anoxygenic photosynthetic purple nonsulfur bacteria (PNSB) have been effectively applied to degrade pollutants due to their metabolic versatility, the use of surface display technology in PNSB remains very limited. In this study, we constructed a surface display system using Rhodopseudomonas palustris CGA009 as the host. The metal-binding protein CadR was fused with outer membrane protein A (OmpA) and expressed in CGA009. SDS-PAGE and immunofluorescence analysis identified the successful expression of the fusion protein on the cell surface. In addition, we used flow cytometry to explore the enhancement effects of linker peptides and different promoters on surface display efficiency under different light intensities. The surface display system enhanced the heavy metal resistance of the host bacteria, and the maximum removal rate of Cd^{2 +} reached 95.6 %. By means of Langmuir isotherm analysis, the maximum biosorption capacity of the system for Cd²⁺ is 101.11 mg/g. The system demonstrates feasibility for application in complex real-world environmental samples. The presence of various metal ions does not interfere with the system's specific adsorption of Cd²⁺. It can stably maintain an adsorption efficiency of over 80 % under conditions of pH 6-8, temperatures of 20-35°C, and light intensity of 1000-6000 lux. Additionally, the system achieves a removal efficiency of 94.3 % in Cd wastewater. In summary, this study provides a reference for the development of photosynthetic bacterial surface display systems and provides an advanced bioremediation strategy for heavy metal contaminated wastewater.

Keywords: Cd bioremediation; Light intensity; Photosynthetic bacterial; Promoter activity; Surface display.