Biochemical and multi-omics analyses of response mechanisms of rhizobacteria to long-term copper and salt stress: Effect on soil physicochemical properties and growth of Avicennia marina

Mangroves are of important economic and environmental value and research suggests that their carbon sequestration and climate change mitigation potential is significantly larger than other forests. However, increasing salinity and heavy metal pollution significantly affect mangrove ecosystem function and productivity. This study investigates the tolerance mechanisms of rhizobacteria in the rhizosphere of Avicennia marina under salinity and copper (Cu) stress during a 4-y stress period. The results exhibited significant differences in antioxidant levels, transcripts, and secondary metabolites. Under salt stress, the differentially expressed metabolites consisted of 30% organic acids, 26.78% nucleotides, 16.67% organic heterocyclic compounds, and 10% organic oxides as opposed to 27.27% organic acids, 24.24% nucleotides, 15.15% organic heterocyclic compounds, and 12.12% phenyl propane and polyketides under Cu stress. This resulted in differential regulation of metabolic pathways, with phenylpropanoid biosynthesis being unique to Cu stress and alanine/aspartate/glutamate metabolism and alpha-linolenic acid metabolism being unique to salt stress. The regulation of metabolic pathways enhanced antioxidant defenses, nutrient recycling, accumulation of osmoprotectants, stability of plasma membrane, and chelation of Cu, thereby improving the stress tolerance of rhizobacteria and A. marina. Even though the abundance and community structure of rhizobacteria were significantly changed, all the samples were dominated by Proteobacteria, Chloroflexi, Actinobacteriota, and Firmicutes. Since the response mechanisms were unbalanced between treatments, this led to differential growth trends for A. marina. Our study provides valuable inside on variations in diversity and composition of bacterial community structure from mangrove rhizosphere subjected to long-term salt and Cu stress. It also clarifies rhizobacterial adaptive mechanisms to these stresses and how they are important for mitigating abiotic stress and promoting plant growth. Therefore, this study can serve as a reference for future research aimed at developing long-term management practices for mangrove forests.