Transcriptome Analysis Unravels Cadmium Stress Tolerance Mechanisms in two Estuarine Bacteria Comamonas sp. Y49 and Aeromonas sp. Y23

Cadmium (Cd) pollution has become an urgent environmental issue in coastal regions of China. Microorganisms, a significant constitution in the marine ecosystem, have evolved various strategies to against and reduce Cd toxicity, which could be applied for bioremediation. In this study, we selected two bacterial strains (Comamonas sp. Y49 and Aeromonas sp. Y23) isolated from the Yangtze River estuary of China, which could tolerate as high as 0.2 mmol/L [Cd2+]. Two strains were individually cultured in a gradient of [Cd2+] (0.1 and 0.2 mmol/L for Comamonas sp. Y49; 0.5 and 0.7 mmol/L for Aeromonas sp. Y23) beyond the minimum inhibition concentration (MIC). We determined phenotypic responses by tracking cellular growth, dissolved [Cd2+] and cellular morphology by Scanning Electron Microscopy (SEM), and applied the transcriptomic analysis for investigating molecular transformation. The results showed that the growth of both strains was inhibited under Cd stress; however, the transcriptomic data indicated that they all enhanced carbon and energy metabolisms to sustain the cellular growth within the MIC range. Besides, both nitrogen and sulfur metabolisms were up-regulated for energy production and amino acid synthesis. In the transcriptomes of Cd-stressed Comamonas sp. Y49, the increased gene regulation related to polysaccharide-like substrate secretion and down-regulated motility-related genes might favour the biofilm formation for counteracting Cd toxicity. There was a correspondent detection of polysaccharide-like substrates under SEM in the culture of Comamonas sp. Y49 under 0.1 mmol/L Cd2+. Differently, the motility seemed to be activated in Aeromonas sp. Y23 to escape harsh conditions, and it formed connected long-chain to reduce the influx of Cd2+. This research could facilitate our understanding of the molecular mechanisms of Cd resistance, providing insights into pollution remediation and ecosystem sustainability.