Cadmium pollution remains a persistent threat to aquatic ecosystems. Although algae play important roles in cadmium transformation, accumulation and bioremediation, how they respond at the molecular level to continuous, environmentally relevant low-dose exposure remains poorly understood.

Recently, a research team led by Prof. BI Yonghong from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences (CAS) has revealed how Synechocystis sp. PCC 6803 responds to low-level cadmium stress over time. The researchers found that the cyanobacterium undergoes a two-stage response, shifting from rapid stress sensing and repair to long-term adaptation and cellular protection.

The study was published in Environmental Pollution.

In this study, the researchers exposed Synechocystis sp. PCC 6803 to 0.05 mg L^-1 Cd²⁺ and monitored its responses over a period of 0.5 to 144 hours. They found that 24 hours marked a key transition from rapid stress response to long-term adaptation.

During the first 24 hours, the cells showed fast and transient activation of pathways involved in metal sensing, photosystem repair, and nitrogen and glutamate metabolism. During the subsequent adaptation phase, 145 commonly differentially expressed proteins formed a stable regulatory module.

Integrated transcriptomic and proteomic analyses revealed that the cells maintained long-term homeostasis through several coordinated responses.

Metal uptake systems such as MntCAB and FeoB were downregulated, which may reduce nonspecific Cd²⁺ influx. Meanwhile, transporters including Slr0944, ZiaA and Sll1725 were induced to enhance cadmium efflux. The cells also strengthened central carbon metabolism, the tricarboxylic acid (TCA) cycle, energy production and antioxidant defence, helping provide adenosine triphosphate (ATP) for active transport while limiting oxidative damage.

Based on these results, the researchers proposed a two-stage adaptation model: an early phase characterized by rapid stress sensing and repair, followed by a later phase marked by reduced metal uptake, enhanced ATP-dependent efflux, metabolic reinforcement and antioxidant protection.

According to the researchers, the findings provide new insights into how cyanobacteria cope with environmentally relevant cadmium exposure and could provide a theoretical basis for algae-mediated cadmium bioremediation.

Schematic illustration of the two-stage response model of Synechocystis sp. PCC 6803 under low-level cadmium stress. (Image by IHB)