Low-Carbon cadmium removal via cyanobacterial MICP: Metabolic mechanisms and species efficiency.

Cadmium (Cd) is widely distributed in contaminated water. Globally, substantial resources and energy are consumed annually to treat Cd-contaminated water, thereby intensifying carbon emissions and contributing to global climate change. Microbially induced carbonate precipitation (MICP) by cyanobacteria has the potential to become a low-carbon method for treating Cd-contaminated wastewater. This study demonstrated that the MICP reaction of Synechococcus sp., Synechocystis sp., and Microcystis aeruginosa could effectively achieve the low-carbon removal of Cd. Irregular spherical CdCO minerals with sizes ranging from 0.5 to 5.0 μm were produced during the MICP process. The removal rate and carbon sequestration amount of Synechococcus sp. were 97.13 ± 1.64 % and 0.101 ± 0.008 mg, respectively, which were significantly higher than those of Synechocystis sp. (47.52 ± 2.53 %, p < 0.01; 0.056 ± 0.001 mg, p < 0.01) and M. aeruginosa (45.73 ± 2.40 %, p < 0.005; 0.051 ± 0.002 mg, p < 0.01) when 10.00 mL of the supernatant was used to treat Cd with an initial concentration of 1.00 mM. On the one hand, Synechococcus sp. exhibited a higher pH (10.667 ± 0.311) and CO concentration (80.995 ± 2.500 mg·L), which facilitated the mineralization of Cd and inorganic carbon fixation. On the other hand, Synechococcus sp. had a higher cell density (OD: 1.451 ± 0.021) and chlorophyll-a concentration (5.262 ± 0.466 mg·L), promoting organic carbon fixation through enhanced cyanobacterial photosynthesis. This phenomenon could be attributed to the elevated transcription levels of key functional genes (including CcmO, CcmK, PsbA, PsaD) involved in CO concentration and photosynthesis processes. Cyanobacterial MICP reduces reliance on energy-intensive treatments, mitigating climate impacts. The integration of Cd remediation with carbon fixation offers a dual-benefit strategy for sustainable wastewater management, addressing hazardous metal pollution while contributing to global carbon neutrality goals.