Kynurenic acid mediates bacteria-algae consortium in resisting environmental cadmium toxicity

Cadmium (Cd²⁺) is a toxic heavy metal in the environment, posing severe damage to animal health and drinking water safety. The bacteria-algae consortium remediates environmental Cd²⁺ pollution by secreting chelating reagents, but the molecular mechanisms remain elusive. Here, we showed that Cellulosimicrobium sp. SH8 isolated from a Cd²⁺-polluted lake could interact with Synechocystis sp. PCC6803, a model species of cyanobacteria, in strengthening Cd²⁺ toxicity resistance, while SH8 or PCC6803 alone barely immobilized Cd²⁺. In addition, the SH8-PCC6803 consortium, but not SH8 alone, could grow in a carbon-free medium, suggesting that autotrophic PCC6803 enabled the growth of heterotrophic SH8. Totally, 12 metabolites were significantly changed when SH8 was added to PCC6803 culture in the presence of Cd²⁺ (PCC6803/Cd²⁺). Among them, kynurenic acid was the only metabolite that precipitated Cd²⁺. Remarkably, adding kynurenic acid increased the growth of PCC6803/Cd²⁺ by 14.1 times. Consistently, the expressions of kynA, kynB, and kynT genes, known to be essential for kynurenic acid synthesis, were considerably increased when SH8 was added to PCC6803/Cd²⁺. Collectively, kynurenic acid secreted by SH8 mitigates Cd²⁺ toxicity for algae, and algae provide organic carbon for the growth of SH8, unveiling a critical link that mediates beneficial bacteria-algae interaction to resist Cd²⁺.