Cyanuric acid in <i>Paramecium</i> secretions is an efficient quorum sensing inducer

Quorum sensing is a process of bacterial chemical communication via extracellular signal molecules known as autoinducers, allowing synchronized collective behavior changes related to population density. However, the ecological significance of quorum sensing in multi-species communities, particularly coexisting with predators, remains unaddressed. In this study, we discovered that cyanuric acid (CA), a compound secreted by the widespread protozoan Paramecium, significantly influences the LuxR-type quorum sensing in Pseudomonas aeruginosa PAO1. Exogenous CA acts as a substitute for autoinducers known as N-acyl homoserine lactones in activating LasR/RhlR regulators that involve promiscuous ligand-receptor interactions, and exerts a specific restoring effect on the PAO1-ΔlasI/ΔrhlI mutant. Further, the inductive capabilities of CA were harnessed to expedite the start-up of the anammox process, substantially enhancing the aggregation and dynamics of the anammox consortium. Treating with 500 nM CA led to a 76.3% increase in anammox bacteria abundance compared to the control, resulting in exceptional nitrogen removal rates of 1.42 g N·L-1·d-1 within 54 days. This study unveils a mode of interspecies information transfer across trophic levels in aquatic organismal communities through CA-stimulated quorum sensing.