Signal molecules in microalgal-bacterial partial nitrification-anammox systems: Roles, mechanisms, and prospects

The partial nitrification-anammox (PN/A) has been widely recognized as a sustainable and energy-efficient alternative to conventional biological nitrogen removal processes. Recently, the integration of photosynthetic microalgae into PN/A systems has given rise to the development of microalgal-bacterial coupled PN/A (MB-PN/A) configurations, which reduce aeration demand, enhance nitrogen removal efficiency, and improve process stability. However, the ecological mechanisms driving microalgal-bacterial interactions in these systems remain poorly understood. Among the potential regulatory factors, signaling molecules (SMs) serve as important mediators of quorum sensing (QS) and intercellular communication. They could coordinate metabolic functions, extracellular polymeric substance (EPS) production, and community organization. This review systematically summarizes the classification and regulatory networks of SMs, including first messengers (e.g., N-acyl-homoserine lactones and autoinducing peptides), second messengers (e.g., cyclic diguanylate monophosphate and cyclic diadenosine monophosphate), and emerging third messengers. It further examines signaling paradigms derived from diverse natural ecosystems as conceptual references for understanding MB-PN/A microbial interactions. On this basis, the review critically evaluates the roles of SMs in guiding functional guild assembly, regulating EPS production, modulating biofilm formation and granulation, and coordinating nitrogen and phosphorus metabolism in MB-PN/A systems, thereby identifying key knowledge gaps in signal transduction and multi-messenger coordination. Future research directions are accordingly proposed, including in situ detection of SMs, causal validation of signal-response relationships, and integration of signaling information into process monitoring and control for optimized MB-PN/A systems.