History of carbon supply shapes the metabolic response of photogranules to light shifts

Oxygenic photogranules mainly composed of cyanobacteria and heterotrophic bacteria, have gained attention for their ability to treat wastewater (removal of C, N, and P) without external aeration. Currently, the metabolic dynamics of photogranules to varying nutrient and light conditions in wastewater treatment systems remains poorly studied. However, understanding how quickly the photogranule metabolism changes, and whether this change is temporary or permanent is important for the optimal use of photogranules. Here, an NMR-based metabolomics approach was applied to investigate the temporal dynamics of photogranule metabolism in the presence or absence of acetate and with or without light. Our findings revealed that under carbon-limited conditions, photogranules relied on alternative carbon sources, such as N-acetylneuraminate (a constituent of EPS) and amino acids like hypotaurine and L-alanine. This adaptation affected key metabolic pathways, including glycolysis, taurine and hypotaurine metabolism, and the tricarboxylic acid cycle. When acetate was provided, both heterotrophic and phototrophic activities were maintained. Notably, the history of carbon supply influenced how photogranules responded to light shifts. Metabolic indicators showed that the lag in carbon addition altered fatty acid metabolism and carbon fixation, leading to shifts in amino acid concentrations and distinct metabolic profiles when the light was turned off. Thus, NMR metabolomics identified metabolic changes, induced by contrasting carbon conditions, lasting for several hours, and significantly affecting the photogranule response to light fluctuations. These results suggest that the history of carbon supplementation may shape metabolic responses of photogranules to other environmental changes or stressors.