Two-way role of boron in microalgal-bacterial granular sludge: Enhanced signal communication for efficient metabolism

Based on the crucial role of boron in the metabolism of algae and bacteria, this study aimed to investigate the effects of boron on microalgae-bacterial granular sludge (MBGS) system. The addition of boron stimulated the secretion of autoinducer-2 to promote bacterial quorum sensing, which increased the extracellular polymeric substances production by more than 12.5 %. Meanwhile, boron significantly impacted the indole-3-acetic acid levels in microalgae to improve photosynthetic efficiency and increased the abundance of target algae (Chlorophyta and Cyanobacteria) by more than 0.23 times and 0.88 times, respectively. However, the influence of boron was not concentration-dependent, with the optimal concentration as 80 µM. Both boron deficiency and excess can heighten oxidative stress and affect the stability of the MBGS system. This study highlights the essentiality of boron in simultaneously influencing signal communication of algae and bacteria to strengthen synergy, and provides a theoretical basis for strengthening the symbiotic relationship within MBGS.