Molecular mechanism underlying biofloc formation of Acidovorax soli Q11 and its application potential

Biofloc technology (BFT) is an emerging microbial-based technology in aquaculture that enhances system productivity by assimilating excessive nitrogenous pollutants, such as ammonia nitrogen and nitrite, into biofloc biomass via the activity of biofloc-forming microbes, rather than relying on conventional nitrification-denitrification or anammox processes. Traditionally, BFT has been implemented by enhancing the carbon/nitrogen ratio (C/N) to stimulate the uptake of ammonia and the biosynthesis of amino acids and proteins of biofloc-forming microbes, whose biomass could be filter-fed and digested by filter-feeding aquatic animals for nutrients. In this study, we isolated a series of biofloc-forming bacteria from multiple aquaculture systems and systematically investigated the molecular mechanisms governing biofloc formation in Acidovorax soli Q11, isolated from a freshwater sturgeon aquaculture pond. Molecular genetic analyses conducted on strain Q11, combined with whole genome sequencing, revealed that biofloc formation is likely regulated by cellulose biosynthesis (bcs) genes. Further characterization of extracellular polymeric substances (EPS) via Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) demonstrated that the secreted EPS exhibited high physicochemical similarity to bacterial cellulose. However, the bioflocs displayed pronounced resistance to cellulase degradation. Notably, the genome of strain Q11 contains two distinct bcs gene clusters, corresponding to Type Ic (harboring bcsD) and Type IIc (harboring bcsE and bcsG), suggesting that this dual bcs operon system may contribute to the enhanced structural stability of the cellulose-like matrix. Furthermore, strain Q11 demonstrated efficient removal of both ammonium (NH₄⁺) and nitrite (NO₂⁻), achieving complete removal of initial NH₄⁺ (39 mg/L) within 36 h and NO₂⁻ (65 mg/L) within 77 h, without detectable accumulation of toxic nitrogenous intermediates. This strain also exhibited versatile carbon source utilization, including effective growth on substrates commonly applied in aquaculture systems, such as glucose, brown sugar, fermentation-derived carbon sources, and rice bran. Collectively, these findings provide new insights into the mechanism of biofloc formation and highlight the potential of Acidovorax soli Q11 as a promising candidate for bioaugmentation in BFT systems, with potential applications in sustainable aquaculture and the treatment of nitrogen-rich wastewater. KEY POINTS: • Biofloc formation mechanism in Acidovorax soli Q11 is mediated by a dual bcs operon system. • EPS secreted by strain Q11 exhibits high structural stability and cellulase resistance. • Strain Q11 enables efficient ammonium and nitrite removal, supporting wastewater bioaugmentation.