Mechanisms of microbial fouling alleviation by controlled-release borates in a recirculating cooling-water system via extracellular polymeric substances composition and metabolomics analysis.

Biofilm accumulation in recirculating cooling-water systems (RCSs) contributes significantly to microbial corrosion and heat transfer impairment. However, traditional chlorine-based bactericides require frequent replenishment for sustained performance and chlorine ions exacerbate pipeline corrosion. This study employs controlled-release borates (CRBs) to ensure continuous boron release and thorough biofilm contact. The release rate of CRBs in the first 10 days was 39.2 mg/(L·d), following zero-order release kinetics. At dosages above 0.4 g-B/L, CRBs reduced total heterotrophic bacteria counts (below 1.0 × 105 CFU/mL) and mitigated corrosion (below 0.075 mm/a). The determination of extracellular polymeric substances (EPS) showed that borates reduced the total EPS content in biofilm by 45.2%-75.2%, particularly protein and polysaccharide, leading to a looser biofilm structure. Aromatic proteins were more sensitive to borates than humic acid-like substances, with preferential disruption of C=O bonds in amide I over aromatic conjugated structures as confirmed by 2D-Fourier transform infrared spectra. Microbial community analysis indicated that borates decreased the abundance of EPS-producing bacteria (e.g., Flavobacterium), antibiotic-resistant bacteria (e.g., Acinetobacter), and corrosion-associated bacteria (e.g., Desulfovibrio). Simultaneously, borates interfered with EPS synthesis by regulating the secretion of endogenous quorum sensing signals. Flow cytometry confirmed that biofilm inhibition stemmed from sustained regulation of microbial physiology by borates rather than acute toxicity. The inactivation of key antioxidant enzymes rendered cells membrane more vulnerable to reactive oxygen species. Metabolomics analysis showed the decreased levels of antioxidative and corrosive metabolites. The CRBs show promise as a viable strategy for biofilm control in RCSs, offering sustained effect without contributing to aquatic eutrophication.