Impact of Hydrodynamic Conditions on the Production and Distribution of Extracellular Polymeric Substance in River Biofilms

The extracellular polymeric substance (EPS) plays a key factor in biofilm formation. However, the research on the importance of each EPS fraction is mainly concentrated in the activated sludge field. In this study, biofilms were cultivated under different hydrodynamic conditions in indoor flumes, and the important regulatory effects of dissolved EPS (SB-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) on biofilm formation were investigated. The results indicated that the ratios of soluble EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) were 27:74:108 in the turbulent flow, 38:48:71 in the transitional flow, and 89:51:51 in the laminar flow. Regarding proportion, TB-EPS and LB-EPS were secreted more in the turbulent flow, while S-EPS was secreted slightly more in the laminar flow. S-EPS lacks the structural strength provided by bound EPS. Under the special bonding effects of LB-EPS and TB-EPS, many microcolonies join to form biofilms. The polysaccharide content in the EPS of biofilms remained dominant under all conditions. Polysaccharides are the core of biofilm formation, which enhance bacterial aggregation and make biofilm dense. Through the mutual verification of the results in the microscopic and macroscopic fields, the mechanism of biofilm formation was further elucidated, especially, in Stage IV, due to the special bonding effects of LB-EPS and TB-EPS, many colonies adhere to the mature biofilm. Further studies are required to investigate the extracellular polysaccharides and proteins in EPS along with their properties in biofilms.