Location of protein and polysaccharide hydrolytic activity in suspended and biofilm wastewater cultures

Macromolecular compounds such as proteins and polysaccharides can comprise a significant portion of dissolved organic carbon in wastewater, but limited information is available on how these compounds are degraded in biological wastewater treatment systems. Bacteria cannot assimilate intact macromolecules but must first hydrolyze them to monomers or small oligomers. Whether this hydrolysis occurs in contact with cells or by enzymes released into bulk solution is critical to an understanding of macromolecule metabolism. This study used the fluorescent model substrate analogs L-leucine-7-amido-4-methylcoumarin . HCl (Leu-MCA) and 4-methylumbelliferyl-alpha-glucoside (MUF-alpha-glc) to determine the location of leucine aminopeptidase and alpha-glucosidase activity in wastewater inoculated biofilm and suspended cultures and in trickling filter effluent. In biofilm cultures, no more than 3% of total hydrolytic activity was located in the cell-free bulk solution. Similar results were obtained in suspended culture where 97% of leucine aminopeptidase and 93% of alpha-glucosidase activity occurred in contact with cells. In trickling filter effluent, hydrolysis was also predominantly cell-associated. Hydrolysis rates were at least five times higher in contact with cells and sloughed biofilm pieces than in cell-free solution. When considered with the results of other experiments demonstrating that hydrolytic fragments of proteins and polysaccharides accumulate in bulk solution during macromolecule degradation, these experiments support a generalized mechanism for macromolecule degradation that features cell-associated hydrolysis followed by the release of hydrolytic fragments back into bulk solution. This cell-associated hydrolysis and release is repeated until hydrolytic fragments are small enough to be assimilated by cells. Use of this macromolecule degradation mechanism can help refine wastewater treatment models so that they can more accurately predict the performance of bioreactors treating complex wastewaters. (C) 1998 Elsevier Science Ltd. All rights reserved.