A conceptual model describing macromolecule degradation by suspended cultures and biofilms

Macromolecular (> 1,000 daltons) compounds such as proteins and polysaccharides can constitute a significant portion of dissolved organic carbon (DOC) in wastewater, but limited information is available on how these compounds are degraded in suspended and fixed-film biological wastewater treatment systems. Bacteria cannot assimilate intact macromolecules but must first hydrolyze them to monomers or small oligomers. Here, we summarize experiments performed in our laboratory which indicate that the enzymes responsible for hydrolysis are primarily those that remain attached to the cell. In biofilm cultures fed macromolecular substrates, for example, no more than 8% of total hydrolytic activity was found to be located in the cell-free bulk solution. These and other experiments support a generalized mechanism for macromolecule degradation by biofilms that features cell-associated hydrolysis, followed by the release of hydrolytic fragments back into bulk solution. The extent of fragment release is larger for proteins (bovine serum albumin) than for carbohydrates (dextrans).