Molecular weight distribution of hydrolysis products during the biodegradation of model macromolecules in suspended and biofilm cultures. II. Dextran and dextrin

Abstract

To improve wastewater treatment models, it is important to consider that wastewater is composed of a variety of complex molecules, many molecules having large molecular weights. Previous experiments have shown that hydrolytic enzymes are cell-associated and that hydrolytic fragments accumulate in bulk solution during the degradation of a model polysaccharide (dextran) in pure culture. These results indicate that incompletely hydrolyzed macromolecules are released into solution prior to their complete degradation. The authors wanted to determine whether the release of incompletely degraded molecules was specific to dextran degradation by pure cultures or whether it could be generalized to mixed culture systems and the degradation of other polysaccharides. To accomplish this, both pure and mixed (wastewater) cultures were used to examine the degradation of dextran and another macromolecular polysaccharide, dextrin, in batch suspended culture, continuous suspended culture and fixed-film reactor systems. Membrane ultrafiltration was used to monitor the molecular weight distribution of polysaccharides in solution during degradation. In all reactor configurations, and for all substrates and inocula investigated, small-molecular-weight (< 1000 amu) oligosaccharides accumulated in solution during polysaccharide degradation. These results, in conjunction with results of enzyme studies, support a generalized model for macromolecular degradation by cells that features cell-bound hydrolysis of polysaccharides and the subsequent release of hydrolytic fragments back into bulk solution. This hydrolysis and release is repeated until fragments are small enough (< 1000 amu) to be assimilated by cells. Essential features of this model are that polysaccharide diffusivity changes during its degradation and that different enzymes, with different methods of operation and different kinetic characteristics, may be used in successive hydrolytic cleavages. These features are particularly important to consider in evaluating macromolecule degradation by aggregates and biofilms and in understanding overall uptake kinetics in bioreactors.