To investigate the effect of fluid shear on uptake rates of low-diffusivity macromolecular substrates by suspended cultures, we measured the uptake of two compounds as models of macromolecules, a protein (bovine serum albumin [BSA]) and a polysaccharide (dextran), using pure cultures of Zoogloea ramigera and Escherichia coli, respectively. Oxygen utilization rates of stirred samples grown on BSA and dextran were 2.3 and 2.9 times higher, respectively, than those of undisturbed (still) samples. Uptake rates of 3H-BSA and [3H]dextran by stirred samples were 12.6 and 6.2 times higher, respectively, than those by still samples. These experimentally obtained increases are larger than those predicted with a mass transfer model. Model results indicated that stirring would increase uptake by factors of 1.6 and 1.8 for BSA and dextran. As predicted by the model, we also found that uptake rates of low-molecular-weight substrates with high diffusivities, such as leucine and glucose, were only slightly affected by fluid shear. Since macromolecules can make up a major portion of bacterial substrate in natural, laboratory, and engineered systems, the demonstrated effect of fluid shear has wide implications for kinetic studies performed in basic metabolic research as well as in the evaluation of engineered bioreactors used for wastewater treatment.
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