Autoradiographic determination of mass-transfer limitations in immobilized cell reactors

Pseudomonas putida cells were grown in confined volumes in dual-membrane immobilized cell reactors constructed from microporous polyethylene hollow fibers and silicone rubber tubules as a model system for the study of mass transport in microbial aggregates. Local cell concentrations in the reactors reached 300 g dry mass/L. Pulse-chase radioisotope labeling with (35)SO(4) (2-) was used to estimate the rates of cell mass synthesis and degradation. Sulfur incorporation consistently exceeded sulfur release, implying that the cell mass concentration continually increases. The location and size of the cell growth region was determined using liquid emulsion autoradiography of thin sections prepared from labeled reactors. Cell growth occurs in a region less than 25 mum in depth adjacent to the oxygen supply, and the expansion of the cells caused by cell growth promotes convection of the cell mass into regions of the reactor where starving cells accumulate. The combination of mass-balance and spatial distribution measurements that can be made using radioisotope tracers provides a versatile method for determining metabolic rates and limitations caused by mass transfer in immobilized cell reactors.