A rotating annular reactor (Roto Torque) was used for qualitative and quantitative studied on biofilm heterogeneity. In contrast to the classic image of biofilms as smooth, homogeneous layers of biomass on a substratum, studies using various pure and mixed cultures consistently revealed more-dimensional structures that resembled dunes and ridges, among others. These heterogeneities were categorized and their underlying causes analyzed. Contrary to expectations, motility of the microorganisms not a decisive factor in determining biofilm homogeneity. Small Variations in substratum geometry homogeneity. Small variations in substratum geometry and flow patterns were clearly reflected in the biofilm pattern. Nonhomogeneous flow and shear patterns in the reactor, together with inadequate mixing resulted in significant, position-dependent differences in surface growth. It was therefore not possible to take representative samples of the attached biomass. Like many other types of reactors, the Roto Torque reactor is valuable for qualitative and morphological biofilm experiments but less suitable for quantitative physiological and kinetics studies using attached microorganisms. (c) 1994 John Wiley & Sons, Inc.

Thiosphaera pantotropha has been reported to denitrify aerobically and nitrify heterotrophically. However, recent evidence has indicated that these properties (particularly aerobic denitrification) have been lost. The occurrence and levels of aerobic denitrification and heterotrophic nitrification by T. pantotropha in chemostat cultures have therefore been re‐evaluated. Only low nitrate reduction rates were observed: the apparent nitrogen loss was of the same order of magnitude as the combined error in the calculated nitrogen consumption. However, 15N mass spectrometry revealed low aerobic denitrification rates (about 10% of the rates originally published by this group). Heterotrophic nitrification rates were about a third of previous observations. N2 and N2O were both produced from NH4+, NO3− and NO2−. Periplasmic nitrate reductase was present in aerobically grown cells.

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Two reactor types, a fluidized bed reactor and a RotoTorque reactor, were tested for their suitability as model systems for immobilization studies of pure cultures of T. pantotropha. T. pantotropha is a very interesting organism for waste water treatment, because of its ability of simultaneous aerobic denitrification and heterotrophic nitrification. Results indicate that the RotoTorque reactor is a more attractive model system for immobilization studies of T. pantotropha than the fluidized bed reactor used in this study. Using the RotoTorque reactor, studies on immobilized growth of T. pantotropha are currently being performed under anaerobic conditions. At a dilution rate of 1.2 h−1 (about four-fold higher than µmax under the experimental conditions) rapid attachment of T. pantotropha can be reproducibly observed. Within a couple of days, the cultures reach a 'steady state', where growth of the attached biomass is balanced by erosion. Microscopic investigations of the attached biomass indicated that T. pantotropha forms microcolonies rather than homogeneous biofilms.