In this study, replacement-series method and contour analysis were applied to investigate optimal bioaugmentation strategies for the treatment of a dye-contaminated aquatic system using a constructed mixed-community for biodecolorization of a model azo dye Reactive Red 22. The novelty emphasizes that a species without essential target functions in a mixed culture could still play a crucial role in influencing the treatment performance. That is, although non-decolorizers (i.e., Escherichia coli DH5 alpha) were considered metabolically '' dormant '' in this model binary-biosystem, their presence still significantly enhanced decolorization performance of the decolorizers (i.e., Pseudomonas luteola). In aerobic growth conditions, E. coli DH5 alpha possessed a growth advantage to out-compete P. luteola due to preferential growth rate of DH5 alpha. However, in static decolorization conditions, DH5 alpha seemed to produce decolorization-stimulating extracellular metabolites to help the major decolorizer (P. luteola) decompose the toxic pollutant (i.e., the azo dye) in a short term for the benefit of total survival in the environment. The experimental results show that the presence of E. coli DH5 alpha increased the decolorization efficiency of P. luteola even though DH5 alpha was an inefficient decolorizer in this microbial community. Thus, addition of DH5 alpha into a mixed culture containing P. luteoa as a major decolorizer may lead to a bioaugmentation effect on decolorization activity. The optimal population ratio for bioaugmentation was determined by the contour analysis. The results indicate that the optimal community species ecology for maximum overall decolorization rate almost maintained at a ratio of one viable P. luteola (0.78 x 10(9) cells/mL) to one DH5 alpha cell (0.70 x 10(9) cells/mL), representing a maximal diversity (i.e., H-max congruent to 1.0). (c) 2006 Elsevier Ltd. All rights reserved.