CONICET | Buscador de Institutos y Recursos Humanos

Understanding the processes that generate patterns of community structure is a central focus of ecological research. With that aim, we manipulated the diversity of bacterial activated sludge to test the influence of the diversity and composition of source bacterial communities on the dynamics of activated sludge floc assembly in lab-scale bioreactors. Three sets of experiments were performed. In the first set of experiments four replicate lab-scale reactors were seeded with sludge from a full-scale wastewater treatment plant (WWTP). Secondly, four laboratory scale reactors were replicates of a mixture of sludge from four different full-scale WWTP, added in equal proportions. In the third experiment, each of four lab-scale reactors contained 70% of sludge from a single full-scale WWTP, and 10% of sludge from the remaining 3 plants. Changes in bacterial community profiles were analyzed for a period of 10 solid retention time using denaturing gradient gel electrophoresis (DGGE) of RT-PCR amplified 16S rRNA. Fingerprinting of each set of reactors, including those started with the same source communities added in different proportions, converged to patterns that were more similar than expected by chance, suggesting a deterministic selection in floc development. Evidence for neutral dynamics was suggested by the dependence of the rate of replacement of species on the number of available species in the source community. Further indication of stochastic dynamics was obtained by the application of the Sloan neutral model for prokaryotes. The fitting of the observed data to the model predictions revealed that the importance of the stochastic component increased with higher diversity. Our results illustrate how both neutral and deterministic dynamics operate together in the assembly of the bacterial floc, and show that the balance of the two depends on the diversity of the source community.