Strategies for the inoculation of bioreactors for long-term space missions include communities of diverse composition or, alternatively, communities of a few organisms selected for their ability to efficiently catalyze reactions of interest in the reactor. The concept of functional redundancy states that in a diverse community, several different organisms may be present that are capable of effecting processes necessary to the maintenance of the system function. The concept implies that if some members of the community are lost, others will be able to keep the system from failing in the critical reactions that take place therein. In a sewage reactor in the laboratory, a diverse community at steady state was perturbed by elimination of aeration for seven days. Chemical pools (NH4+, NO3-, dissolved O2), pH, and CO2 evolution were monitored before, during, and after the perturbation. CO2 evolution remained relatively stable, throughout the one-month incubation, although there were strong deviations at the time when the oxygen was initially excluded, and again when O2 was reintroduced. During the anoxic period, NO3- disappeared, and ammonia increased substantially, along with the pH. When the aeration was resumed, reactor conditions approached those of the pre-disturbance period. The microbial community, analyzed by TRFLP fingerprinting, changed substantially during the anaerobic period, and changed again when aeration was resumed; however, the final community was not similar in composition to the initial community, even though the functional ability of the post-disturbance community became similar to that of the initial assemblage. We conclude that redundancy of function within the community members accounted for the similarity of function under similar environmental conditions, although the community composition did not recover its original form.