STRESS-RESPONSE DYNAMICS OF AUTOTROPHS UNDER DIFFERENT PHYSICAL AND ENVIRONMENTAL CONDITIONS

Previous research has shown that nitrification rates can be increased if alternating anoxic/aerobic conditions are utilized in lab-scale sequencing batch reactors (SBRs) as opposed to solely aerobic conditions. The aim of this paper was to evaluate the mechanism responsible for higher nitrification rates under alternating conditions. Two hypotheses were proposed to explain why nitrification rates were greater in alternating reactors. First, the population of nitrifiers in the alternating anoxic/aerobic reactors might be larger than in the fully aerobic reactors. Secondly, the feast/famine phenomena, a result of the stress imposed on periodically starving bacteria is responsible for the higher nitrification rates occurring in alternating reactors. Experimental results indicated no significant differences in the number of nitrifiers present between aerobic and alternating anoxic/aerobic reactors, discounting the first hypothesis. Examination of the stress protein, GroEL, revealed that the stress is present in both aerobic and alternating SBRs. A time dependant sampling strategy was employed to track the concentration of the protein in the reactors over the length of a SBR cycle. In aerobic reactors GroEL was found to remain relatively constant throughout the length of a cycle. In alternating SBRs the concentration of GroEL increased linearly during the initial anoxic period. A similar linear increase also occurred in the subsequent aerobic period, but at a dramatically higher rate. The maximum concentration of GroEL in alternating SBRs was twice that of aerobic reactors, indicating that the stress response of the microorganisms is much greater. These results lend validity to the proposed theory that increased nitrification rates may occur due to the stress encountered in the alternating conditions.