Functional stability and adaptation of nitrifying bioreactor communities stressed by nickel [electronic resource]

The functional stability of microbial communities is the key to reliable system performance in many applications of environmental biotechnology. These communities are often subject to changing operational and environmental conditions, resulting in episodes of instability. The processes and time scales for functional recovery depend upon the nature of the perturbations and the resistance of the community. This research investigated the relationships between the intensity and duration of a defined perturbation and the functional stability and adaptation of the perturbed community. The model community chosen was ammonia-oxidizing bacteria (AOB) in a nitrifying community derived from the activated sludge, and nickel was the model stressor used for perturbation. The effects of nickel perturbation were explored in both batch cultures and continuous flow reactors. The extent and time scale of nickel stress were examined in batch cultures. An intraparticle diffusion model coupled with a noncompetitive inhibition model suggested that the slow release of nickel from biomass accounted for the continued inhibition after nickel removal from the aqueous phase. Moreover, depending on the dosage and duration of exposure, nickel could also affect transcription of the amoA gene and community dynamics of AOB. Nickel stress immediately inhibited ammonia oxidation at all dosages, but a significant decrease in amoA gene transcript levels only occurred at the highest nickel dosages (50 mg/L and 150 mg/L) and after a delay of at least 12 hours. An extended exposure (6 -- 8 weeks) to a sufficiently high nickel dosage (150 mg/L) was required for a shift in AOB community structure, from an initial community dominated by a Nitrosomonas-like cluster to one dominated by a Nitrosospira lineage. The time scales required for recovery of ammonia oxidation, therefore, also varied due to the time required for the recovery of enzyme activity, transcription and growth of resistant lineages. The functional stability of ammonia oxidation -- and particularly its recovery after a nickel stress -- was further investigated in continuous flow reactors exposed to two dosages of nickel (1 and 150 mg/L) for two exposure times (10 and 60 days). Ammonia oxidization was affected in all reactors under nickel stress, but amoA gene expression was only significantly affected by high dosage. For prolonged exposure to low dosages of nickel, recovery of ammonia oxidation was accompanied by increased relative abundance of terminal restriction fragments (T-RFs) associated with Nitrosomonas oligotropha lineages. At high dosages, restoration of ammonia oxidation was accompanied both by a rebound in amoA transcription levels and by the dominance of T-RFs associated with Nitrosospira and/or N. oligotropha lineages. These results underscore the importance of redundancy and diversity of a community in conferring functional stability under stress conditions. Finally, the effects of exposure history on the adaptation of a community to nickel stress were investigated in continuous flow reactors. Same level and duration of nickel stress were applied to communities with different histories of nickel exposure. The functional stability of these communities varied, indicating different levels of adaptation. High levels of adaptation resulted from extended exposure to a low dosage of nickel and from exposure to a high dosage followed by a long post-exposure period. These well-adapted communities were dominated by T-RFs associated with N. oligotropha and/or Nitrosospira lineages. These same T-RFs were present in communities with limited or no adaptation, but were not dominant, suggesting that the success of strategies for enhancing functional stability will depend upon the maintenance of sufficient resistant populations in the community.

Advisor: Criddle Craig primary advisor. Thesis advisor
Boehm Alexandria advisor. Thesis advisor
Francis Christopher advisor. Thesis advisor