Biological wastewater treatment plays a major role in the protection of water quality and public health. Wastewater also constitutes a renewable source of freshwater in urban centres with scarce water supplies. Improving the stability and performance of wastewater treatment processes is a main focus of environmental engineering and biotechnology, to help achieve the sustainable development goal of guaranteeing availability and perdurability of water and sanitation for everyone. Microbial communities in bioreactors are responsible for the removal of organic carbon and nutrients. Disturbance is deemed a main factor affecting the stability of microbial community assembly and function, but a predictive understanding of its outcomes remains unclear. Several relevant aspects are currently under debate including whether disturbance can lead to reproducible or variable states of function and microbial composition, whether a more diverse community implies a better ecosystem performance, what drives the balance between stochastic and deterministic mechanisms of community assembly, and how pulses versus sustained perturbations affect microbial community structure and performance. Additionally, trait-based approaches from community ecology offer a way to find general rules to understand the mechanisms driving changes in community assembly and function under perturbations, but their applicability to microbial ecology and environmental biotechnology has only been suggested so far.
This dissertation aims to integrate the concepts of disturbance-diversity-performance in sludge bioreactors from an interdisciplinary perspective involving the fields of engineering, molecular biology, ecology and statistics. The goals of this research were to (1) study the effect of defined disturbance events on bioreactor community assembly; (2) investigate the effect of disturbances on key functions for process performance in bioreactors; (3) examine temporal dynamics of sludge bacterial communities under succession; and (4) incorporate frameworks from ecological theory to aid in the understanding of these three prior objectives, with the additional potential of making its findings useful for studies across other complex microbial communities. To address these goals, controlled experiments using activated sludge bioreactors (inoculated from a full-scale facility) and different disturbance agents were designed and performed at different scales of volume, time and replication. For all studies, variations in ecosystem function were tracked, while a combination of distance-based multivariate tests and ordination methods, alpha-diversity indices, general linear multivariate models, and null model analysis from ecology, was employed to evaluate changes in microbial community structure.
The first study involved a 35-day microcosm experiment using 24 replicated wastewater bioreactors subjected to a range of different disturbance frequencies in the form of toxic 3-chloroaniline. A stronger temporal decrease in alpha-diversity at the extreme, undisturbed and press-disturbed, sides of the disturbance range led to an intermediate disturbance hypothesis (IDH) pattern, with the highest diversity found at intermediate levels of disturbance. Undisturbed and press-disturbed levels displayed the highest community and functional similarity across replicates, suggesting deterministic processes dominating. The opposite was observed amongst intermediately disturbed levels, implying stronger stochastic assembly mechanisms. Tradeoffs were observed in community function between organic carbon removal and both nitrification and biomass productivity, as well as between diversity and function, hence not every ecosystem function was favoured by higher community diversity.
The second study employed trait-based frameworks from ecology to enhance the understanding of the response of this complex microbial system after 35 days of succession under a disturbance gradient. The concept of ‘community aggregated traits’ was incorporated to assess ecosystem function parameters and to identify ‘trait complexes’ from the genotypic information gathered from the metagenomics dataset. Different community-level life-history strategies were associated across the disturbance range, with competitors (C) at the undisturbed level and stress-tolerants (S) at the press-disturbed level, while intermediate levels of disturbance harboured ruderal (R) strategies. Such three-way CSR strategies were proposed in the lights of trade-offs in community function and genotypic potential, and changes in bacterial genera composition.
The third study tested the effect of a non-toxic sustained variation in organic loading, involving two sets of four replicate mesocosm reactors that were operated at two different food to biomass (F:M) (0.19 and 0.36) and carbon to nitrogen (C:N) (3.5 and 6.3) ratios for a period of 127 days, including 53 days of sludge acclimation. The results showed that disturbed reactors with high F:M and C:N ratios exhibited different ecosystem function, community and genotypic assembly compared to the control reactors. Treatment reactors displayed quantifiable and initially variable functional resistance...
|Commitee:||Holyoak, Marcel, Scow, Kate M., Young, Thomas M.|
|School:||University of California, Davis|
|Department:||Civil and Environmental Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 79/10(E), Dissertation Abstracts International|
|Subjects:||Ecology, Microbiology, Environmental engineering|
|Keywords:||Bacterial communities, Intermediate disturbance hypothesis, Intermediate stochasticity hypothesis, Life-history strategies, Stochastic deterministic assembly, Trait based approach|
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