| الوصف: |
Static liquid microcosms have been used to study evolutionary and ecological dynamics of bacterial populations, where metabolic activity produces a heterogenous environment with a low-O2 region in the liquid column, and a high-O2 region directly below the air-liquid (A-L) interface. This system has been used to study adaptive radiation in Pseudomonas fluorescens SBW25, where non-biofilm forming wild-type populations diverge and biofilm-forming mutants known as Wrinkly Spreaders (WS) dominate the A-L interface where they are better able to exploit the ecological opportunity created by the high-O2 region. Although this system is well understood, it is not clear why biofilm-formation is such a successful strategy for colonising the high-O2 region. Similarly, this model system needs to be developed to reflect the complexity of microbial communities, where multiple species exist together and selective pressures may alter community composition, dynamics and emergent properties. Such better developed models can help understand changes in diverse natural occurring communities, enhancing our understanding of the progression of human infections, changes in ecologically important communities in agriculture, food production, and technology in applying microbial communities to mitigate the effects of anthropogenic pressures. In this thesis I show that A-L interface biofilm-formation is the best strategy for colonising the high-O2 region, where biofilms retain cells in position and overcome physical displacement including Brownian motion and bioconvection currents which otherwise would move aerotaxic wild-type cells away from the high-O2 region. WS mutant cells also possess an additional mechanism to associate and penetrate the A-L interface, where the production of additional surface-active agents further lower surface tension allowing cells to break through the interface. Biofilm-formation can then initiate above the interface and explain the dry phenotype and strength of WS biofilms as the biofilm is situated at the air-side of the interface. These key adaptive changes of the WS mutant allows highly efficient cell localisation at the high-O2 region to be achieved. However, biofilm strength is not directly related to fitness in static liquid microcosms. Other A-L interface biofilm-forming mutants within the SBW25 lineage can out-compete the WS mutant, producing weaker biofilms but maximising productivity by also significantly colonising the low-O2 liquid column compared to the WS mutant, suggesting colonising both regions provides a fitness advantage in microcosms. Ecosystem engineering was further explored to fully capture the ecological dynamics of diversifying SBW25 populations. Initial colonists not only generate O2 gradients but they were found to further alter the chemical environment through the uptake of nutrients and production of secondary metabolites and toxic waste products, which effect the diversification, biofilm-characteristics and fitness of evolved WS mutants. This additional aspect to ecosystem engineering within the microcosms system was also reflected in community-level work. I developed the microcosm model system for biofilm-forming communities using a soil-wash as the inoculum. The effects of heterogenous and O2 -limiting conditions on selection within bacterial communities were investigated and short-term serial-transfer experiments revealed changes in community productivity and biofilm characteristics. Productivity decreased in communities subject to longer incubation periods reflecting a tragedy of the commons with nutrient depletion and toxic waste-accumulation restricting growth. Final-transfer communities were stratified but retained phenotypic plasticity as isolates could form A-L interface biofilms as well as colonise the liquid column. Motility and cell localisation assays revealed isolates could migrate between both regions. This suggests a resource allocation trade-off between fast but competitive growth within the A-L interface biofilm and high-O2 region and slower but less competitive growth in the low-O2 liquid column, with community members maximising productivity by utilising the entire ecosystem. My research contributes to the growing body of knowledge aiming to understand the evolutionary and ecological processes driving change in biofilm-forming populations and communities in static liquid microcosms. It shows the value of continuing to ask deeper questions surrounding biofilm-formation within model systems, and the importance of developing model systems to reflect the complexity of naturally occurring microbial communities. A-L interface biofilm-formation has been the main subject of interest within this research, however extending my focus to the liquid column has shown the importance of colonising the region below the biofilm to improve productivity, competitive fitness and community resilience. This suggests understanding the influence of non-biofilm space in biofilm-forming communities is important in understanding the complex dynamics and persistence of multi-species microbial communities. |
