| Biofilms, the most common form of bacterial growth, are defined as surface-associated microorganisms embedded in a highly-hydrated, dynamic matrix of exopolymeric substances (EPS). Biofilms grow under both saturated conditions, where they have been extensively studied, and under unsaturated conditions, where much less is known about their physiology. In unsaturated environments such as soil, biofilms face unique challenges such as low water potentials and heterogeneous nutrient distribution. These biofilms are important to nutrient cycling and bioremediation. Bioremediation of organic pollutants often involves bacteria that can use the pollutants as carbon and energy sources, but is often complicated by the poor bioavailability of pollutants. I hypothesized that P. aeruginosa biofilms adapt to improve access to poorly bioavailable substrates. In this dissertation, the cell shape, macromolecular composition, physiochemical properties, and ecology of unsaturated biofilms were observed for variations with carbon source, substratum, time, and other stimuli. I found that when faced with either reduced solubility or restricted diffusional resupply of the carbon and energy source, P. aeruginosa growing in a biofilm elongates to maximize its cross section perpendicular to the nutritional gradient, increasing its nutrient acquisition surface area. Biofilm EPS macromolecular composition varies with time and carbon source, suggesting that extracellular chemical shifts were systematic and could feedback to improve pollutant bioavailability. Both the amount of EPS produced and the hydrophobicity of the EPS increased in response to increased hydrophobicity of both carbon sources and substrata. However, when using extremely hydrophobic carbon sources, only the hydrophobicity of the EPS increased. A significant portion of the EPS in both pure P. aeruginosa and mixed-species biofilms was extracellular DNA (eDNA) of apparently chromosomal origin. Collectively, this work suggests that unsaturated biofilm EPS is not simply a passive reservoir for cellular turnover, but rather a selective agent in unsaturated biofilm metabolism, and thus relevant to bioremediation. This dissertation represents the first extensive exploration of unsaturated biofilm EPS and contributes to the understanding of microbial ecology and bioremediation. |
