Publisher's Synopsis
The activity of microbes impacts the biogeochemical cycling of virtually all nutrients on Earth. Unlike the open water column, the sedimentary environment tends to be layered, with micro- and macro-gradients selecting for microbes with robust metabolic strategies and gradient-dependent adaptations. In contrast to the eukaryotic world, where hexose sugars are the major source of energy for the biochemical machinery, sedimentary prokaryotes use a far wider array of electron donors (EDs), and in the absence of oxygen, may be capable of using a wider array of electron acceptors (EAs) too. One such group of bacteria, the Shewanella (represented here by Shewanella oneidensis, strain MR-1) is capable of extracellular electron transport (EET) to insoluble minerals, as well as about 15 known soluble EAs. The metal (iron or manganese) oxide minerals are referred to as insoluble electron acceptors (IEA). Motility and taxis offer microbes a significant advantage in electron donor/acceptor rich conditions, where relocating toward favorable hot spots of specific soluble electron donors (EDs) or electron acceptors (EAs) over distances of 100-1000 μm can make the difference between survival and death. The scope of this dissertation includes three accomplishments: i) discovery and characterization of electrokinesis, a process whereby bacteria increase swimming speed in response to the contact with insoluble electron acceptors (IEAs); ii) development of the congregation model, through examination of cell swimming trajectories, determination of how a series of runs-and-reversals allows cells to repeatedly come into contact with IEA surfaces, thus congregating around them, and the identification of the genes involved in the regulation of this mechanism; and, iii) discovery of the correlation between the congregation behavior on one hand, and attachment to, and growth upon IEAs on the other hand, for MR-1. Congregation genes are then shown to also be essential for motility enabled accumulation and attachment. Lastly, quantification of congregation, accumulation and attachment in different Shewanella strains and species reveals that each species exhibits preference for IEA with specific redox potential.
