Mechanisms and Evolution of Magnetotactic Bacteria

Citation

Nash, Cody Zane (2008) Mechanisms and Evolution of Magnetotactic Bacteria. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9M6S-MS85. https://resolver.caltech.edu/CaltechETD:etd-05282008-140316

Abstract

Magnetotactic bacteria (MB) contain intracellular magnetic crystals of iron oxides and/or iron sulfides. These crystals and the membranes which enclose them are together known as magnetosomes. The crystals formed by MB fall into a narrow size range and have species-specific crystal morphologies. Magnetosomes are physically connected to the rest of the cell by actin-like filaments that are thought to allow the MB to take advantage of their passive orientation in the Earth’s magnetic field to navigate more efficiently across chemical gradients. The large excess of crystals in most strains suggests that magnetosomes may also function as an iron reservoir or as a redox battery.

This thesis describes a number of investigations of the MB. First, a set of genes was identified as being conserved uniquely among the MB by comparative genomics. This method was validated by finding many of the genes already known to be involved in magnetotaxis. Many additional genes were identified and some of these genes were found to cluster together. Three of these clusters were genetically interrupted to determine their role in magnetite biomineralization.

Second, a transposon mutagenesis was undertaken to identify genes necessary for the magnetic phenotype of MB. Out of 5809 mutants screened, nineteen were found to be non- or partially magnetic. Fourteen of these have insertion sites in genes known to be involved in magnetotaxis. Five more were found to have insertions in previously unsuspected genes. The mutant phenotypes of the five mutants include the complete absence of magnetosomes, elongate crystals, reduced numbers of crystals and incomplete mineralization. These mutant strains were used to develop ferromagnetic resonance theory of isolated single-domain particles and biogenic particle identification.

Third, MB were discovered in hot springs and in hyper-saline, hyper-alkaline Mono Lake, CA. This extends the environmental range of MB to astro- and paleobiologically relevant environments. Magnetotactic Archaea were tentatively identified from Mono Lake, CA and are the first magnetotactic representatives of that domain.

Thesis Files