Dependence of bacterial magnetosome morphology on chemical conditions in deep-sea sediments

Abstract

Magnetotactic bacteria (MTB) play an important role in biogeochemical cycles of iron and remanent magnetization acquisition of sediments. It is generally considered that magnetite-producing MTB prefer chemical conditions in the oxic-anoxic transition zone (OATZ). We studied the distribution of fossil MTB magnetite (magnetofossils) and microbes phylogenetically related to MTB within deep-sea surface sediments across the OATZ. Transmission electron microscope (TEM) observations and rock-magnetic proxies show that magnetofossils commonly occur both within the OATZ and above it, even at the top of surface oxidized layers, and that the proportion of magnetofossils with a teardrop morphology increases within the OATZ. Consistent results were obtained from the pyrosequencing of 16S rRNA gene fragments. Operational taxonomic units (OTUs) related to Nitrospirae and OP3 MTB, which produce teardrop-shape magnetosomes, were detected only within the OATZ, whereas OTUs related to alphaproteobacterial MTB, which produce equant or elongated magnetite particles, were distributed above the OATZ. These results suggest that magnetofossil morphology can be used to detect past OATZs, and that some MTB lineages can occur ubiquitously in marine sediments rather than being limited within the OATZ. Our results imply that the timing of remanent magnetization acquisition may depend on the inhabiting MTB lineages.