The biomineralization and fossilization of magnetotactic bacteria: Insights from experimental and field studies

Abstract:

Magnetotactic bacteria (MTB) are widespread prokaryotes which can navigate along the Earth’s magnetic field lines and produce tens to hundreds of nanocrystals of magnetite (Fe₃O₄) or/and greigite (Fe₃S₄) aligned in chain(s) within a cell. The remains of MTB (i.e. magnetofossils) within geological records have therefore been considered as potential recorders of paleomagnetic, paleoenvironmental and ancient-life signals. These intracellularly-formed nanocrystals, called magnetosomes, generally have distinctively physical, chemical and crystallographic features from those magnetic minerals produced by abiotic or extracellular mineralization processes, and therefore could be distinguished by rock magnetic and electron microscopic approaches. However, identification and quantification of magnetofossils from sediments or sedimentary rocks are nevertheless not straightforward not only due to their tiny sizes, relatively low concentration, always mixing with abiotic magnetic minerals, but also the chain collapse and crystal maghemization during post-depositional processes. Comprehensive studies on the biomineralization and fossilization of magnetosomes are therefore essential for unambiguously identifying and quantitating magnetofossils from geologic samples. In this presentation, we summarize the biomineralization processes and magnetic properties of magnetosome chains within modern cultured and uncultured MTB. Experimental studies on the effects of the chain aligning and collapsing on the magnetic properties of magnetosomes are discussed, which give useful clues to understand the possible occurrence of magnetofossils within natural materials and their corresponding magnetic changes. Recent findings in magnetofossils from marine and lake sediments, showing how to identify magnetofossils from sediments by using the comprehensive rock magnetism, ferromagnetic resonance, and transmission electron microscopy approaches, as well as their implications for sedimentary magnetism, and paleoclimatic and paleoenvironmental processes will be discussed.