August 2023

Some organisms have the unique capacity to geolocate and navigate in response to the Earth’s magnetic field lines. Migratory birds and fishes are the best-documented animals that evolved this capacity to guide their movements. In the microbial world, magnetotactic bacteria (MTB) and multicellular magnetotactic prokaryotes (MMPs) have been the only known magnetoreceptive microorganisms for decades. Some microeukaryotes also orient their motility axis along magnetic field lines thanks to the exploitation of MTB magnetism. The magnetic guidance of these prokaryotes and eukaryotes is due to the biomineralization of magnetic crystals. This article provides a brief overview of the current knowledge concerning the different multicellular prokaryotes and micro/macroeukaryotes capable of magnetoreception. We also discuss the evolution of this unique ability.

Magnetite is the most abundant magnetic iron mineral on the Earth’s surface. Its formation in natural ecosystems is mainly due to microbial activity. Microbially synthesized magnetite, commonly called “biogenic magnetite,” has many beneficial properties for a wide range of environmental and commercial applications. Its high surface reactivity facilitates interactions with (in)organic pollutants in anthropic and natural ecosystems, as well as with reagents in industrial catalysis. Due to its magnetic properties and good biocompatibility, biogenic magnetite is also well suited for biomedical applications such as cancer treatment or drug delivery. Biomineralization of magnetite offers an inexpensive and sustainable method for the production of this highly functional material. Moreover, this biomineralization process results in a biomolecule coating of the magnetite, making it highly amenable to further functionalization. This chapter reviews the application of biogenic magnetite across environmental, medical, and industrial settings. Existing challenges and future opportunities in these applications are also discussed.

Magnetite is a redox-active mineral that can form from both abiotic and biotic processes, and plays an active role in different biogeochemical cycles. Biogenic magnetite particles have properties that differ from their abiogenic counterparts in a variety of ways, including their size, chemical purity, magnetic properties, and association with biomass-derived organic matter. These properties directly influence magnetite reactivity—in particular its sorbent and redox behavior—affecting its association with metals, oxyanions, and other compounds in the environment. Biogenic (and abiogenic) magnetite particles are involved in redox processes by storing electrons, functioning as biogeobatteries, and by transferring electrons between microbial cells or between cells and inorganic constituents. Thus, magnetite influences the fate of contaminants and nutrients in the environment.

Magnetofossils are magnetic nanoparticles that represent the fossil remains of microorganisms that biomineralize magnetic minerals in a genetically controlled manner. Most magnetofossils found in the geologic record are produced by magnetotactic bacteria, which use them for navigating within their living environment. Magnetofossils can be identified using a combination of magnetic and imaging techniques. A common attribute of magnetofossils, although not pervasive, is that they are arranged in chains, which determines their specific magnetic properties. Magnetofossil signatures have been reported from ancient rocks to modern sediments and even in extraterrestrial materials. They provide a window into biomineralization, past environments, and ancient magnetic fields, as well as supplying fuel for questions on the origin of life in the Solar System.

Biomagnetism describes the biological origin of magnetism within living organisms. This phenomenon occurs due to the formation of iron-based minerals that exhibit magnetic ordering at room temperature. Perhaps the most studied form of biomagnetism originates in bacteria, especially magnetotactic bacteria that produce internal magnetite and greigite grains and iron-reducing bacteria that produce magnetite nanoparticles externally as a byproduct of iron respiration. These bacteria likely contribute to a significant proportion of environmental magnetite. The emergence of biomagnetism remains unclear, although it is thought that magnetotactic bacteria evolved this mechanism several billion years ago. Understanding how and why micro-organisms generate biomagnetism is helping to shed light on the origin of life on Earth and potentially on other planets. Biomagnetism is also of broad interest to industrial and environmental applications.