Biology subjects

Amor, M.

Publications and source records attributed to Amor, M..

4 recordsLinked to original sources

bioRxiv2026

Magnetic organelle gene clusters enable bacterial growth and ATP homeostasis under oxidizing conditions

Biological organisms can produce magnetic nanoparticles made of magnetite to detect the Earth's magnetic field for orientation. The evolutionary mechanisms that led to the acquisition of this capacity remain unknown. The most ancient organisms known to perform magnetic orientation are magnetotactic bacteria. Under tight genetic control, they produce chains of magnetite nanoparticles in dedicated organelles to allow efficient navigation in aquatic environments containing molecular oxygen. Yet, these bacteria have been proposed to emerge before molecular oxygen became abundant on Earth, at a time when this navigation system could not have served its current function. Here, we show that gene clusters responsible of magnetite formation and organization enable resistance against hydrogen peroxide, a toxic oxygen-derived compound that was produced during the early stages of Earth's oxygenation when iron oxidation consumed molecular oxygen. Characterization of the nanoparticle's oxidation state supported the involvement of magnetite in resistance to hydrogen peroxide-induced oxidative stress. These findings are compatible with an early emergence of magnetotactic bacteria, before natural waters became oxygenated. We propose a conceptual framework to explain the origin of magnetic orientation in bacteria, and offer tools to identify the initiation of Earth's oxygenation in a time-resolved manner.

microbiology↗

bioRxiv2022

McaA and McaB control the dynamic positioning of a bacterial magnetic organelle

Magnetotactic bacteria (MTB) are a diverse group of microorganisms that use intracellular chains of ferrimagnetic nanocrystals, produced within their magnetosome organelles, to align and navigate along the geomagnetic field. The cell biological and biochemical properties of magnetosomes make them a powerful model for studying the molecular mechanisms of biomineralization and compartmentalization in bacteria. While several conserved magnetosome formation genes have been described, the evolutionary strategies for their species-specific diversification remain unknown. Here, we demonstrate that the fragmented nature of magnetosome chains in Magnetospirillum magneticum AMB-1 is controlled by two genes named mcaA and mcaB. McaA recognizes the positive curvature of the inner cell membrane while McaB localises to magnetosomes. Along with the MamK actin-like cytoskeleton, they create space for addition of new magnetosomes in between pre-existing magnetosomes. Phylogenetic analyses suggest that McaAB homologs are widespread and may represent an ancient strategy for organelle positioning in MTB.

microbiology↗

bioRxiv2020

Magnetotactic bacteria accumulate a large pool of iron distinct from their magnetite crystals

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that form intracellular nanoparticles of magnetite (Fe3O4) or greigite (Fe3S4) in a genetically controlled manner. Magnetite and greigite synthesis requires MTB to transport a large amount of iron from the environment which is subsequently concentrated in organelles called magnetosomes for crystal precipitation and maturation. X-ray absorption analysis of MTB suggests that the intracellular iron is mainly contained within the crystals, thus preventing potential toxic effects of free iron. In contrast, recent mass spectrometry studies suggest that MTB may contain a large amount of iron that is not precipitated in crystals. Here, we attempt to resolve these descrepancies by performing chemical and magnetic assays to quantify the different iron pools in the magnetite-forming strain Magnetospirillum magneticum AMB-1 cultivated at varying iron concentrations. AMB-1 mutants showing defects in crystal precipitation were also characterized following the same approach. All results show that magnetite represents at most 30 % of the total intracellular iron under our experimental conditions. We further examined the iron speciation and subcellular localization in AMB-1 using the fluorescent indicator FIP-1 that is designed for detection of labile Fe(II). Staining with this probe suggests that unmineralized reduced iron is found in the cytoplasm and associated with magnetosomes. Our results demonstrate that, under our experimental conditions, AMB-1 is able to accumulate a large pool of iron distinct from magnetite. Finally, we discuss the biochemical and geochemical implications of these results. ImportanceMagnetotactic bacteria (MTB) are a group of microorganisms producing iron-based intracellular magnetic crystals. They represent a model system for studying iron homeostasis and biomineralization in bacteria. MTB contain an important mass of iron, about 10 to 100 higher than other bacterial model such as Escherichia coli, suggesting efficient iron uptake and storage systems. Accordingly, MTB have been proposed to significantly impact the iron biogeochemical cycle in sequestering a large amount of soluble iron into crystals. Recently, several studies proposed that MTB could also accumulate iron in a reservoir distinct from their crystals. Here, we present a chemical and magnetic methodology for quantifying the fraction of the total cellular iron contained in the magnetic crystals of the magnetotactic strain Magnetospirillum magneticum AMB-1. Comparison of the mass of iron contained in the different cellular pools showed that most of the bacterial iron is not contained in AMB-1 crystals. We then adapted protocols for the fluorescent detection of Fe(II) in bacteria, and showed that iron could be detected outside of crystals using fluorescence assays. This work suggests a more complex picture for iron homeostasis in MTB than previously thought. Because iron speciation controls its solubility, our results also provide important insights into the geochemical impact of MTB. A large pool of unmineralized iron in MTB could be more easily released in the environment than magnetite, thus limiting iron sequestration into MTB crystals.

microbiology↗

bioRxiv2019

Single-cell determination of iron content in magnetotactic bacteria: implications for the iron biogeochemical cycle

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that biomineralize dissolved iron from the environment into intracellular nanoparticles of magnetite [Fe(II)Fe(III)2O4] or greigite [Fe(II)Fe(III)2S4] in a genetically controlled manner. After cell death, these magnetite and greigite crystals are trapped into sediments which effectively removes iron from the soluble pool. MTB may significantly impact the iron biogeochemical cycle, especially in the ocean where dissolved iron limits nitrogen fixation and primary productivity. Although MTB are ubiquitous in the environment, their impact on the biogeochemical cycling of metallic elements is still poorly constrained. A thorough assessment of the mass of iron incorporated by MTB has been hampered by a lack of methodology to accurately measure the amount of, and variability in, their intracellular iron content. Here, we quantify the mass of iron contained in single MTB cells of the model organism, Magnetospirillum magneticum sp. AMB-1, using a time-resolved mass spectrometry methodology. Bacterial iron content depends on the external iron concentration, and reaches a maximum value of 10-6 ng of iron per cell when bacteria are cultivated with initial iron concentrations of 100 M or higher. From our experimental results, we calculated the flux of dissolved iron incorporation into natural MTB populations and conclude that MTB may mineralize a significant fraction of environmental dissolved iron into crystals.

microbiology↗