Magnetoreception is a remarkable ability found across a diverse range of organisms, including bacteria, birds, fish, insects, and mammals, enabling them to detect and harness the Earth's geomagnetic field. Recently, the recruitment of biomineralizing ectosymbionts by euglenozoans was evidenced as an ecological strategy for microeukaryotes to acquire this sense. Here, we report a case of magnetosymbiosis involving a ciliate and four populations of endosymbiotic bacteria experiencing genome reduction. Among these bacteria, one group of sulphate-reducing Desulfovibrionales was found to biomineralize bundles of bullet-shaped magnetite crystals. The ciliate's magnetotaxis mirrors that of free-living magnetotactic bacteria and euglenozoans, enabling efficient navigation in chemically stratified aquatic environments. However, in this case, magnetotaxis arises from an endosymbiotic interaction. Using a combination of optical-, confocal-, electron- and X-ray-based microscopy techniques, together with genomic analyses, these findings demonstrate that magnetosymbiosis can emerge in unicellular eukaryotic lineages through endosymbiotic integration, expanding our understanding of such interactions in aquatic ecosystems. More broadly, this work contributes to the ongoing debate on the origins of magnetoreception in eukaryotes.

Some bacteria genetically control the biomineralisation of intracellular amorphous calcium carbonates (iACC) with potential implications for microbial physiology, evolution, bioremediation and biogeochemical cycling. Until now, this capacity has been documented in Cyanobacteria, the giant gammaproteobacterium Achromatium and a few magnetotactic Pseudomonadota and Nitrospirota. Here, we report the discovery of iACC biomineralisation in members of the Methylococcaceae, a family of aerobic methanotrophic Gammaproteobacteria. A homologue of the ccyA gene, a diagnostic marker for iACC formation in Cyanobacteria, was identified in several Methylococcaceae genomes, based on the conserved C-terminal (GlyZip)3 domain of the encoded calcyanin protein. Moreover, two cultivated strains, Methylococcus geothermalis and Methylococcus mesophilus, whose genomes contained the ccyA gene, were consistently shown to form iACC. The ccyA genes of Methylococcaceae and Microcystis share higher sequence similarity than with other Cyanobacteria, suggesting horizontal gene transfer (HGT) from an ancestral Microcystis-like cyanobacterium to Methylococcaceae. This finding extends the known taxonomic distribution of ccyA and suggests that the capability to biomineralize iACC was acquired by HGT. The discovery of iACC in methane-oxidising Methylococcaceae highlights a previously unrecognised coupling between calcium carbonate biomineralisation and methane cycling in aquatic environments, suggesting an overlooked role of iACC formation in microbial carbon storage and local geochemical regulation.

Domain wall (DW) motion is a crucial process involved in magnetization reversal, be it under magnetic field or spin-polarized current stimulus. In most cases DW speed does not exceed ≈100m/s and collapses above a given threshold of the stimulus, an effect known as Walker breakdown. A few specific material properties have been identified to delay the breakdown of speed by increasing the energy barrier preventing internal precession. We show that in a 3D nanomagnetic system, here with vortex-state domains, the topology of the magnetization distribution may intrinsically and robustly delay the Walker breakdown due to an exchange-spring effect. In addition, curvature induces a major non-reciprocal effect, delaying or not the Walker breakdown depending on the chirality of the azimuthal domain versus the direction of motion of the DW.

Abstract Nanostructured synthetic bone grafts offer a promising strategy to prevent bacterial colonisation while supporting bone regeneration. Here, calcium-deficient hydroxyapatite (CDHA) nanopillars synthesized from α-tricalcium phosphate exhibit contact-killing activity against Gram-positive Bacillus subtilis . Unlike inert bactericidal surfaces, CDHA exchanges ions with the surrounding environment, introducing chemical interactions alongside mechanical damage. Using synchrotron cryo-soft X-ray tomography and spectromicroscopy, we visualise bacterial ultrastructure and intracellular ionic changes in fully hydrated cells at subcellular resolution. Some bacteria exposed to the nanotopographies display membrane rupture, cytosolic leakage, and multivesicular body formation, indicating severe stress responses. XANES cryo-spectromicroscopy and linear absorption coefficient analysis identify a bacterial subpopulation with increased intracellular calcium, correlating with calcium release from the substrate and confirmed by confocal microscopy. However, this calcium accumulation does not affect viability, consistent with the halotolerant nature of B. subtilis . These findings provide insights into the combined effects of nanotopography and surface chemistry on bacterial responses.

Bilayers of NdCo5/Ni8Fe2 can act as reconfigurable racetracks thanks to the parallel stripe-domain configuration present in the hard magnetic material with weak perpendicular anisotropy (NdCo5), and its imprint on the soft magnetic layer (Ni8Fe2). This pattern hosts domain walls with well-defined vortex-antivortex configurations and establishes paths for their deterministic propagation under the effect of pulsed currents. Magnetic transmission x-ray microscopy experiments show guided domain-wall propagation events within the Ni8Fe2 layer above a threshold current of 3×1011A/m2. Propagation direction is governed by the change of in-plane magnetization, with opposite displacements for head-to-head and tail-to-tail domain walls. A comparison between domain-wall propagation events at remanence and under an applied in-plane field reveals the presence of an exchange bias field that keeps memory of the last saturated state. The link between the magnetic history stored in the hard magnetic layer and the direction of domain-wall propagation in the soft magnetic layer is a magnetic spring at the NdCo5/Ni8Fe2 interface, as shown by micromagnetic simulations. This leads to a system, which behaves as a hard-soft magnetic composite with reconfigurable capabilities for the controlled propagation of magnetic spin textures. locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon Physics Subject Headings (PhySH)Domain wall motionMagnetic anisotropyMagnetic domainsMagnetic vorticesSpin textureSpintronicsSurface & interfacial phenomenaMagnetic multilayers

Bilayers of NdCo5/Ni8Fe2 can act as reconfigurable racetracks thanks to the parallel stripe-domain configuration present in the hard magnetic material with weak perpendicular anisotropy (NdCo5), and its imprint on the soft magnetic layer (Ni8Fe2). This pattern hosts domain walls with well-defined vortexantivortex configurations and establishes paths for their deterministic propagation under the effect of pulsed currents. Magnetic transmission x-ray microscopy experiments show guided domain-wall propagation events within the Ni8Fe2 layer above a threshold current of 3 x 1011 A/m2. Propagation direction is governed by the change of in-plane magnetization, with opposite displacements for head-to-head and tail-to-tail domain walls. A comparison between domain-wall propagation events at remanence and under an applied in-plane field reveals the presence of an exchange bias field that keeps memory of the last saturated state. The link between the magnetic history stored in the hard magnetic layer and the direction of domain-wall propagation in the soft magnetic layer is a magnetic spring at the NdCo5/Ni8Fe2 interface, as shown by micromagnetic simulations. This leads to a system, which behaves as a hard-soft magnetic composite with reconfigurable capabilities for the controlled propagation of magnetic spin textures.

Hepatitis C virus (HCV) replication causes a profound remodeling of the host endomembrane system. The availability of direct-acting antiviral (DAA) drugs provides an opportunity to define the ultrastructural events that follow viral replication blockade using confocal immunofluorescence, transmission electron microscopy (TEM) as well as correlative cryogenic light-soft X-ray tomography (CLSXT). Study of DAA-treated HCV replicons using CLSXT indicates that HCV-induced membranous alterations are no longer visible after 24 h of treatment and that a component of the replicase is located in pleomorphic, high-absorption contrast acidic organelles. TEM studies confirmed the rapid elimination of the viral machinery, and the concurrent appearance of large endo-lysosomes in DAA-treated cells. These and results by others suggest that HCV replication compartment may constantly be recycled by the endo-lysosomal system and that this equilibrium is unbalanced by DAA treatment, resulting in a transient activation of the endo-lysosomal system to achieve rapid viral machinery removal.

Cryo-imaging in cellular biology provides the means to visualize the cellular interior at close-to-native conditions. A cornerstone in the field has been cryo-correlative light and electron microscopy (cryo-CLEM), with cryo-visible light fluorescent microscopy (cryo-VLFM) providing the specificity by tagging macromolecules or structures and cryo-electron tomography (cryo-ET) for the structural details at molecular level. The large resolution gap between these techniques, however, is limiting this correlative workflow as cryo-ET targets are often smaller than the resolution limit of cryo-VLFM. Here we introduce cryo-soft X-ray tomography (cryo-SXT) as an intermediate step that can compensate for the partial view caused by the lost cellular material due to FIB-milling and limited resolution of cryo-VLFM by providing invaluable cellular context information in 3D to the cryo-ET dataset within an integrated workflow. This work shows that X-ray and electron imaging are not mutually exclusive, creating opportunities for further correlative imaging strategies.

Nanoparticle-based drug carriers offer a promising alternative to conventional cancer therapies by enabling targeted delivery and reducing off-target toxicity. Here, we used synthesised and characterised carbon-based nanoparticles derived from 2-acrylamido-2-methylpropanesulfonic acid (AMPS-CDs), demonstrating biocompatibility with both human astrocytes and glioblastoma cells. We assessed their potential to enhance riluzole's efficacy through synergistic interaction (AMPS-CDs@RZ) using live-cell synchrotron-based FTIR spectroscopy and cryo-soft X-ray tomography to monitor biochemical and structural changes at the single-cell level. While AMPS-CDs nanoparticles alone were non-toxic, the combination with riluzole significantly enhanced cell death in glioblastoma cells, with a significantly lower impact in non-cancerous astrocytes. Treatment with AMPS-CDs@RZ induced significant changes in bio-macromolecules, including DNA, protein conformation, and lipid metabolism. Notably, the treatment triggered nuclear envelope (NE) blebbing in glioblastoma cells, likely due to the interaction of the nanoparticle formulation with the nuclear membrane. This initiated stress signals that disrupted the cell's inner intracellular membrane system, including the endoplasmic reticulum and mitochondria. To our knowledge, this is the first report linking NE blebbing to this mechanism involving membrane disassembly and nuclear envelope blebbing in riluzole-induced toxicity in glioblastoma is novel, providing a new therapeutic strategy and insights into cellular stress responses. These findings suggest that AMPS-CDs nanoparticles are a promising carrier for riluzole, potentially enhancing the specificity and efficacy of glioblastoma treatments while minimising damage to healthy tissues.

Topology is a powerful tool for categorizing magnetization textures by defining a topological index in both two-dimensional (2D) systems, such as thin films or curved surfaces, and in 3D bulk systems. In the emerging field of 3D nanomagnetism, both volume and surface topological numbers must be considered, requiring the identification of a proper global topological invariant to support categorization. Here we consider domain walls in cylindrical nanowires as an excellent playground for 3D nanomagnetic systems, excited by a charge current, that generates an OErsted field. We first provide experimental evidence of previously unreported domain-wall transformations of topology occurring at the nanosecond timescale. We investigate these transformations with micromagnetic simulations, tracking both bulk and surface topological signatures.We demonstrate a topological invariant combining both signatures, while the topological charge varies from bulk to surface during the dynamics. The experimental change of topology is reproduced when the pulse duration matches the timescale of the internal transformations of the wall, and the current is switched off before the transformation is complete. We expect that the topological categorization and dynamical exploitation apply to any 3D nanomagnetic system.

Cryo-imaging in cellular biology provides the means to visualize the cellular interieur at close-to-native conditions. A cornerstone in the field has been cryo-correlative light and electron microscopy (cryo-CLEM), with cryo-visible light fluorescent microscopy (cryo-VLFM) providing the specificity by tagging macromolecules or structures and cryo-electron tomography (cryo-ET) for the structural details at molecular level. The large resolution gap between these techniques, however, is limiting this correlative workflow as cryo-ET targets are often smaller than the resolution limit of cryo-VLFM. Here we introduce cryo-soft X-ray tomography (cryo-SXT) as an intermediate step that can compensate for the partial view and limited resolution of cryo-VLFM by providing invaluable cellular context information in 3D to the cryo-ET dataset within an integrated workflow. ### Competing Interest Statement Sergey Kapishnikov is part of SiriusXT, a company developing a cryo-SXT laboratory system. The other authors declare no competing interests. ANR PIA grant, EquipEx CACSICE: ANR-11-EQPX-0008 ANR PRC RabReprogram PFR6 Programme Féderateur de recherche

The expansion of nanomagnetism to the third dimension leads to phenomena such as curvature-induced magnetochirality and anisotropy, which can significantly influence the behavior of magnetic textures. One of the most promising systems is the magnetic nanotube - where intrinsic curvature effects are present. However, studies of magnetic nanotubes remain limited to straight systems, and little is known about the influence of 3D geometries. In this work, three dimensional (3D) complex-shaped nanotubes are fabricated by combining nanoprinting with the conformal deposition of magnetic films. Specifically, 3D conductive non-magnetic tungsten scaffolds are fabricated using focused electron beam induced deposition and subsequently coated with a nickel magnetic shell, resulting in complex-shaped magnetic nanotubes whose geometry can be controlled by tuning the electron-beam parameters and electrodeposition conditions. Performing X-ray microscopy revealed that nanotubes of various geometries host a vortex-like azimuthal state, and that the energy landscape of the magnetic configuration can be tailored geometrically. Specifically, the pinning of magnetic domain walls at curved vertices is observed experimentally and confirmed with micromagnetic simulations, offering geometrical control of magnetic configurations in nanotube architectures. This approach provides a new pathway to fabricate and study complex 3D core-shell magnetic structures, facilitating experimental investigations of their fundamental properties, key for the next-generation of spintronic devices.

Phase changes of macromolecules in living cells gained recently a major interest in cell biology as liquid liquid phase separation or liquid solid phase transition phenomenon being observed in increasing number of biological or pathological processes. Comprehensive characterization of these phases remains challenging and requires new methodological approaches for their complete description across spatial and temporal scales. We propose a combination of imaging methods applied to a model system and report the in vivo crystallization pathways of a macromolecule from its solution to crystalline states at the cell population level down to the meso scale. Combining various live fluorescence based techniques and a high resolution cryo imaging technique within unaltered cryopreserved cells, we could described the unexpectedly wide landscape of the in vivo crystalline states of the fluorescent coral derived protein xpa, gain information of crystal growth dynamics in cellulo, and provide hypothesis of the crystal nucleation requirements of this stochastic process. ### Competing Interest Statement The authors have declared no competing interest.

This study integrates multiple microscopy techniques to investigate the effect of introducing axial compositional gradients on the magnetic properties of Fe-Ni ferromagnetic nanowires. We study the chemical structure of the nanowires using photoemission electron microscopy and transmission (X-ray and electron) microscopy. We explore the magnetic properties through magnetic force microscopy, complemented by micromagnetic simulations, and we characterize the 3D magnetization vector by measuring magnetic circular dichroism using X-ray microscopies. We observe that variations in the Fe/Ni ratio induce localized magnetic curling in Fe-rich regions, while Ni-rich segments predominantly exhibit axial magnetization, demonstrating how compositional gradients can control magnetic domain configurations on the nanoscale. Our results highlight the value of multimodal imaging for uncovering the interplay among structural, chemical, and magnetic properties in complex nanostructures. These findings represent a significant step toward the manipulation of magnetic domains in nanowires, which is essential for future devices based on 3D nanomagnetic architectures.

Magnetic vector tomography allows for visualizing the 3D magnetization vector of magnetic nanostructures and multilayers with nanometric resolution. In this work, we present MARTApp (Magnetic Analysis and Reconstruction of Tomographies Application), a software designed to analyze the images obtained from a full-field or scanning transmission X-ray microscope and reconstruct the 3D magnetization of the sample. Here, its workflow and main features are described. Moreover, a synthetic test sample consisting of a hopfion is used to exemplify the workflow from raw images to the final 3D magnetization reconstruction.

Heterochromatin organization is critical to many genome-related programs including transcriptional silencing and DNA repair. While super-resolution imaging, electron microscopy, and multiomics methods have provided indirect insights into the heterochromatin organization, a direct measurement of mesoscale heterochromatin ultrastructure is still missing. We use a combination of correlative light microscopy and cryo-soft X-ray tomography (CLXT) to analyze heterochromatin organization in the intact hydrated state of human mammary fibroblast cells. Our analysis reveals that the heterochromatin ultra-structure has a typical mean domain size of approximately 80 nm and a mean separation of approximately 120 nm between domains. Functional perturbations yield further insights into the molecular density and alterations in the mesoscale organization of the heterochromatin regions. Furthermore, our polymer simulations provide a mechanistic basis for the experimentally observed size and separation distributions of the mesoscale chromatin domains. Collectively, our results provide direct, label-free observation of heterochromatin organization in the intact hydrated state of cells.