Cells of magnetotactic bacteria are used as model systems for studying the magnetic properties of ferrimagnetic nanocrystals. Each individual bacterial strain produces magnetosomes (membrane-bounded magnetic crystals) that have distinct sizes, shapes, crystallographic orientations and spatial arrangements, thereby providing nanoparticle systems whose unique magnetic properties are unmatched by synthetic chemically-produced crystals. Here, we use off-axis electron holography in the transmission electron microscope to study the magnetic properties of isolated and closely-spaced bullet-shaped magnetite (Fe3O4) magnetosomes biomineralized by the following magnetotactic bacterial strains: the cultured Desulfovibrio magneticus RS-1 and the uncultured strains LO-1 and HSMV-1. These bacteria biomineralize magnetite crystals whose crystallographic axes of elongation are parallel to <100> (RS-1 and LO-1) or <110> (HSMV-1). We show that the individual magnetosome crystals are single magnetic domains and measure their projected in-plane magnetization distributions and magnetic dipole moments. We use analytical modelling to assess the interplay between shape anisotropy and the magnetically preferred <111> magneto-crystalline easy axis of magnetite.

Sulfidated nanoscale zerovalent iron (S-nZVI) materials show enhanced reactivity and selectivity towards chlorinated solvents compared to non-sulfidated nZVI, thus they have a high potential for subsurface chlorinated solvent remediation. However, little is known about the possible toxic effects of S-nZVI towards microbial communities, which is of particular concern with regard to combined abiotic–biotic chlorinated solvent treatment scenarios. In this study, the toxicity of two different S-nZVI materials towards Shewanella oneidensis MR-1 (S. MR-1) was examined under anaerobic and aerobic conditions using colony forming units (CFU) and adenosine triphosphate (ATP) measurements, and the results were then compared to identical exposures performed with non-sulfidated nZVI. In a second step, the toxicity of S-nZVI and nZVI materials was tested on the commercial bioremediation culture KB-1® and on an in-house trichloroethylene enrichment culture. Under aerobic conditions, S. MR-1 viability was less affected by S-nZVI materials compared to non-sulfidated nZVI materials (up to three times higher viability) and it was generally lower compared to anaerobic conditions where little difference in S. MR-1 viability was observed between the tested materials. In terms of the two dechlorinating cultures, they exhibited significantly higher ATP viability during anaerobic exposures to S-nZVI and nZVI materials. Particularly for KB-1®, which retained comparable ATP-viability after ~60 hours exposure as S. MR-1 after two hours. Moreover, the ATP viability of the mixed cultures was generally higher in S-nZVI exposures compared to nZVI exposures (up to three times higher viability). The observed viability patterns are explained by differences in the shell structure, chemistry and stability of the tested S-nZVI and nZVI materials towards corrosion, while the substantially enhanced resilience of KB-1® is argued to stem from its year-long cultivation in the presence of reduced FeS particulates.

Extracellular polymeric substances (EPS) are high molecular weight polymers that microorganisms secrete into their extracellular environment. EPS serves as the carrier of the structural integrity of microbial biofilms, determining the physicochemical properties and the functional complexity of biofilms. EPS creates an ideal environment for interfacial reactions and nutrient trapping around microbial cells, while also acting as a buffer zone against environmental stresses. EPS in soil can contribute to soil health through its own properties such as adhesion, hygroscopicity and complexing ability. Here, we first introduce the concept, components, properties and controlled factors of EPS in the soil environment, and outline current advances in extraction methods and characterization techniques for soil EPS. EPS form a dynamic biophysical-chemical interface between microbes and the soil matrix. We explore the role of EPS in the colonization and survival of microorganisms, aggregation and weathering of soil minerals, and cross-linking with soil organic matter. We then summarize the soil ecological functions of microbial EPS: 1) promoting aggregate formation and stabilization; 2) enhancing water retention and holding capacity; 3) mediating nutrient storage and trapping; and 4) regulating contaminant sequestration and transformation. Finally, we propose several future research interests for microbial EPS in soil, thereby calling for more attention and research on microbial EPS and its functions in soil ecosystems, and exploring their potential applications in the development of environment-friendly agriculture.

Iodine (I) is a trace element with health and environmental significance. Iodate (IO3-), iodide (I-) and organic iodine (org-I) are the major species of iodine that exist in the environment. Dissimilatory IO3--reducing bacteria reduce IO3- to I- directly under anoxic conditions via their IO3- reductases that include periplasmic iodate reductase IdrABP1P2, extracellular DMSO reductase DmsEFAB and metal reductase MtrCAB. IdrAB and DmsEFAB reduce IO3- to hypoiodous acid (HIO) and H2O2. The reaction intermediate HIO is proposed to be disproportionated abiotically into I- and IO3- at a ratio of 2:1. The H2O2 is reduced to H2O by IdrP1P2 and MtrCAB as a detoxification mechanism. Additionally, dissimilatory Fe(III)- and sulfate-reducing bacteria reduce IO3- to I- directly via their IO3- reductases and indirectly via the reduction products Fe(II) and sulfide in the presence of Fe(III) and sulfate, respectively. I--oxidizing bacteria oxidize I- to molecular iodine (I2) directly under oxic conditions via their extracellular multicopper iodide oxidases IoxAC. In addition to I2, a variety of org-I compounds are also produced by the I--oxidizing bacteria during I- oxidation. Furthermore, ammonia-oxidizing bacteria oxidize I- to IO3- directly under oxic conditions, probably via their intracellular ammonia-oxidizing enzymes. Many bacteria produce extracellular reactive oxygen species that can oxidize I- to triiodide (I3-). Bacteria also accumulate I- during which I- is oxidized to HIO by their extracellular vanadium iodoperoxidases. The HIO is then transported into the bacterial cells. Finally, bacteria methylate I- to org-I CH3I, probably via their methyltransferases. Thus, bacteria play crucial and versatile roles in the global biogeochemical cycling of iodine via IO3- reduction, I- oxidation and accumulation and org-I formation.