Role of indigenous microorganisms and organics in the release of iron and trace elements from sediments impacted by iron mine tailings from failed Fundão dam.

After Fundão Dam failure in 2015, most of Gualaxo do Norte River in Doce River Basin in Brazil became silted by iron mining tailings consisting mainly of fine-grained quartz, hematite, and goethite. Previous work pointed to the possibility of reductive dissolution of iron and manganese from tailings, leading to mobilization of iron, manganese and trace elements. Several microorganisms were shown to reduce Fe(III) to Fe(II) and Mn(III, IV) to Mn(II) "in vitro", but their roles in mobilization of Fe and trace elements from freshwater sediments are poorly understood. In this work, bottom sediments and water collected in Gualaxo do Norte River were used to build anoxic microcosms amended with acetate, glucose or yeast extract, in order to access if heterotrophic microorganisms, either fermenters or dissimilatory Fe reducers, could reduce Fe(III) from minerals in the sediments to soluble Fe(II), releasing trace elements. The Fe(II) concentrations were measured over time, and trace elements concentrations were evaluated at the end of the experiment. In addition, minerals and biopolymers in bottom sediments were quantified. Results showed that organic substrates, notably glucose, fuelled microbial reduction of iron minerals and release of Fe(II), Mn, Ba, Al and/or Zn from sediments. In general, higher concentrations of organic substrates elicited mobilization of larger amounts of Fe(II) and trace elements from sediments. The results point to the possibility of mobilization of huge amounts of iron and trace elements from sediments to water if excess biodegradable organic matter is released in rivers affected by iron mine tailings.

Microcystin drives the composition of small-sized bacterioplankton communities from a coastal lagoon.

Cyanobacterial blooms affect biotic interactions in aquatic ecosystems, including those involving heterotrophic bacteria. Ultra-small microbial communities are found in both surface water and groundwater and include diverse heterotrophic bacteria. Although the taxonomic composition of these communities has been described in some environments, the involvement of these small cells in the fate of environmentally relevant molecules has not been investigated. Here, we aimed to test if small-sized microbial fractions from a polluted urban lagoon were able to degrade the cyanotoxin microcystin (MC). We obtained cells after filtration through 0.45 as well as 0.22 µm membranes and characterized the morphology and taxonomic composition of bacteria before and after incubation with and without microcystin-LR (MC-LR). Communities from different size fractions (< 0.22 and < 0.45 µm) were able to remove the dissolved MC-LR. The originally small-sized cells grew during incubation, as shown by transmission electron microscopy, and changed in both cell size and morphology. The analysis of 16S rDNA sequences revealed that communities originated from < 0.22 and < 0.45 µm fractions diverged in taxonomic composition although they shared certain bacterial taxa. The presence of MC-LR shifted the structure of < 0.45 µm communities in comparison to those maintained without toxin. Actinobacteria was initially dominant and after incubation with MC-LR Proteobacteria predominated. There was a clear enhancement of taxa already known to degrade MC-LR such as Methylophilaceae. Small-sized bacteria constitute a diverse and underestimated fraction of microbial communities, which participate in the dynamics of MC-LR in natural environments.

Swimming behavior of the multicellular magnetotactic prokaryote 'Candidatus Magnetoglobus multicellularis' near solid boundaries and natural magnetic grains.

The magnetotactic yet uncultured species 'Candidatus Magnetoglobus multicellularis' is a spherical, multicellular ensemble of bacterial cells able to align along magnetic field lines while swimming propelled by flagella. Magnetotaxis is due to intracytoplasmic, membrane-bound magnetic crystals called magnetosomes. The net magnetic moment of magnetosomes interacts with local magnetic fields, imparting the whole microorganism a torque. Previous works investigated 'Ca. M. multicellularis' behavior when free swimming in water; however, they occur in sediments where bumping into solid particles must be routine. In this work, we investigate the swimming trajectories of 'Ca. M. multicellularis' close to solid boundaries using video microscopy. We applied magnetic fields 0.25-8.0 mT parallel to the optical axis of a light microscope, such that microorganisms were driven upwards towards a coverslip. Because their swimming trajectories approach cylindrical helixes, circular profiles would be expected. Nevertheless, at fields 0.25-1.1 mT, most trajectory projections were roughly sinusoidal, and net movements were approximately perpendicular to applied magnetic fields. Closed loops appeared in some trajectory projections at 1.1 mT, which could indicate a transition to the loopy profiles observed at magnetic fields ≥ 2.15 mT. The behavior of 'Ca. M. multicellularis' near natural magnetic grains showed that they were temporarily trapped by the particle's magnetic field but could reverse the direction of movement to flee away. Our results show that interactions of 'Ca. M. multicellularis with solid boundaries and magnetic grains are complex and possibly involve mechano-taxis.

Effect of applied magnetic fields on motility and magnetotaxis in the uncultured magnetotactic multicellular prokaryote 'Candidatus Magnetoglobus multicellularis'.

Magnetotactic bacteria are found in the chemocline of aquatic environments worldwide. They produce nanoparticles of magnetic minerals arranged in chains in the cytoplasm, which enable these microorganisms to align to magnetic fields while swimming propelled by flagella. Magnetotactic bacteria are diverse phylogenetically and morphologically, including cocci, rods, vibria, spirilla and also multicellular forms, known as magnetotactic multicellular prokaryotes (MMPs). We used video-microscopy to study the motility of the uncultured MMP 'Candidatus Magnetoglobus multicellularis' under applied magnetic fields ranging from 0.9 to 32 Oersted (Oe). The bidimensional projections of the tridimensional trajectories where interpreted as plane projections of cylindrical helices and fitted as sinusoidal curves. The results showed that 'Ca. M. multicellularis' do not orient efficiently to low magnetic fields, reaching an efficiency of about 0.65 at 0.9-1.5 Oe, which are four to six times the local magnetic field. Good efficiency (0.95) is accomplished for magnetic fields ≥10 Oe. For comparison, unicellular magnetotactic microorganisms reach such efficiency at the local magnetic field. Considering that the magnetic moment of 'Ca. M. multicellularis' is sufficient for efficient alignment at the Earth's magnetic field, we suggest that misalignments are due to flagella movements, which could be driven by photo-, chemo- and/or other types of taxis.

Ferromagnetic resonance of intact cells and isolated crystals from cultured and uncultured magnetite-producing magnetotactic bacteria.

Most magnetotactic bacteria (MB) produce stable, single-domain magnetite nanocrystals with species-specific size, shape and chain arrangement. In addition, most crystals are elongated along the [111] direction, which is the easy axis of magnetization in magnetite, chemically pure and structurally perfect. These special characteristics allow magnetite crystal chains from MB to be recognized in environmental samples including old sedimentary rocks. Ferromagnetic resonance (FMR) has been proposed as a powerful and practical tool for screening large numbers of samples possibly containing magnetofossils. Indeed, several studies were recently published on FMR of cultured MB, mainly Magnetospirillum gryphiswaldense. In this work, we examined both uncultured magnetotactic cocci and the cultured MB M. gryphiswaldense using transmission electron microscopy (TEM) and FMR from 10 K to room temperature (RT). The TEM data supported the FMR spectral characteristics of our samples. The FMR spectra of both bacteria showed the intrinsic characteristics of magnetite produced by MB, such as extended absorption at the low field region of the spectra and a Verwey transition around 100 K. As previously observed, the spectra of M. gryphiswaldense isolated crystals were more symmetrical than the spectra obtained from whole cells, reflecting the loss of chain arrangement due to the small size and symmetrical shape of the crystals. However, the FMR spectra of magnetic crystals isolated from magnetotactic cocci were very similar to the FMR spectra of whole cells, because the chain arrangement was maintained due to the large size and prismatic shape of the crystals. Our data support the use of FMR spectra to detect magnetotactic bacteria and magnetofossils in samples of present and past environments. Furthermore, the spectra suggest the use of the temperature transition of spectral peak-to-peak intensity to obtain the Verwey temperature for these systems.

Elemental composition of biomineralized amorphous mineral granules isolated from ants: correlation with ingested mineral particles from the soil.

Amorphous mineral granules are formed by concentric mineral layers containing polyphosphate, pyrophosphate and/or orthophosphate and several metallic cations such as Mg(2+), Ca(2+), K(+), Mn(2+), Fe(3+), Cu(2+), and Zn(2+). In this work, we analyzed amorphous mineral granules isolated from the ant species Camponotus abdominalis, Camponotus sp., Acromyrmex subterraneus and Pachycondyla marginata by energy-dispersive X-ray analysis. The elemental composition of the granules was compared to that of mineral particles, probably soil particles, to access the influence of the environment and of specific characteristics of each ant species in the elemental composition of the amorphous mineral granules. Both the granules and mineral particles presented Mg, Ca, Fe, and Zn in the four species. Additionally, Al tended to be present in both (or none) of the two types of material in a given ant species, suggesting that the aluminum found in the amorphous mineral granules could be derived from ingested soil particles. On the other hand, Sr was found in the amorphous mineral granules of some of the studied ant species, but not in the mineral particles. The fact that 3/4 of the elements found in the granules were found also in the mineral particles suggests that the mineral composition of the soil plays a fundamental role in the accumulation of some elements in the amorphous mineral granules of ants. These results suggest a major role of soil particles as a source of micronutrients for the four ant species.

Swimming behaviour of the multicellular magnetotactic prokaryote 'Candidatus Magnetoglobus multicellularis' under applied magnetic fields and ultraviolet light.

Magnetotactic bacteria move by rotating their flagella and concomitantly are aligned to magnetic fields because they present magnetosomes, which are intracellular organelles composed by membrane-bound magnetic crystals. This results in magnetotaxis, which is swimming along magnetic field lines. Magnetotactic bacteria are morphologically diverse, including cocci, rods, spirilla and multicellular forms known as magnetotactic multicellular prokaryotes (MMPs). 'Candidatus Magnetoglobus multicellularis' is presently the best known MMP. Here we describe the helical trajectories performed by these microorganisms as they swim forward, as well as their response to UV light. We measured the radius of the trajectory, time period and translational velocity (velocity along the helix axis), which enabled the calculation of other trajectory parameters such as pitch, tangential velocity (velocity along the helix path), angular frequency, and theta angle (the angle between the helix path and the helix axis). The data revealed that 'Ca. M. multicellularis' swims along elongated helical trajectories with diameters approaching the diameter of the microorganism. In addition, we observed that 'Ca. M. multicellularis' responds to UV laser pulses by swimming backwards, returning to forward swimming several seconds after the UV laser pulse. UV light from a fluorescence microscope showed a similar effect. Thus, phototaxis is used in addition to magnetotaxis in this microorganism.

Magnetosome chain superstructure in uncultured magnetotactic bacteria.

Magnetotactic bacteria produce magnetosomes, which are magnetic particles enveloped by biological membranes, in a highly controlled mineralization process. Magnetosomes are used to navigate in magnetic fields by a phenomenon called magnetotaxis. Two levels of organization and control are recognized in magnetosomes. First, magnetotactic bacteria create a spatially distinct environment within vesicles defined by their membranes. In the vesicles, the bacteria control the size, composition and purity of the mineral content of the magnetic particles. Unique crystal morphologies are produced in magnetosomes as a consequence of this bacterial control. Second, magnetotactic bacteria organize the magnetosomes in chains within the cell body. It has been shown in a particular case that the chains are positioned within the cell body in specific locations defined by filamentous cytoskeleton elements. Here, we describe an additional level of organization of the magnetosome chains in uncultured magnetotactic cocci found in marine and freshwater sediments. Electron microscopy analysis of the magnetosome chains using a goniometer showed that the magnetic crystals in both types of bacteria are not oriented at random along the crystal chain. Instead, the magnetosomes have specific orientations relative to the other magnetosomes in the chain. Each crystal is rotated either 60°, 180° or 300° relative to their neighbors along the chain axis, causing the overlapping of the (1 1 1) and [Formula in text] capping faces of neighboring crystals. We suggest that genetic determinants that are not present or active in bacteria with magnetosomes randomly rotated within a chain must be present in bacteria that organize magnetosomes so precisely. This particular organization may also be used as an indicative biosignature of magnetosomes in the study of magnetofossils in the cases where this symmetry is observed.

Manganese in biogenic magnetite crystals from magnetotactic bacteria.

Magnetotactic bacteria produce either magnetite (Fe(3)O(4)) or greigite (Fe(3)S(4)) crystals in cytoplasmic organelles called magnetosomes. Whereas greigite magnetosomes can contain up to 10 atom% copper, magnetite produced by magnetotactic bacteria was considered chemically pure for a long time and this characteristic was used to distinguish between biogenic and abiogenic crystals. Recently, it was shown that magnetosomes containing cobalt could be produced by three strains of Magnetospirillum. Here we show that magnetite crystals produced by uncultured magnetotactic bacteria can incorporate manganese up to 2.8 atom% of the total metal content (Fe+Mn) when manganese chloride is added to microcosms. Thus, chemical purity can no longer be taken as a strict prerequisite to consider magnetite crystals to be of biogenic origin.

Greigite magnetosome membrane ultrastructure in 'Candidatus Magnetoglobus multicellularis'.

The ultrastructure of the greigite magnetosome membrane in the multicellular magnetotactic bacteria 'Candidatus Magnetoglobus multicellularis' was studied. Each cell contains 80 membrane-enclosed iron-sulfide magnetosomes. Cytochemistry methods showed that the magnetosomes are enveloped by a structure whose staining pattern and dimensions are similar to those of the cytoplasmic membrane, indicating that the magnetosome membrane likely originates from the cytoplasmic membrane. Freeze-fracture showed intramembrane particles in the vesicles surrounding each magnetosome. Observations of cell membrane invaginations, the trilaminar membrane structure of immature magnetosomes, and empty vesicles together suggested that greigite magnetosome formation begins by invagination of the cell membrane, as has been proposed for magnetite magnetosomes.

Ultrastructure and cytochemistry of lipid granules in the many-celled magnetotactic prokaryote, 'Candidatus Magnetoglobus multicellularis'.

Conspicuous cytoplasmic granules are reported in a magnetotactic multicellular prokaryote named 'Candidatus Magnetoglobus multicellularis'. Unfortunately, this microorganism, which consists of an assembly of gram-negative bacterial cells, cannot yet be cultivated, limiting the biochemical analysis of the granules and preventing in vitro studies with starvation/excess of nutrients. In this scenario, light and electron microscopy techniques were used to partially address the nature of the granules. Besides magnetosomes, three types of inclusions were observed: small (mean diameter=124 nm) polyhydroxyalkanoate-like (PHA) granules, large (diameters ranging from 0.11 to 2.5 microm) non-PHA lipid granules, and rare phosphorus-rich granules, which probably correspond to polyphosphate bodies. The PHA granules were rounded in projection, non-reactive with OsO(4), and suffered the typical plastic deformation of PHAs after freeze fracturing. The nature of the large granules, consisting of round globular structures (mean diameter=0.76 microm), was classified as non-PHA based on the following data: (a) multilayered structure in freeze-fracture electron microscopy, typical of non-PHA lipids; (b) Nile blue fluorescence imaging detected non-PHA lipids; (c) imidazole buffered osmium tetroxide and ruthenium red cytochemistry stained the globules, which appeared as electron-dense granules instead of electron lucent as PHAs do. Most likely, 'Candidatus Magnetoglobus multicellularis' stores carbon mainly as unusual lipid granules, together with smaller amounts of PHAs.

Greigite magnetosome membrane ultrastructure in «Candidatus Magnetoglobus multicellularis»

The ultrastructure of the greigite magnetosome membrane in the multicellular magnetotactic bacteria «Candidatus Magnetoglobus multicellularis» was studied. Each cell contains 80 membrane-enclosed iron-sulfide magnetosomes. Cytochemistry methods showed that the magnetosomes are enveloped by a structure whose staining pattern and dimensions are similar to those of the cytoplasmic membrane, indicating that the magnetosome membrane likely originates from the cytoplasmic membrane. Freeze-fracture showed intramembrane particles in the vesicles surrounding each magnetosome. Observations of cell membrane invaginations, the trilaminar membrane structure of immature magnetosomes, and empty vesicles together suggested that greigite magnetosome formation begins by invagination of the cell membrane, as has been proposed for magnetite magnetosomes (AU)

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Magnetic optimization in a multicellular magnetotactic organism.

Unicellular magnetotactic prokaryotes, which typically carry a natural remanent magnetic moment equal to the saturation magnetic moment, are the prime example of magnetically optimized organisms. We here report magnetic measurements on a multicellular magnetotactic prokaryote (MMP) consisting of 17 undifferentiated cells (mean from 148 MMPs) with chains of ferrimagnetic particles in each cell. To test if the chain polarities of each cell contribute coherently to the total magnetic moment of the MMP, we used a highly sensitive magnetization measurement technique (1 fAm(2)) that enabled us to determine the degree of magnetic optimization (DMO) of individual MMPs in vivo. We obtained DMO values consistently above 80%. Numerical modeling shows that the probability of reaching a DMO > 80% would be as low as 0.017 for 17 randomly oriented magnetic dipoles. We simulated different scenarios to test whether high DMOs are attainable by aggregation or self-organization of individual magnetic cells. None of the scenarios investigated is likely to yield consistently high DMOs in each generation of MMPs. The observed high DMO values require strong Darwinian selection and a sophisticated reproduction mechanism. We suggest a multicellular life cycle as the most plausible scenario for transmitting the high DMO from one generation to the next.

Intracellular inclusions of uncultured magnetotactic bacteria.

Magnetotactic bacteria produce magnetic crystals in organelles called magnetosomes. The bacterial cells may also have phosphorus-containing granules, sulfur globules, or polyhydroxyalkanoate inclusions. In the present study, the ultrastructure and elemental composition of intracellular inclusions from uncultured magnetotactic bacteria collected in a marine environment are described. Magnetosomes contained mainly defect-free, single magnetite crystals with prismatic morphologies. Two types of phosphorus-containing granules were found in magnetotactic cocci. The most common consisted of phosphorus-rich granules containing P, O, and Mg; and sometimes also C, Na, Al, K, Ca, Mn, Fe, Zn, and small amounts of S and Cl were also found. In phosphorus-sulfur-iron granules, P, O, S, Na, Mg, Ca, Fe, and frequently Cl, K, and Zn, were detected. Most cells had two phosphorus-rich granules, which were very similar in elemental composition. In rod-shaped bacteria, these granules were positioned at a specific location in the cell, suggesting a high level of intracellular organization. Polyhydroxyalkanoate granules and sulfur globules were less commonly seen in the cells and had no fixed number or specific location. The presence and composition of these intracellular structures provide clues regarding the physiology of the bacteria that harbor them and the characteristics of the microenvironments where they thrive.

Intracellular inclusions of uncultured magnetotactic bacteria / Inclusiones intracelulares de bacterias magnetotáticas no cultivadas

Magnetotactic bacteria produce magnetic crystals in organelles called magnetosomes. The bacterial cells may also have phosphorus-containing granules, sulfur globules, or polyhydroxyalkanoate inclusions. In the present study, the ultrastructure and elemental composition of intracellular inclusions from uncultured magnetotactic bacteria collected in a marine environment are described. Magnetosomes contained mainly defect-free, single magnetite crystals with prismatic morphologies. Two types of phosphorus-containing granules were found in magnetotactic cocci. The most common consisted of phosphorus-rich granules containing P, O, and Mg; and sometimes also C, Na, Al, K, Ca, Mn, Fe, Zn, and small amounts of S and Cl were also found. In phosphorus-sulfur-iron granules, P, O, S, Na, Mg, Ca, Fe, and frequently Cl, K, and Zn, were detected. Most cells had two phosphorus-rich granules, which were very similar in elemental composition. In rod-shaped bacteria, these granules were positioned at a specific location in the cell, suggesting a high level of intracellular organization. Polyhydroxyalkanoate granules and sulfur globules were less commonly seen in the cells and had no fixed number or specific location. The presence and composition of these intracellular structures provide clues regarding the physiology of the bacteria that harbor them and the characteristics of the microenvironments where they thrive (AU)

Las bacterias magnetotácticas producen cristales magnéticos en orgánulos llamados magnetosomas. Además, pueden contener gránulos de fósforo, glóbulos de azufre o inclusiones de polihidroxialcanoatos. En este estudio se describe la ultraestructura y la composición elemental de las inclusiones intracelulares de bacterias magnetotácticas no cultivables extraídas de un medio marino. Los magnetosomas contenían principalmente cristales de magnetita individuales de morfología prismática sin defectos. En los cocos magnetotácticos se encontraron dos tipos de gránulos que contenían fósforo. Los más frecuentes fueron los gránulos ricos en fósforo que contenían P, O, Mg y, a veces también, C, Na, Al, K, Ca, Mn, Fe, Zn y pequeñas cantidades de S y Cl. En los gránulos de fósforo-azufre-hierro se detectó P, O, S, Na, Mg, Ca, Fe, y con frecuencia Cl, K y Zn. La mayoría de las células tenían dos gránulos ricos en fósforo, cuya composición elemental era muy parecida. En las bacterias de forma bacilar, estos gránulos estaban situados en determinados lugares de la célula, sugiriendo un alto nivel de organización intracelular. Los gránulos de polihidroxialcanoatos y los glóbulos de azufre eran menos frecuentes y no mostraban ninguna localización especial dentro de la célula ni tenían un número fijo. La presencia y composición de estas estructuras intracelulares proporciona pistas sobre la fisiología de la bacteria que las hospeda y sobre las características de los microambientes donde se desarrollan (AU)

Multicellular life cycle of magnetotactic prokaryotes.

Most multicellular organisms, prokaryotes as well as animals, plants, and algae have a unicellular stage in their life cycle. Here, we describe an uncultured prokaryotic magnetotactic multicellular organism that reproduces by binary fission. It is multicellular in all the stages of its life cycle, and during most of the life cycle the cells organize into a hollow sphere formed by a functionally coordinated and polarized single-cell layer that grows by increasing the cell size. Subsequently, all the cells divide synchronously; the organism becomes elliptical, and separates into two equal spheres with a torsional movement in the equatorial plane. Unicellular bacteria similar to the cells that compose these organisms have not been found. Molecular biology analysis showed that all the organisms studied belong to a single genetic population phylogenetically related to many-celled magnetotactic prokaryotes in the delta sub-group of the proteobacteria. This appears to be the first report of a multicellular prokaryotic organism that proliferates by dividing into two equal multicellular organisms each similar to the parent one.

Zinc detoxification by a cyanobacterium from a metal contaminated bay in Brazil

O presente trabalho descreve o acúmulo de zinco em grânulos de polifosfato da cianobactéria Synechocystis aquatilis NPBS-1 como mecanismo de detoxificação de metais pesados. O microrganismo foi isolado de uma baía contaminada com metais, a Baía de Sepetiba, próxima à cidade do Rio de Janeiro. As células foram cultivadas em 25 µM de cloreto de zinco e preparadas para microscopia eletrônica e análise de dispersão de energia de raios-X. Algumas mudanças morfológicas ocorreram após a exposição ao zinco. A principal modificação foi o aumento no número de grânulos de glicogênio, como provável mecanismo fisiológico de adaptação. Os grânulos de polifosfato continham fósforo, enxofre, cálcio, ferro e zinco. O acúmulo de zinco nesses grânulos parece ser um mecanismo efetivo de detoxificação do metal para sobrevivência da cianobactéria na baía. Devido à sua não-especificidade na captação de cátions metálicos, os grânulos podem ser igualmente efetivos em acumular outros eventuais metais em excesso, além do zinco.

Cell organization and ultrastructure of a magnetotactic multicellular organism.

Magnetotactic multicellular aggregates and many-celled magnetotactic prokaryotes have been described as spherical organisms composed of several Gram-negative bacteria capable to align themselves along magnetic fields and swim as a unit. Here we describe a similar organism collected in a large hypersaline lagoon in Brazil. Ultrathin sections and freeze fracture replicas showed that the cells are arranged side by side and face both the external environment and an internal acellular compartment in the center of the organism. This compartment contains a belt of filaments linking the cells, and numerous membrane vesicles. The shape of the cells approaches a pyramid, with the apex pointing to the internal compartment, and the basis facing the external environment. The contact region of two cells is flat and represents the pyramid faces, while the contacts of three or more cells contain cell projections and represent the edges. Freeze-fracture replicas showed a high concentration of intramembrane particles on the edges and also in the region of the outer membrane that faces the external environment. Dark field optical microscopy showed that the whole organism performs a coordinated movement with either straight or helicoidal trajectories. We conclude that the organisms described in this work are, in fact, highly organized prokaryotic multicellular organisms.

Envelope ultrastructure of uncultured naturally occurring magnetotactic cocci.

Magnetotactic bacteria are microorganisms that respond to magnetic fields. We studied the surface ultrastructure of uncultured magnetotactic cocci collected from a marine environment by transmission electron microscopy using freeze-fracture and freeze-etching. All bacteria revealed a Gram-negative cell wall. Many bacteria possessed extensive capsular material and a S-layer formed by particles arranged with hexagonal symmetry. No indication of a metal precipitation on the surface of these microorganisms was observed. Numerous membrane vesicles were observed on the surface of the bacteria. Flagella were organized in bundles originated in a depression on the surface of the cells. Occasionally, a close association of the flagella with the magnetosomes that remained attached to the replica was observed. Capsules and S-layers are common structures in magnetotactic cocci from natural sediments and may be involved in inhibition of metal precipitation on the cell surface or indirectly influence magnetotaxis.