Dominance of 'Gallionella capsiferriformans' and heavy metal association with Gallionella-like stalks in metal-rich pH 6 mine water discharge.

Heavy metal-contaminated, pH 6 mine water discharge created new streams and iron-rich terraces at a creek bank in a former uranium-mining area near Ronneburg, Germany. The transition from microoxic groundwater with ~5 mm Fe(II) to oxic surface water may provide a suitable habitat for microaerobic iron-oxidizing bacteria (FeOB). In this study, we investigated the potential contribution of these FeOB to iron oxidation and metal retention in this high-metal environment. We (i) identified and quantified FeOB in water and sediment at the outflow, terraces, and creek, (ii) studied the composition of biogenic iron oxides (Gallionella-like twisted stalks) with scanning and transmission electron microscopy (SEM, TEM) as well as confocal laser scanning microscopy (CLSM), and (iii) examined the metal distribution in sediments. Using quantitative PCR, a very high abundance of FeOB was demonstrated at all sites over a 6-month study period. Gallionella spp. clearly dominated the communities, accounting for up to 88% of Bacteria, with a minor contribution of other FeOB such as Sideroxydans spp. and 'Ferrovum myxofaciens'. Classical 16S rRNA gene cloning showed that 96% of the Gallionella-related sequences had ≥ 97% identity to the putatively metal-tolerant 'Gallionella capsiferriformans ES-2', in addition to known stalk formers such as Gallionella ferruginea and Gallionellaceae strain R-1. Twisted stalks from glass slides incubated in water and sediment were composed of the Fe(III) oxyhydroxide ferrihydrite, as well as polysaccharides. SEM and scanning TEM-energy-dispersive X-ray spectroscopy revealed that stalk material contained Cu and Sn, demonstrating the association of heavy metals with biogenic iron oxides and the potential for metal retention by these stalks. Sequential extraction of sediments suggested that Cu (52-61% of total sediment Cu) and other heavy metals were primarily bound to the iron oxide fractions. These results show the importance of 'G. capsiferriformans' and biogenic iron oxides in slightly acidic but highly metal-contaminated freshwater environments.

Two types of morphologically distinct fibers comprising Gallionella ferruginea twisted stalks.

Two morphologically distinct extracellular stalk fibers produced by Gallionella ferruginea were compared by electron microscopy and elemental analysis. The thick- and fine-fiber stalks were different in structure on a micrometer scale and in the site on the mother cell to which they were attached, but on a nanometer scale they were similar in ultrastructure and in the elemental composition of their basic fiber matrix.

Silicon and phosphorus linkage with iron via oxygen in the amorphous matrix of Gallionella ferruginea stalks.

Bacterial species belonging to the genus Gallionella are Fe-oxidizing bacteria that produce uniquely twisted extracellular stalks consisting of iron-oxide-encrusted inorganic/organic fibers in aquatic environments. This paper describes the degree of crystallinity of Gallionella stalks and the chemical linkages of constituent elements in the stalk fibers. Transmission electron microscopy revealed that the matrix of the fiber edge consisted of an assembly of primary particles of approximately 3 nm in diameter. Scanning transmission electron microscopy revealed the rough granular surfaces of the fibers, which reflect the disordered assembly of the primary particles, indicating a high porosity and large specific surface area of the fibers. This may provide the surface with broader reactive properties. X-ray diffractometry, selected-area electron diffraction, and high-resolution transmission electron microscopy together showed that the primary particles had an amorphous structure. Furthermore, energy-dispersive X-ray analysis and Fourier transform infrared spectroscopy detected the bands characteristic of the vibrational modes assigned to O-H, Fe-O-H, P-O-H, Si-O-H, Si-O-Fe, and P-O-Fe bonds in the stalks, suggesting that the minor constituent elements P and Si could affect the degree of crystallinity of the fibers by linking with Fe via O. This knowledge about the mutual associations of these elements provides deeper insights into the unique inorganic/organic hybrid structure of the stalks.

Structural and spatial associations between Fe, O, and C in the network structure of the Leptothrix ochracea sheath surface.

The structural and spatial associations of Fe with O and C in the outer coat fibers of the Leptothrix ochracea sheath were shown to be substantially similar to the stalk fibers of Gallionella ferruginea, i.e., a central C core, probably of bacterial origin, and aquatic Fe interacting with O at the surface of the core.

Nanometer-scale visualization and structural analysis of the inorganic/organic hybrid structure of Gallionella ferruginea twisted stalks.

The so-called Fe/Mn-oxidizing bacteria have long been recognized for their potential to form extracellular iron hydroxide or manganese oxide structures in aquatic environments. Bacterial species belonging to the genus Gallionella, one type of such bacteria, oxidize iron and produce uniquely twisted extracellular stalks consisting of iron oxide-encrusted inorganic/organic fibers. This paper describes the ultrastructure of Gallionella cells and stalks and the visualized structural and spatial localization of constitutive elements within the stalks. Electron microscopy with energy-dispersive X-ray microanalysis showed the export site of the stalk fibers from the cell and the uniform distribution of iron, silicon, and phosphorous in the stalks. Electron energy-loss spectroscopy revealed that the stalk fibers had a central carbon core of bacterial exopolymers and that aquatic iron interacted with oxygen at the surface of the carbon core, resulting in deposition of iron oxides at the surface. This new knowledge of the structural and spatial associations of iron with oxygen and carbon provides deeper insights into the unique inorganic/organic hybrid structure of the stalks.

Biological treatment of Mn(II) and Fe(II) containing groundwater: kinetic considerations and product characterization.

In the present article, the treatment of groundwater containing Mn(II) and Fe(II) has been investigated. The biological oxidation of Mn(II) and Fe(II) in upflow filtration units comprised the applied experimental technique. The oxidation processes were mediated by specific bacteria, namely the Leptothrix ochracea and Gallionella ferruginea, which belong to the general category of manganese and iron oxidizing bacteria. This work was focused on the characterization of the products of biological oxidation and to the examination of the kinetics of Mn(II) removal as compared with Fe(II) removal from groundwaters. The products of biological oxidation were characterized using the spectroscopic techniques XRD, XPS and SEM-EDS and comprised a mixture of biogenic hydrous manganese and iron oxides. The oxidation state of manganese in the precipitates was found to be between 3 and 4. Iron oxides were mainly in the form of amorphous ferrihydrite. The kinetic results indicated that the rates of manganese and iron oxidation were several orders of magnitude greater than the respective for abiotic oxidation. The bacterially mediated oxidation of iron was faster than manganese oxidation, presenting half-lives of reaction 0.9 and 3.98 min, respectively.