Fe (II) and Mn (II) oxidation

Found in quiet water, edge of lakes

Orange, red surface film or floc-like, slimy deposits in slow moving water or pond. This color indicates the presence of oxidized iron.

Genera that exhibit the property: Chemollithoautotrophic iron oxidation -Thiobacillus and Leptothrix (Beta Proteobacter) or by photoferrotrophy Rhodomicrobium -alpha proteobacter, Chlorobium, Chloroflexus

Chemollithoautrophy:
Thiobacillus also involved in sulfide respriation (see box below)

Oxidation of ferrous iron occurs anaerobically with nitrate (pH 7)

Photoferrotrophy:
anoxygenic phototrophic bacteria

Authors for literature search: Konhauseretal, Hiesing et al, 1998, 1999

Leptothrix is a well know genus that oxidizes FeII, (These are not photosynthetic. Many of the other Fe oxidzers are photosynthetic.) They produce sheaths, and are motile by way of flagella. Cells are straight rods, Gram-negative and motile by flagella. Growth is filamentous under natural conditions, owing to the formation of tubular sheaths that surround single and linear chains of cells. Sheaths are produced by excretion of fibrillar polymeric substances that are crosslinked to form a mesh-like fabric closely fitting to the cells. In contrast to slimes or capsules, this matrix is not in intimate contact with the cells.A linear arrangement of single cells within a tubular sheath enables bacteria to form filaments, without actual enlargement of cell size. It has been shown that filamentous growth is an effective strategy of bacteria to exceed the size limit of particles edible by protozoa, thereby allowing them to escape from grazing (Sommaruga and Psenner, 1995). In addition, sheaths provide cells with physical protection from infection by bacteriophages (Winston and Thompson, 1979), bacterial predators (Venosa, 1975), and bacterivorous metazoa. Obviously tough sheaths impregnated with ferric oxides cannot be consumed by metazoa and are not penetrated by bdellovibrios or phages.At circumneutral pH, biological oxidation of iron is difficult to distinguish from the chemical oxidation by oxygen. Only with the identification of an iron-oxidizing protein with a molecular weight of 150 kDa in spent culture medium of the sheathless strain Leptothrix discophora SS-1, the capability of biological iron-oxidation in Leptothrix species could be clearly demonstrated (Corstjens et al., 1992).

Websites relevant to Fe oxidation:

http://www.splammo.net/bact102/102ironbact.html

http://pubs.usgs.gov/publications/text/Norriemicrobes.html

http://www.riversalive.org/iron_bacteria.htm

Thiobacillus can use ferrous iron from the oxidation of reduced sulfur compounds if provided with a terminal electron acceptor, but the conditions required (e.g., low pH) may vary somewhat.

Certain strains of purple bacteria use ferrous iron as the sole electron donor for photosynthesis. Two different types of purple bacteria, represented by strains L7 and SW2, were isolated which oxidized colorless ferrous iron under anoxic conditions in the light to brown ferric iron. Strain L7 had rod-shaped, nonmotile cells (1.3 by 2 to 3 microns) which frequently formed gas vesicles. In addition to ferrous iron, strain L7 used H2 + CO2, acetate, pyruvate, and glucose as substrate for phototrophic growth. Strain SW2 had small rod-shaped, nonmotile cells (0.5 by 1 to 1.5 microns). Besides ferrous iron, strain SW2 utilized H2 + CO2, monocarboxylic acids, glucose, and fructose. Neither strain utilized free sulfide; however, both strains grew on black ferrous sulfide (FeS) which was converted to ferric iron and sulfate. Strains L7 and SW2 grown photoheterotrophically without ferrous iron were purple to brownish red and yellowish brown, respectively; absorption spectra revealed peaks characteristic of bacteriochlorophyll a. The closest phototrophic relatives of strains L7 and SW2 so far examined on the basis of 16S rRNA sequences were species of the genera Chromatium (gamma subclass of proteobacteria) and Rhodobacter (alpha subclass), respectively. In mineral medium, the new isolates formed 7.6 g of cell dry mass per mol of Fe(II) oxidized, which is in good agreement with a photoautotrophic utilization of ferrous iron as electron donor for CO2 fixation. Dependence of ferrous iron oxidation on light and CO2 was also demonstrated in dense cell suspensions. In media containing both ferrous iron and an organic substrate (e.g., acetate, glucose), strain L7 utilized ferrous iron and the organic compound simultaneously; in contrast, strain SW2 started to oxidize ferrous iron only after consumption of the organic electron donor. Ferrous iron oxidation by anoxygenic phototrophs is understandable in terms of energetics. In contrast to the Fe3+/Fe2+ pair (E0 = +0.77 V) existing in acidic solutions, the relevant redox pair at pH 7 in bicarbonate-containing environments, Fe(OH)3 + HCO3-/FeCO3, has an E0' of +0.2 V. Ferrous iron at pH 7 can therefore donate electrons to the photosystem of anoxygenic phototrophs, which in purple bacteria has a midpoint potential around +0.45 V. The existence of ferrous iron-oxidizing anoxygenic phototrophs may offer an explanation for the deposition of early banded-iron formations in an assumed anoxic biosphere in Archean times