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This paper presents mineralogical, chemical and morphological information on the oxyhydroxides from …


Biology Articles » Geobiology » Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea » Iron oxidizing bacteria

Iron oxidizing bacteria
- Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea

The sample of a low temperature (20–30°C), slightly acidic (pH 6.26) fluid, obtained from an actively venting chimney at Franklin Seamount (Table 1), allows physical and chemical constraints to be placed on the types of iron oxidizing bacteria that should thrive in the vent environment. Fig. 5 shows the redox conditions for common iron bearing compounds together with favorable environments for the growth of various genera of iron oxidizing bacteria. Lundgren and Dean[21] noted that iron-rich substrates such as iron carbonates and sulfides nurture Leptothrix and Gallionella under conditions of neutral pH and medium to low f(O2). The bacterial genus Leptothrix produces filamentous sheaths that can become heavily encrusted with hydrated ferric and manganic oxyhydroxides.

A sheath is defined as a hollow cylindrical structure made up predominantly of encrusted iron and possibly manganese oxyhydroxides that surrounds a chain of cells. It is closely fitting but not in intimate contact with the cells so the bacterium may move out leaving behind a preserved cast. [22] The width, length and rough surface of the described filaments suggest that Leptothrix is the most common genus at the Franklin Seamount vent sites.

The samples from Franklin Seamount contain very little organic carbon (0.18 to 0.94 wt.% Corg, ref. [4]), so it is probable that the organisms had died and decomposed, leaving behind their fossil form. Submersible observations of the chimneys and shipboard examination of the samples found no indication of macro-biological activity except for a few worm casts. [4,5] The genus Gallionella is identified at the vent sites by its spirally twisted stalk structure (Figs. 3d and 4e) formed by the rotation of the individual filaments of its apical cells. [23]

In summary, the morphologies of ferric iron casts suggest that the iron oxidizing bacteria Leptothrix and Gallionella once thrived within the iron oxyhydroxide chimneys at Franklin Seamount. A near neutral pH, low f(O2) and ferrous iron-rich vent fluid combined with a seawater environment rich in organic carbon favored the growth of these genera. Recognition of the casts made by these genera, and specifically the textures of the manganese and iron, is good evidence that microbially mediated deposition of iron is occurring at Franklin Seamount.


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