Iron Cycle

The production of H2S is environmentally incompatible with dissolved iron because of the formation of sulfides. Thus in terrestrial wetlands, where sulfate is limiting, iron cycle develops. Bacterial Fe-cycle takes place now in swamps. It includes oxidation of Fe2+-bicarbonate under O2-limited conditions with formation of Fe(OH)3 ferrihydrite. Energy of oxidation might be used for chemosynthesis by Gallionella with precipitation of Fe(ni) on the slimy stalks. Precipitation of Fe(iii) on slimy structures is well known for the number of so-called 'iron bacteria', among which Leptothrix ochracea is best known for large deposits of ochre in slow-flowing water. Historically, that was the first example of geological activity of microorganisms described by Ehrenberg. Ochre-forming deposits were used as a 'swamp-ore' in the beginning of the Iron Age. However, two processes should be distinguished: chemosynthetic oxidation of iron and precipitation of iron hydroxides on mucous polysac-charides. Both processes are geologically significant. Ferrihydrite is readily reduced by iron-reducing bacteria, which use H2, acetate, and a number of other Corg-compounds as electron donors. There are two possible end products: siderite FeCO3 is formed in excess of organic matter and magnetite Fe3O4 under more restricted conditions. Iron-reducing bacteria substitute nitrate reducers in moderately reductive habitats. There are also thermophilic iron reducers. For formation of ferrihydrite in anoxic environment, there are two possible pathways, both phototrophic: one possibility is oxidation of Fe2+ by cyanobacteria but it is unclear if it is direct or indirect and caused by O2 production; and the other is definitely direct and is performed by nonsulfur purple bacteria such as Rhodomicrobium. Oxidation of Fe2+ by anoxygenic photo-trophic nonsulfur bacteria was described only recently. Product of oxidation in the light is ferrihydrite. This process closes the iron cycle in anoxic environment. Large deposits of layered silicified iron oxides composed of alternating layers of hematite and magnetite known as banded-iron formations (BIFs) were formed during the Early Proterozoic 1.8 billion years ago. Their origin remains unclarified. Fe is of hydrothermal origin. Total amount of iron oxides contains about 40% of O2 evolved corresponding to Corg of kerogen. Iron migrated in the ancient ocean most probably as bicarbonate. Period of BIF is clearly incompatible with sulfate reduction.

Oxidation of sulfides, first of all pyrite, involves both cycles of iron and sulfur. Oxidation involves two functions. At low pH Fe2+ is stable in the air. Oxidation of sulfide produces sulfuric acid with a drop to pH <2. Some pyrite-oxidizing chemosynthetic bacteria such as Acidothiobacillus ferrooxidans use energy of both sulfur and iron oxidation. However in nature, these two functions are often divided between iron-oxidizing Leptospirillum and sulfur-oxidizing Acidothiobacteria working in concert. There are also other examples ofthese bacteria, especially thermophilic, which are most important in bacterial hydrometallurgy because they are able to dissolve various sulfides, and copper, gold, and other metals that also come into solution.

In addition to the cycles of major elements used by chemosynthetic bacteria, it is worth mentioning cycles of arsenic, manganese, and selenium, where both oxidative and reductive pathways operate. The general rule is, che-mosynthetic microbes use oxido-reductive reaction and develop in the thermodynamic field of stability of the product of this reaction. Energy generated in the reaction must be sufficient to support the formation of ATP.

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