General Microbiology - Schlegel, H. 1987

Microorganisms and the Environment
Microorganisms and the Geological History of the Earth

Microorganisms have played a vital role in shaping the Earth's crust. To a large extent, their activity drove the partial Separation of chemical elements and compounds that originally occurred as mixtures within bedrock. Many commercially valuable mineral deposits being mined today owe their existence, entirely or in part, to microbial activity.

Iron Deposition. Major iron ore deposits are known as banded iron formations (BIFs). The precipitation of iron oxides here occurred primarily between 2.8 and 1.6 billion years ago. Prior to that era, iron leaching from seafloor magmatic rocks accumulated in vast quantities as Fe²⁺ ions, alongside other reduced ions (S^2-, Mn²⁺), in the oceans. When cyanobacterial oxygenic Photosynthesis began, S^2- ions were oxidized to SO^2-₄, and Fe²⁺ was oxidized to Fe³⁺. The latter are highly insoluble. Iron oxide precipitation over extensive areas took place wherever iron-rich deep waters met oxygen-containing surface waters. Banded iron formations consist of alternating layers of iron oxide and silica, ranging from 0.2 to 2.0 mm in thickness. This banding is believed to result from the seasonal rhythm of photosynthesis in the basins where the sediments accumulated. It was only after The oxidation of sulfur and iron in seawater was complete that oxygen began to accumulate in the atmosphere, starting about 1.6 billion years ago.

Microorganisms are likewise involved in the mobilization of iron from granitic rocks and its subsequent precipitation. When sulfur in pyrite or marcasite is oxidized to sulfuric acid by the action of Thiobacillus thiooxidans and T. ferrooxidans, iron is solubilized as an Fe (II) salt and then oxidized by T. ferrooxidans into an Fe (III) salt (for details on iron leaching, see Section 11.3).

Upon Water neutralization, trivalent iron precipitates as Fe(OH)₃. It is highly probable that many deposits of exceptionally pure iron oxide were formed through millions of years of microbial leaching. In other locations, organic (humic) acids clearly participate in iron solubilization. Subsequent Biological Oxidation of Fe (II) to Fe (III) can be mediated by iron Bacteria such as Gallionella or Siderocapsa (at neutral pH), resulting in The formation of bog iron and meadow iron ore.

Calcium Carbonate Deposition. In many aquatic environments, calcium is present as Ca(HCO₃)₂ or CaSO₄. Driven by shifts in pH or the removal of CO₂ by photosynthetic organisms, bicarbonate converts into sparingly soluble calcium carbonate and precipitates. Under anaerobic conditions, sulfate is reduced to hydrogen sulfide by sulfate-reducing bacteria, precipitating calcium carbonate in the process:

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Most limestone likely formed when calcium bicarbonate was transported into tropical water bodies and precipitated there as CaCO₃ as elevated temperatures drove off CO₂:

Sulfur Deposition. The formation of commercially viable sulfur deposits is linked to bacterial sulfate reduction. During the anaerobic decomposition of Organic compounds in the presence of sulfate, the latter serves as the preferred hydrogen acceptor. The resulting hydrogen sulfide inhibits any potentially competing Anaerobic Respiration processes. Isotope studies have confirmed that the sulfur deposits in Texas and Louisiana, for instance, are of biogenic origin.

Seawater sulfur consists primarily of two stable isotopes: ³²S (95%) and ³⁴S (4%). During bacterial sulfate reduction—a process limited mainly by the availability of hydrogen Donors—the ³²SO^2-₄ sulfate containing the lighter isotope is more readily taken up and reduced by Cells than the ³⁴S-bearing sulfate. Consequently, the resulting hydrogen sulfide is depleted in ³⁴S compared to seawater sulfate. Subsequent biological or abiotic oxidation of this "light" hydrogen sulfide yields "light" sulfur. The sulfur isotope ratios found in these deposits point directly to a biogenic origin. The isotopic composition of biogenic sulfur differs significantly from that determined for volcanic sulfur (e.g., in Sicily).

Our Structure/133.html">Discussion of biogeochemical transformations could readily be extended to the genesis of other deposits, such as coal, petroleum, natural gas, diatomite, and bauxite. Microorganisms drive these various transformations through core metabolic activities—including oxidation, Fermentation, acid production, reduction, CO₂ assimilation, and the release of volatile products. The ultimate outcomes include mineralization, dissolution, mobilization, and immobilization of various substances. The Study of how microorganisms participate in the formation, alteration, and weathering of rocks falls under the purview of geomicrobiology.