Iron oxidizing microorganisms

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The term iron-oxidizing microorganisms includes bacteria and archaea , which gain energy through the oxidation of bivalent iron (Fe (II)). Bacteria with such an energy metabolism are often referred to as iron bacteria for short , but this group is not a phylogenetically closely related unit. Oxygen is the predominant oxidizing agent for iron oxidation . In the absence of oxygen ( anoxic conditions), nitrate or perchlorate can also be used by some species . Some anoxygenic phototrophic bacteria oxidize Fe (II) by using it as a reducing agent in building metabolism, using light as an energy source.

Aerobic iron oxidizers

Environment conditions

The oxidation of divalent iron (Fe (II)) to trivalent iron (Fe (III)) depends to a large extent on the pH value and the oxygen concentration. Under neutral pH conditions (pH 7) and atmospheric oxygen concentration, abiotic oxidation takes place so quickly, without the involvement of microorganisms, that divalent iron is only present for a few minutes (the half-life is about 6-7 minutes). Under these conditions, the reaction speed is so high that microorganisms cannot compete with it. Iron-oxidizing microorganisms therefore only occur under environmental conditions in which the reaction rate of the abiotic Fe (II) oxidation is reduced, so that it is possible for them to gain energy through Fe (II) oxidation. This is the case when the pH, oxygen concentration, or both are low.

Acidophilic iron oxidizers

Bacteria and archaea occur as acidophilic Fe (II) oxidizers. All representatives of this group carry out an Fe (II) oxidation under acidic conditions (<pH 4). Under these conditions, the abiotic oxidation of iron with oxygen only plays a subordinate role. The currently best described organisms from the group of acidophilic iron oxidizers are the bacterial species Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans . In addition to these two, there are other acidophilic Fe (II) oxidizers, both bacteria and archaea. While Acidithiobacillus and Leptospirillum species are often found in acid mine waste water ( Río Tinto , acid mine water ), species of the Archaea genus Sulfolobus prefer hot and acidic springs. The temperature of these springs ( solfataras ) often reaches boiling point. In the acidophilic Fe (II) oxidizers, the energy for the formation of ATP and NADPH is obtained exclusively from the oxidation of Fe (II) with oxygen. At pH 2, 33 kJ mol −1 can be obtained from this reaction . This amount of energy is only insignificantly higher than the amount of energy of 31.8 kJ mol −1 , which is required for the formation of 1  mol of ATP. Sulfolobus acidocaldarius is used in industry for high-temperature leaching of copper and iron ores.

Neutrophil iron oxidizers

Neutrophil iron oxidizers prefer environments with a reduced oxygen concentration (<1 mg per liter) and a redox potential between 200 and 300 mV at a medium pH value. The bacterial genera Gallionella and Leptothrix belong to this group . One problem with iron oxidation at medium pH values ​​is that under these conditions Fe (III) precipitates in the form of solid compounds that encrust the microorganisms and thus hinder their exchange of substances with the environment. Both of the named bacteria avoid this by depositing the oxidation product in a certain form: Gallionella in the form of a band directed away from the cell, Leptothrix on a tube made of organic material in which the cells are located and in which they can move freely.

Since the products of bacterial iron oxidation are almost insoluble in water at medium pH values ​​and are yellow-brown ( ocher ) to red-brown in color, they are very noticeable. Neutrophil iron oxidizers were therefore investigated and described early on. An early summary description by N. Cholodny was published in 1926.

Anaerobic iron oxidizers

Anaerobic, non-phototrophic iron oxidizers

While the aerobic and phototrophic Fe (II) oxidizers are almost exclusively autotrophic organisms, autotrophic and heterotrophic organisms occur in the anaerobic non-phototrophic Fe (II) oxidizers.

This reaction and the organisms responsible for it have so far been demonstrated in a wide range of different ecosystems such as marine sediments , brackish water lagoons, deep sea sediments or rivers . This range of ecosystems and the great phylogenetic variability of the microorganisms involved allow the conclusion that the Fe (II) oxidation coupled to nitrate reduction plays an important role in the iron and nitrogen cycle on a global level.

Anaerobic, phototrophic iron oxidizers

With the discovery of phototrophic Fe (II) -oxidizing microorganisms, it was possible for the first time to detect biological Fe (II) oxidation in oxygen-free ecosystems. This discovery shed new light on the development of the earth's history, especially on the formation conditions of iron band ore in geological time. Until then, there was a consensus in geology that molecular oxygen for the oxidation of Fe (II) had to be available for the formation of iron band ores, so the cyanobacteria responsible for this had to have already existed. without the need for molecular oxygen to be present. This raised well-founded doubts as to whether the first appearance of ribbon iron ores is a reliable signal for the existence of molecular oxygen and the presence of oxygen phototrophic organisms such as cyanobacteria. So far, Chlorobium ferrooxidans , Rhodovulum robiginosum , Rhodomicrobium vannielii , Thiodictyon sp., Rhodopseudomonas palustris and Rhodovulum spp. Have been derived from freshwater and marine habitats . isolated and described in more detail. Except for Rhodomicrobium vannielii , all known species can completely oxidize Fe (II) to Fe (III) with light. In Rhodomicrobium vannielii , on the other hand, the cells become encrusted with Fe (III), which prevents any further metabolic activity. How encrustation is avoided in the other species is not known. It is assumed that here small soluble compounds dissolve the Fe (III) crust that forms around the cells. However, this thesis has not yet been confirmed.

The Fe (III) formed in the phototrophic Fe (II) oxidation forms a weakly crystalline mineral, which over time changes into the more crystalline minerals goethite or lepidocrocite . Phototrophic Fe (II) oxidation probably only plays an important role for ecosystems locally, but not globally. This is due to two main reasons: On the one hand, the depth of light penetration into soils or sediments is only about 200 µm, on the other hand, the organisms cannot actively dissolve Fe (II) -containing iron minerals and are therefore solely dependent on their solubility.

Importance in geochemistry

Rusticles on an anchor of the Titanic

The microbial iron oxidation plays an important role in the geochemical cycle of iron, since the iron oxidizers responsible for it are ubiquitous. They can be found in a variety of different iron-containing habitats. So far, iron bacteria have been detected in sediments and in the water bodies of lakes and rivers, in sources of ferrous groundwater in the marine area in coastal sediments, fallow water areas and in the vicinity of hydrothermal springs. Most noticeably, iron oxidizers affect habitats in acid mine water and in the immediate vicinity of plant roots in wetlands.

The habitats are in some cases significantly affected by the activity of the iron oxidizers. Most of the compounds of divalent iron are readily soluble in water, while those of trivalent iron are almost all sparingly water-soluble at medium pH values. This means that the microbial Fe (II) oxidation leads to precipitation, i.e. immobilization of iron. The genesis of some oxidic iron deposits can thus be explained.

A very impressive result of the microbial iron (II) oxidation is the formation of rusticles on shipwrecks, for example the Titanic .

Individual evidence

  1. ^ PC Singer, W. Stumm: Acidic mine drainage. Rate-determining step. In: Science. Volume 167, 1970, pp. 1121-1123.
  2. ^ R. Blake, DB Johnson: Phylogenetic and biochemical diversity among acidophilic bacteria that respire on iron. In: DR Lovley (Ed.): Environmental Microbe-Metal Interactions. ASM Press, Washington, DC 2000, pp. 53-78.
  3. ↑ N. Cholodny: The iron bacteria - contributions to a monograph. (= Plant Research . Issue 4). Gustav Fischer, Jena 1926.
  4. ↑ KJ Edwards, DR Rogers, CO Wirsen, TM McCollom: Isolation and characterization of novel psychrophilic, neutrophilic, Fe-oxidizing, chemolithoautotrophic alpha- and gamma-Proteobacteria from the deep sea. In: Appl. Environ. Microbiol. 69, 2003, pp. 2906-2913.
  5. ^ KL Straub, M. Benz, B. Schink, F. Widdel: Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. In: Appl. Environ. Microbiol. 62, 1996, pp. 1458-1460.
  6. ↑ ES Shelobolina, CG VanPraagy, DR Lovley: Use of ferric and ferrous iron Containing minerals for respiration by Desulfitobacterium frappieri. In: Geomicrobiol. J. 20, 2003, pp. 143-156.
  7. ^ S. Ratering, S. Schnell: Nitrate-dependent iron (II) oxidation in paddy soil. In: Environ. Microbiol. 3, 2001, pp. 100-109.
  8. ↑ F. Widdel, S. Schnell, S. Heising, A. Ehrenreich, B. Assmus, B. Schink: Ferrous iron oxidation by anoxygenic phototrophic bacteria. In: Nature (London). 362, 1993, pp. 834-835.
  9. ^ S. Heising, L. Richter, W. Ludwig, B. Schink: Chlorobium ferrooxidans sp. nov., a phototrophic green sulfur bacterium that oxidizes iron in coculture with a 'Geospirillum' sp. strain. In: Arch. Microbiol. 172, 1999, pp. 116-124.
  10. ↑ a b K. L. Straub, FA Rainey, F. Widdel: 'Rhodovulum iodosum' sp. nov, and 'Rhodovulum robiginosum' sp. nov., two new marine phototrophic ferrous-ironoxidizing purple bacteria. In: Int. J. Syst. Bacteriol. 49, 1999, pp. 729-735.
  11. ↑ S. Heising, B. Schink: Phototrophic oxidation of ferrous iron by a 'Rhodomicrobium vannielii' strain. In: Microbiology. 144, 1998, pp. 2263-2269.
  12. ↑ a b c A. Kappler, DK Newman: Formation of Fe (III) -minerals by Fe (II) -oxidizing photoautotrophic bacteria. In: Geochim. Cosmochim. Acta. 68, 2004, pp. 1217-1226.
  13. ↑ Y. Jiao, A. Kappler, LR Croal, DK Newman: Isolation and characterization of a genetically tractable photoautotrophic Fe (II) -oxidizing bacterium, 'Rhodopseudomonas palustris' strain TIE-1. In: Appl. Environ. Microbiol. 71, 2005, pp. 4487-4496.
  14. ^ KL Straub, M. Benz, B. Schink: Iron metabolism in anoxic environments at near neutral pH. In: FEMS Microbiol. Ecol. 34, 2001, pp. 181-186.
  15. ↑ A. Ciania, K.-U. Gossa, RP Schwarzenbach: Light penetration in soil and particulate minerals. In: Eur. J. Soil Sci. 53, 2005, pp. 561-574.
  16. ↑ S. Hauck, M. Benz, A. Brune, B. Schink: Ferrous iron oxidation by denitrifying bacteria in profundal sediments of a deep lake (Lake Constance). In: FEMS Microbiol. Ecol. 37, 2001, pp. 127-134.
  17. ↑ S. Heising, L. Richter, W. Ludwig, B. Schink: ' Chlorobium ferrooxidans' sp. nov., a phototrophic green sulfur bacterium that oxidizes ferrous iron incoculture with a 'Geospirillum' sp. strain. In: Arch. Microbiol. 172, 1999, pp. 116-124.
  18. ^ S. Heising, B. Schink: Phototrophic oxidation of ferrous iron by a Rhodomicrobium vannielii strain. In: Microbiology. 144, 1998, pp. 2263-2269.
  19. ↑ F. Widdel, S. Schnell, S. Heising, A. Ehrenreich, B. Assmus, B. Schink: Ferrous iron oxidation by anoxygenic phototrophic bacteria. In: Nature. 362, 1993, pp. 834-836.
  20. ↑ D. Emerson, C. Moyer: Isolation and characterization of novel iron-oxidizing bacteria that grow at circumneutral pH. In: Appl. Environ. Microbiol. 63, 1997, pp. 4784-4792.
  21. ↑ M. Blöthe, EE Roden: Microbial iron redox cycling in a circumneutral-ph groundwater seep. In: Applied and Environmental Microbiology. 75 (2), 2009, pp. 468-473.
  22. ^ KL Straub, FA Rainey, F. Widdel: ' Rhodovulum iodosum' sp. nov. and 'Rhodovulum robiginosum' sp. nov., two new marine phototrophic ferrous-ironoxidizing purple bacteria. In: Int. J. Syst. Bacteriol. 49, 1999, pp. 729-735.
  23. ↑ KJ Edwards, DR Rogers, CO Wirsen, TM McCollom: Isolation and characterization of novel psychrophilic, neutrophilic, Fe-oxidizing, chemolithoautotrophic alpha- and gamma-proteobacteria from the deep sea. In: Appl. Environ. Microbiol. 69, 2003, pp. 2906-2913.
  24. ↑ D. Emerson, CL Moyer: Neutrophilic Fe-oxidizing bacteria are abundant at the Loihi Seamount hydrothermal vents and play a major role in Fe oxide deposition. In: Appl. Environ. Microbiol. 68, 2002, pp. 3085-3093.
  25. ^ AR Colmer, KL Temple, ME Hinkle: An iron-oxidizing bacterium from the drainage of some bituminous coal mines. In: J. Bacteriol. 59, 1949, pp. 317-328.
  26. ↑ D. Emerson, JV Weiss, JP Megonigal: Iron-oxidizing bacteria are associated with ferric hydroxide precipitates (Fe-plaque) on the roots of wetland plants. In: Appl. Environ. Microbiol. 65, 1999, pp. 2758-2761.
  27. ↑ D. Sobolev, EE Roden: Suboxic deposition of ferric iron by bacteria in opposing gradients of Fe (II) and oxygen at circumneutral pH. In: Appl. Environ. Microbiol. 67, 2001, pp. 1328-1334.