Iron Cycle or Transformation
Iron exists in nature either as ferrous (Fe++) or ferric (Fe+++) ions. Ferrous iron is oxidized spontaneously to ferric state, forming highly insoluble ferric hydroxide. Plants as well as microorganisms require traces of iron, manganese copper, zinc, molybdenum, calcium boron, cobalt etc. Iron is always abundant in terrestrial habitats, and it is oftenly in an unavailable form for utilization by plants and leads to the serious deficiency in] plants.
Soil microorganisms play important role in the transformations of iron in al number of distinctly different ways such as:
Certain bacteria oxidize ferrous iron to ferric state which precipitate as ferric hydroxide around cells
Many heterotrophic species attack on in soluble organic iron salts and convert into inorganic salts
Oxidation-reduction potential decreases with microbial growth and which leads to the formation of more soluble ferrous iron from highly insoluble ferric ions
Number of bacteria and fungi produce acids such as carbonic, nitric, Sulphuric and organic acids which brings iron into solution
Under anaerobic conditions, the sulfides formed from sulphate and organic sulphur compounds remove the iron from solution as ferrous sulfide
As microbes liberate organic acids and other carbonaceous products of metabolism which results in the formation of soluble organic iron complex. Thus, iron may be precipitated in nature and immobilized by iron oxidizing bacteria under alkaline soil reaction and on the other hand solubilization of iron may occur through acid] formation.
Some bacteria are capable of reducing ferric iron to ferrous which lowers the oxidation-reduction potential of the environment (eg. Bacillus, Clostridium, Klebsiella etc). However, some chemoautotrophic iron and sulphur bacteria such as Thiobacillus ferroxldans and Ferrobacitlus ferrooxidans can oxidize ferrous iron to ferric hydroxide which accumulates around the cells.
Most of the aerobic microorganisms live in an environment where iron exists in the oxidized, insoluble ferric hydroxide form. They produce iron-binding compounds in order to take up ferric iron. The iron-binding or chelating compounds / ligands produced by microorganisms are called "Siderophores". Bacterial siderophores may act as virulence factors in pathogenic bacteria and thus, bacteria that secrete siderophores are more virulent than non- siderophores producers. Therefore, siderophore producing bacteria can be used as biocontrol agents eg. Fluorescent pseudomonads used to control Pythium, causing damping-off diseases in seedlings. Recently Vascular – Arbusecular – Mycorrhiza (VAM) has been reported to increase uptake of iron.