Table of Contents
What are Siderophores?
Siderophores are organic compound with low molecular masses. They are released by plants and microorganisms under iron limiting conditions. Their basic function is to bind with ferric ions (Fe3+) that are present in surrounding environment and transport them to microbial and plant cells. Siderophores not only have high affinity for ferric ions but they also have the ability to bind with a variety of metals.
Ferric form of iron is insoluble at physiological pH. When siderophore forms a complex with iron it is transported to cytosol where Fe3+ is reduced to Fe2+ (ferrous iron) and becomes accessible to living cells.
Siderophores are produced by following:
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- Bacteria e.g. E.coli
- Fungi e.g. Aspergillus Fumigatus
- Plants e.g. Members of poaceae
- Mammals
- Cyanobacteria e.g. Prochlorococcus
Roles of Siderophores in Environment
Soil Mineral Weathering
Weathering is the formation of soil upon breakdown of rocks and minerals when in contact with Earth’s biological organisms. This process occurs in situ.
The soil is colonised by microbial communities that are attached to mineral surfaces forming microenvironment. The minerals in these microenvironments can be taken up by microorganisms but are in insoluble form so siderophores are released by microbes/plants. These siderophores bind to these minerals forming siderophore-Fe (III) complex at mineral surface and is transferred into surrounding soil solution thereby making it available for uptake by living cells.
Biogeochemical Cycling of Fe in Ocean
Biogeochemical cycle is the pathway followed by an element essential to living organisms as it travels through the biotic and abiotic compartments of earth. Fe is an essential micronutrient as it controls the productivity and community structure of phytoplankton but it is present in low concentration. Marine bacteria play a significant role in the cycling of Fe as it competes with phytoplankton for Fe by releasing different siderophores. The siderophores form complexes with Fe that increases the concentration of Fe making it available for phytoplankton.
Biotechnological Applications of Siderophores
Enhance Growth and Pathogen Biocontrol of Plants
Microbial siderophores form complex with Fe that is stable and soluble therefore increasing the availability of Fe to plants and encouraging their growth. In addition, siderophores act as biocontrol agent since they deprive the pathogens of Fe by competing for it so there is no need of synthetic pesticides too. Some pseudomonads species do both.
Biocontrol of Fish Pathogens
Pathogenic bacteria infect the host fish by either releasing harmful enzymes like proteases or by producing transferrin. The biocontrol mechanism is actually a competition between transferrin produced by pathogens and siderophores released by biocontrol agents. Due to high affinity for Fe, siderophore wins and reduces Fe levels necessary for pathogens to cause infections. Pseudomonas fluorescens is used as probiotics in fish farming.
Microbial Ecology and Taxonomy
The types of siderophores produced can be used to classify microbial strains and this is known as siderotyping. In this way, siderophores can be used as a chemotaxonomic marker for identifying other types of bacteria based on variation in their chemical structure. This is quick and certain identification at species level.
Bioremediation of Environmental Pollutants
Bioremediation is the cleaning up of a contaminated site by microbes which in this case is done by releasing siderophores, that increases the solubility and mobility of a wide range of metals like Cu, Ni, Pb, Zn etc. Since siderophores have affinity for metals other than Fe, the clean-up is carried in a cost-effective and eco-friendly method. Microbial siderophores also degrade pollutants like petroleum hydrocarbons in marine ecosystems. Phytosiderophores also contribute to bioremediation and are more efficient than other siderophores at mobilizing metals in soil.
Bioleaching of Pulps
Siderophores are effective agents in pulp treatment and can be used during bleaching process in pulp and paper industry. This reduces 70% of the chemicals required to bleach pulp therefore, a lot less percentage of pollutants are released into air and waste water.
Micronutrient Fertilizers Production
Fertilizers have soluble salts, oxides that have high leaching rates so are of less use to crops. Adding siderophores to these fertilizers produces chelated micronutrients fertilizers that not only provides micronutrient to plants more readily but also ensures that soil environment isn’t contaminated by indissoluble materials.
Bioleaching of Metals
Bioleaching is the extraction of metals from their ores with the help of microorganisms that release siderophores for this action. These siderophores form complexes with metals like uranium, thorium and rare earth elements(REEs) and bioleach them. This is much better than traditional methods like smelting since it’s cost-effective and environmentally friendly as well.
Biosensing
Siderophores can be used in the construction of optical biosensors and therefore can identify the concentration of metals in the ocean. Siderophores are sensitive and selective as well for a particular element, therefore providing a good potential.
