Iron requirement of various organisms
Iron (Fe) is a metal and an essential micronutrient for animals, plants, human beings and microorganisms. It is found naturally in earth's crust at about 5% concentration as hematite and magnetite ore form. In aquatic environments under anoxic conditions, iron is present in elemental form or as iron oxide when oxygen is available. Iron is also an important constituent of plant sap. It is very oldest mineral found on the earth; it was there when the earth was born. When earth's early atmosphere turned oxidative with the emergence of photosynthetic plants, iron acquired trivalent state. Reduction of Fe+3 to Fe+2 is also important action in geochemical cycling of iron. We are well aware of prehistoric presence of iron which is popularly known as 'Iron Age'. Meteorites and dying stars are also great source of iron. Thus it is universal element. Let's see an essentiality of iron for various types of living organisms from kingdoms Plantae, Animalia and Protista.
Plants: Iron at 10-5 M or less than concentration forms the essential micronutrient requirement of green plants. Otherwise, its deficiency leads to severe chlorosis of leaves. It is insoluble in neutral and alkaline soils. Grasses like wheat, barley, maize produce phytosiderophores or mugineic acids via roots to sequester iron from soil and rhizosphere. Plants also assimilate iron via microbial siderophores. In plants iron solubility and uptake is enhanced by acidification of surrounding environment or via extracellular reduction of Fe+3 to Fe+2. Iron is also essential for growth of green algae. More iron is required for autotrophic than heterotrophic growth of algae. Iron nutrition is also critical for in vitro culturing of algae and it is essential component of tissue culture media used for micropropagation of plants. Function of iron is associated with iron containing enzymes like peroxidase, catalase, cytochrome C, cytochrome oxidase, pyridine nucleotide reductase and cytochromes f and b6 of chloroplasts. Iron is also functional component of photosynthetic electron transfer chain.
Animals: Large amount of iron is already present in tissues such as liver of animals including humans. Iron is present in cells, respiratory pigments hemoglobin and myoglobin, chromatin, cytochromes and blood. Iron containing sulfur (Fe-S) proteins are involved in electron transport and energy conservation. Ferritin, lactoferrin and transferrin are important iron chelating proteins present in animal tissues. Lactoferrin is found in human and bovine milk and most of the secretions of human mucosal surfaces like saliva, bronchial, nasal, tears, bile, seminal fluid, pancreatic juice and even urine. It is also present in PMN leukocytes. Lactoferrin is often present to reduce virulence of pathogenic bacterial infections. Free iron (Fe+2) in humans can be harmful which can be oxidized in presence of peroxide to form Fe+3 and OH- radicals that can damage macromolecules. To prevent this, free iron and excess iron is sequestered in transferrin, ferritin and hemoproteins. When needed, transferrin chelate and transports iron to blood where it is absorbed to tissues for future use. These proteins also limit iron availability to invading microbes and opportunistic pathogenic flora. Iron is present usually as protein bound form even in food. It is absorbed in small intestine and enters the circulation via blood, lymph and deposited in spleen, liver and bone marrow. Unwanted iron is eliminated through walls of intestines. Useful form is used for regeneration of hemoglobin. Without iron cells suffer metabolic stress and disrupted growth patterns. Deficiency of iron in diet causes acute to chronic anemia. Iron tablets prescribed to treat anemia actually stimulate hemoglobin biosynthesis instead of taking part in actual biosynthesis process. Iron in animals is also essential for respiration, DNA synthesis and all the processes of reproduction, oxidations, secretions and development.
Microbes: About 10-8M free iron is essential for microbial growth. Iron in microbes is involved in ATP synthesis, heme biosynthesis and virulence. Most of the iron is present in oxidized/ferric forms are insoluble which microbes cannot assimilate. So they produce siderophores that bind, solubilize and release iron from host proteins, soil particles or other environments. Chelated free iron form is toxic and hence stored as polyphosphate, ferritins and siderophores for further metabolic use. Iron acquisition is important step for induction of pathogenesis. Iron is required by pathogenic strains for successful competition. Their siderophores chelate free iron making it unavailable for other competitive flora thus reducing competition and establishing in the host environment for successful colonization. Bacteria show great variety of iron acquisition, uptake and storage methods. Some outer membrane proteins of Neisseria remove directly bound Fe-protein. Iron containing leg-hemoglobin is required for rhizobial respiration in aerobic environment inside root nodules. Leg-hemoglobin also protects oxygen sensitive nitrogen fixing nitrogenase enzyme system in these bacteria. Staphylococcus aureus lyses red blood cells and acquire heme-bound iron and transferrin via siderophores. Shigella and Escherichia produce more than one type of enterochelin; alcaligin siderophore of Bordetella; FeO, Fe transport systems and enterobactin in enteric bacteria; acinetoferrin of Acinetobacter; ferritin and actinoferrin of Bacteriodes and Actinomyces respectively, Citrate chelators of Erwinia and Bradyrhizobium are some of the most studied examples of bacterial siderophores. Species specific siderophores of some bacterial genera: Pseudomonas aeruginosa (pyoverdine), Azotobacter vinelandii (azotobactin), Vibrio cholera (vibriobactin), Yersinia pestis (yersinabactin), Bacillus cepacia (ornibactin), Agrobacterium tumefaciens (agrobactin), Bacillus anthracis (bacillibactin) have been exploited for various medicinal and environmental applications. Vibrio and Aeromonas use heme as a source of iron via heme transporter receptor present in their outer membrane and inner membrane. They also synthesize siderophores in aquatic systems. Bacillus strains are most versatile as they can produce catecholate, petrobactin, ferric citrate, heme, iron permease and xenosiderophores types of iron chelators. In aerobic and aerotolerant bacteria iron also acts cofactor of enzymes like catalase and cytochromes. Microbes growing under anaerobic conditions have less difficulty in obtaining iron as it is in reduced and soluble ferrous form. Siderophores are also are able to chelate metals other than iron such as aluminium, copper, zinc, manganese, and chromium. Reduction and chelation of insoluble iron compounds is common way among pathogenic fungi and yeasts. Fungi produce hydroxamate and polycarboxylate type of siderophores. In fungi and yeasts, iron is stored in vacuoles, siderophores or ferritin like proteins. Conidiophores of Neurospora, Saccharomyces cerevisiae don't produce siderophores but have Fe specific ferric reductase.
Iron is thus important for growth and vitality of all living things but low solubility is the limiting factor for its availability.
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