The use of chemical fertilizers in agriculture poses severe environmental pollution problems, affecting soil, ground water and the health of agricultural workers. In addition to this, at toxic concentrations, heavy metals such as aluminium, cadmium, chromium and lead, amongst others, cause different harmful effects on plants growing in contaminated soils. These metals have been found in significant amounts in foods/feedstock coming from crops grown in such soil. However, iron (Fe)-one such heavy metal- is important for the survival of humans, plants, and most micro organisms. For example, in plants, Fe is essential for the formation of the green pigment (chlorophyll) and hence in photosynthesis. It also plays an important role in energy transfer, in several enzymatic reactions where it serves as enzyme co-factor; as well as in nitrogen fixation by crops such as beans and soybeans. In spite of its bio-importance,

Fe can become toxic under flooded (as obtained in lowland rice cultivation) and acidic soil conditions. This is because ferric iron, the form in which the metal exists in the soil, becomes more easily reduced to ferrous, the bio-usable form of Fe, and accumulates to high concentrations that are harmful to the plant. Between 12 % and 100 % crop losses have been reported in flooded rice production in West Africa . The opposite is true for non-acid soils where Fe remains highly insoluble. Apart from the non-acidic soil-induced insolubility, Fe deficiency symptoms in plants have also been linked to the presence of other heavy metals in the soil. Certainly, this complicates matters for crop production. Thus, in addition to stress from excessive or limited levels of Fe, plants suffer from increased stress when heavy metals other than Fe are sources of soil pollution.

To circumvent the problem of Fe scarcity, many soil-living micro organisms and grass crops (e.g. maize, rice, millet, wheat, etc) release low-molecular-weight, high-affinity, iron-binding compounds known as siderophores. Unfortunately, not all soil micro organisms are capable of producing these siderophores. Such micro organisms survive Fe deficiency stress by using siderophores produced by others. Dicotyledonous (e.g. beans, soybean, etc) and non-grass monocot plants (e.g. cassava, plantain, okra, etc) don’t produce siderophores, a reason for which the possible role of microbial siderophores in Fe acquisition by these plants have been a topic of intense global research. Based on their affinity for Fe and to a lesser degree other heavy metals, it is evident that the role of microbial siderophores can be two-pronged. Firstly, by scavenging for Fe, they can make the metal available when it is scarce. Secondly, they can potentially be used to immobilise heavy metals when in abundance in soil, thereby preventing, or at least, alleviating metal toxicity. Indeed, in humans, but not in plants, a siderophore known as desferrioxamine B (sold as desferal tablets) is currently in use as drug for treating Fe overload-related diseases. It should therefore be a wise strategy to evaluate if/how siderophores can influence plant production in soils where plants grow poorly due to Fe deficiency (biofertilisation); or to soils in need of repair from heavy metal pollution (rhizoremediation).

A Nigerian, Christian Dimkpa began his PhD research in the area of microbial siderophore-mediated plant-heavy metal interactions in March 2006, under the auspices of the International Max Planck Research School (IMPRS) which is funded by the Max Planck Society, Germany . He is supervised by Professor Erika Kothe, a renowned Microbiologist and head of the Microbial Phytopathology research group of the Friedrich Schiller University and Dr. Ales Svatos, an expert in analysing proteins/secondary metabolites (two of the so-called ‘‘omics’’: proteomics and metabolomics) at the Max Planck Institute for Chemical Ecology, both in Jena, Germany. His research involves studying the role of siderophores produced by a specific soil-living micro organism in promoting plant growth and inhibiting heavy metal uptake in contaminated environments. Following series of laboratory and soil experiments, results were obtained indicating that siderophores from this bacterium can serve both as organic fertilisers, and tools for remediating heavy metal pollution in soil. This was supported by the significant plant growth promotion obtained from plants treated with microbial siderophores, in comparison to untreated plants, both grown in soil heavily-laden with iron, aluminium and other heavy metals. This work is significant in the sense that it shows for the first time that compounds (siderophores) similar to those which are the active ingredients in drugs used for the treatment of heavy metal contamination in humans, can serve a similar purpose in plants. Part of this work has already being submitted for publication in an international scientific journal.

What then could be the ecological implications of such study for a country like Nigeria ? The answer is not far-fetched: most of the soils in the southern agricultural belt of Nigeria are known to be acidic in nature, while soils in the northern belt are rather non-acidic. Therefore, Fe levels will be respectively high and low in these agro-ecological zones, to the detriment of plants grown in these zones. Moreover, most rice production in Nigeria is done under flooded conditions, which means that toxic levels of Fe will be present in such soils. Furthermore, the federal government is currently establishing mechanisms that will expedite the exploitation of the abundant solid minerals. Mining activities are however known to release toxic levels of heavy metals to the surrounding environment. Yet, it is not clear what counter-measures if any, are being put in place with regards to the pollution that is often associated with mineral exploitation and the effect on the environment and agriculture. It is important to create a balance in Fe levels in Nigerian soils, as well as protect the soil from heavy metal pollution; thus conserving the environment for sustainable agricultural production. In the light of this, research in the use of microbial siderophores as biofertilisers and for remediating heavy metal pollution is a necessity, especially when industry-scale solid minerals exploitation finally takes off. Good enough, being a natural product of biological origin, siderophores are themselves not potential sources of persistent pollution as they degrade quite easily with time.

Christian Dimkpa holds an MSc in Molecular Biology obtained from universities in Belgium, and is currently a Max Planck doctoral fellow of Plant Microbiology in Germany. He was previously with the prestigious International Institute of Tropical Agriculture (IITA) based in Onne, and Ibadan , Nigeria . He also holds a Higher National Diploma (HND) in Crop Production Technology obtained from Nigerian institutions. He has co-authored papers in plant genetics and molecular biology.

Robot  # 1 | 20.10.2007 08:00

The use of chemical fertilizers in agriculture poses severe environmental pollution problems, affect...Read the full article.