Essay: Metals for plants, animals and microorganisms

Iron is one of the most essential growth elements for all living organisms. PGPB produce low-molecular-weight compounds called siderophores in soil free from iron to acquire ferric ion (Raymond et al., 2003). Bacteria, fungi and grasses produce and secrete Siderophores which are small, low-molecular-weight compounds and high-affinity iron chelating compound. It is called “iron carrier” in Greek and they are the strongest binders to Fe3+ known, with enterobactin which is one of the strongest (Raymond et al., 2003).
Several siderophores are biosynthesized independently by microbes and non-ribosomal peptides (Miethke and Marahiel , 2007). Siderophores are released by microorganisms to scavenge iron from mineral phases through the formation of soluble Fe3+ complexes that are taken up through active transport mechanisms. Chrome azurol assay is a general test to detect the production of siderophores by rhizobacteria. Siderophores are usually classified by the ligands used to chelate the ferric iron. catecholates (phenolates), carboxylates and hydroxamates are the major groups of siderophores (e.g. derivatives of citric acid). Some Pseudomonas sp. have the ability to utilize siderophores that are produced by species of bacteria and fungi, example is Pseudomonas putida have the capacity to utilize heterologous siderophores that is produced by PGPR to enhance the level of iron availability in the natural habitat. Fluorescent Pseudomonas, Pseudomonas fluorescens NCIM 5096 and P. putida NCIM 2847 produce maximum yield of hydroxamate, a type of siderophore in succinic acid medium (SM) and they are used as seed inoculants on plant to enhance growth an increase yields.
Rhizobacteria isolates such as Bacillus cereus UW 85, Azotobacter vinelandii MAC 259 and Bacillus megaterium from tea rhizosphere have the ability to produce siderophores and they can be used to increase the yield, promote plant growth and reduce disesase intensity (Chakraborty et al., 2006). In our environment, heavy metal contamination has become a crucial problem due to the industrial activities and sewage sludge applications in the terrestrial environment and this have seriously contributed to the spread of heavy metals. These metals are undegradable and they persist in the environment having accumulated in different parts of the food chain (Igwe et al., 2005). These heavy metals influence the microbial population by affecting their morphology, biochemical activities, growth and also results in decreasing biomass and diversity also in plants and animals but the degree of toxicity varies for different organisms.
Heavy metals may cause decrease in metabolic activity and diversity including adversely affecting the qualitative and quantitative structure of microbial communities (Yogendra et al., 2013).
Some metals such as Zinc, Copper, Nickle, and Chromium are necessary and beneficial micronutrients for plants, animals and microorganisms, while others, such as Cadium, Mecury and lead have no biological and physiological functions; Hence, all these metals could be toxic at relative low concentrations. When soil microorganisms were exposed to moderate heavy metal concentrations, they were found to be very sensitive (Yogendra et al., 2013). Some bacteria have developed mechanisms to detoxify heavy metals, and some even use them for respiration. Microbial interactions with metals may have many implications for the environment. Microbes may play a wide role in the biogeochemical cycling of toxic heavy metals by cleaning up or remediating metal-contaminated environments (Yogendra et al., 2013).
Heavy metal ions, in high concentrations react to form toxic compounds in cells. For toxic effect, heavy metal ions must first enter the cell, because some heavy metals are essential for enzymatic functions and bacterial growth. Several research work have reported two general uptake systems, one is quick and unspecific, driven by a chemiosmotic gradient across the cell membrane and requires no ATP, while the other is slower and more substrate-specific, driven by energy from ATP hydrolysis. The first mechanism is more energy efficient, it results in an influx of a larger variety of heavy metals, and when these metals are present in the cell in high concentrations, they are likely to have toxic effects. (Nies, 1999).For the bacteria to survive under metal-stressed conditions, they evolve in several types of mechanisms to tolerate the uptake of heavy metal ions. These mechanisms include the efflux of metal ions outside the cell, reduction of the heavy metal ions to a less toxic state and accumulation and complexation of the metal ions or using them as terminal electron acceptors in anaerobic respiration inside the cell (Nies, 1999).
However, tolerance mechanisms for metals such as zinc, copper, chromium, arsenic, nickel and cadmium have been identified. Most of the mechanisms studied involve the efflux of metal ions outside the cell, and genes for this general type of mechanism have been present on both chromosomes and plasmids. This is due to the intake and subsequent efflux of heavy metal ions by microorganisms usually includes a redox reaction involving the metal (that some bacteria use for energy and growth), bacteria that are resistant to and grow on metals can also play an important role in the biogeochemical cycling of those metal ions. (Yogendra et al., 2013).