Examples of toxic heavy metals accumulating microorganisms
Mercury is a highly toxic metal which, once released into water, accumulates in the food chain, damaging ﬁsh, shrimps and poisoning people who eat them. The infamous Minamata accident (called after a town on the Japanese island of Kyushu where the inhabitants suffered the toxic effects of ﬁsh poisoned by mercury-rich industrial efﬂuents) is an example of the devastating effects of mercury on the central nervous system. Existing techniques of mercury removal, such as precipitation or ion exchange, are expensive and not sufﬁciently efﬁcient, as small but signiﬁcant amounts of mercury still remain in the water. Researchers discovered that many bacteria had developed high tolerance to heavy metals, which related to the binding of these metals to proteins, e.g. metallo-thionein that binds mercury. As naturally thriving mercury-tolerant bacteria are rare and cannot be grown easily in culture, researchers at Cornell University, Ithaca, New York, inserted the metallo-thionein gene into Escherichia coli. A sufﬁciently large number of genetically engineered bacteria could thus treat mercury polluted water inside a bioreactor. The efﬁciency of the procedure was high, as mercury was removed from polluted water down to a few monograms per liter. Once the bacteria died, they were incinerated to recuperate the accumulated pure mercury. Mercury emissions were predicted to increase by 30 per cent throughout Europe between 1990 and 2010. The European Commission funded a demonstration project to show the feasibility and proﬁtability of the microbial remediation technology under real time conditions. A plant was set up at Usti-nad-Labem in the Czech Republic, and has been operating since July 2000. While phycoremediation is bioremediation based on the use of micro- and macro-algae, phytoremediation, relies on higher plants to clean water and soils from heavy metals and other pollutants, or to recolonize former mining areas (e.g. in South Africa, Australia, USA, Canada, France, etc.). For instance, heavy metals in industrial efﬂuents can be concentrated in aquatic plants (e.g. the Azolla fern and water lentils – Lemna spp.) and thereafter recovered. In 2003 in Western Europe, there were about 1.4 million sites discovered to be polluted. Current remediation techniques are chemical or physico-chemical extraction techniques; they are costly and destroy soil structure. Phytoremediation techniques use “hyper-accumulating plants” which can store 10 to 500 more pollutants in their leaves and stems, which are thereafter harvested, incinerated, and metals recovered from ashes and reused in metallurgy.The survey of hyper-accumulating plants started by the early 1990s. They are often small plants, such as Alyssum murale, which grows on metamorphic rocks, Brassica juncea, the Indian mustard, which extracts lead, or Thlaspi, which accumulates zinc and nickel. About 400 species have been identiﬁed, including 300 that accumulate only nickel. An endemic tree in New Caledonia, Sebertia acuminata, contains up to 20 per cent of nickel in its sap and is coloured in green (nickel is generally toxic to plants at a concentration of 0.005 per cent). In the Democratic Republic of Congo, the number of plants accumulating copper and cobalt is highest: 24 and 26 species, respectively. The accumulation efﬁciency is not generally very high. For some metals like silver, mercury and arsenium, there are yet no plants known to accumulate them. However, in 2000, the team of Lena Ma of the University of Florida, Gainesville, identiﬁed a fern, Pteris vittata, which tolerates and accumulates arsenium, while conserving a very rapid growth and a high biomass. Edenspace, a company from Virginia specialized in phytoremediation, acquired the rights to commercialize the fern (now called edenfern™) by signing an exclusive license agreement in 2000 with the University of Florida which patented the use of the fern in phytoremediation. In the US, seven or eight similar companies in phytoremediation were already in existence in 2002, where the value of the potential market for phytoremediation was estimated at $100 million.
Metal processing mechanisms of microorganisms
Sites of biosorption of toxic metals
Two-stage biosorption process
7.2 Bioremediation and genetic engineering The molecular basis of heavy metal accumulation is being studied with a view to transferring the relevant genes to plant species having a wider geographic and ecological distribution. Transgenes is applied to phytoremediation is certainly incipient. Its application on a large scale is confronted with the evaluation of risks relating to the transfer of the bacterial transgenes to plants consumed by herbivorous animals that might acquire the property of hyper-accumulating toxic metals or compounds. Genetic transformation of the microorganisms involved in bioremediation could enhance the process through the introduction of genes controlling speciﬁc degradation pathways; it also aims at degrading recalcitrant compounds such as pesticides and other xenosubstances. A team of US researchers led by Richard Meagher of the University of Georgia, Athens, were able to introduce into the genome of Arabidopsis thaliana, two foreign genes from Escherichia coli for the synthesis of two enzymes: one which catalyzes the transformation of arsenate into arsenite, the other which induces the formation of a complex with arsenate, that is retained in the leaves. These remarkable results led to transgenic Arabidopsis plants which could.
Bioremediation is the development of the field of environmental biotechnology by utilizing biological processes in controlling pollution. Bioremediation can be performed in situ on the spot and natural process. The rate of microbial degradation of the heavy metals depends on several factors, including microbial activity, nutrition, soil acidity and environmental factors.
There are two types of bioremediation technologies, namely ex-situ and in situ. Ex-situ is the management which include physical removal of contaminated materials to a location for further treatment. The use of bioreactors, land (land farming), composting and some other forms of solid-phase treatment is an example of ex-situ technology, whereas in situ technology is directly applicable to the treatment of materials contaminated with contaminants at the site .