Most of the heavy metals such as cadmium, vanadium, manganese, lead, arsenic, iron, mercury, zinc, nickel, chromium, copper, antimony etc. are toxic to biological systems, not only due to their inert nature but also due to their persistence and long run cumulative effects. The occurrence of Itai-itai disease due to cadmium, minamata disease due to arsenic are some of the examples of heavy metal pollution.
Fungi, bacteria, actinomycetes, algae etc. are the major microorganisms reported to tolerate heavy metals which universally occur in diverse ecosystems.
The species of Chlorella, Anabaena inequalis, Westiellopsis prolific, Stiego clonium tenue, Synechococcus species, Selenastrum capricornutum etc. tolerate heavy metals.
Bacteria-Resistance to mercury compounds being a common property of both Gram-positive and Gram-Negetive Bacteria.Bacillus cereus, Mycobacterium scrofulceum, Streptococcus agalactiae, Streptomyces lividans, Thiobacillus ferrooxidans, Pseudomonas aeruginosa, Yersinia enterocolitica, Staphylococcus aureus etc. tolerate both cadmium and mercury. Bacterial species of Arthrobacter, Bacillus, Brevibacterium, Corynebacterium, Nocardia, Serrantia etc. absorb mercury and lead alongwith other heavy metals in the solution.
Actinomyces flavoviridis and several species of Streptomyces exhibited high ability to absorb mercury and lead alongwith other heavy metals from mixed metal solution of manganese, cobalt, nickel, copper, zinc, cadmium, mercury, lead and uranium. Actinomyces levoris and S. viridochromogenes were shown to accumulate large amount of uranium from aqueous systems.
Heavy metals tolerance phenomenon exhibited by a anumber of fungal species. Fungi accumulate heavy metals from dilute background concenterations. Lead and copper were more readily accumulated by fungi and actinomycetes in comparision to zinc, manganese, cobalt, nickel and cadmium which make selective accumulation of these heavy metals by fungi different from many bacteria and yeasts. Yeasts are least sensitive to heavy metals. Trichoderma viridae, Aspergillus niger, A.giganteus tolerate nickel concenteration but showed prolongation of growth and inhibited spore formation and spore germination. Selective absorption of mercury and lead from a mixed metal solution along with other heavy metals by A. niger, A.oryzae, Chaetomium globosum, Fusarium oxysporum, Giberrella fujikuroi, Mucor hiemalis, Neurospora sitophila, Penicillium chrysogenum, P.lilacium and Rhizopus oryzae is in studies. Besides yeast species of the genera Candida, Hensenula, Saccahromyces and Torulopsis are also beneficial.
Plants capable of accumulating high concenteration of metals such as of cleaning-up heavily polluted soils. There is an excellent potential for using hyper-accumulator plants to remove metals through this process. Certain types (Thlaspi caerulescens of family Brassicaceae) has been found to be a strong hyper- accumulator of zinc and cadmium.Certain plant species of family Brassicaceae such as Brassica oleracea , Raphanus sativus, Thlaspi caerulescens are a strong Zn accumulator. Alyssum lesbiacum and A.murale, both are known as Ni hyper accumulators.
Several species of micro algae including the green alga, Chlorella, Blue green algae, Anabaena, marine algae, bacteria, mosses, and macrophytes have been used for heavy metal removal. Microcystis showed biosorption of Cd+2 and Ni+2.
Mechanism of Heavy Metal Resistance
There are four main physiological mechanisms of heavy metal resistance: inactivation, Impermeability, by-pass, altered target sites.
Many plasmids responsible for heavy metal tolerance were reported in bacteria. Mediation of penicillase plasmid in importing resistance to divalent metal ions of mercury and cadmium is reported in Staphylococcus aureus. DNA sequence analyses of several heavy metal resistance systems gives knowledge about mercury, cadmium, and arsenic resistance mechanisms.
Chromosomal mutations to heavy metal resistance can be produced in the laboratory but do not generally occur in nature.Toxic heavy metal resistance is comparable to antibiotic resistancee.g. streptomycin resistance found in natural isolates is always due to aminoglycoside-inactivating enzymes and not to the modified ribosomal protein that is responsible for laboratory-selected resistance to streptomycin. Several mercury resistance transposons reported from environmental bacteria include:Tn501, Tn21, Tn1861 and Tn3403.
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