Algae, bacteria, actinomycetes, fungi, higher plants, cyanobacteria., are the major microorganisms reported to tolerate heavy metals which universally occur in diverse ecosystems with meaning frequency.
1. Algae:
The species of Chlorella, Anabaena inaequalis, Westiellopsis prolifica, Stigeo clonium tenue, Synechococcus species, Selenastrum capricornutum etc. tolerate heavy metals.
2. Bacteria:
Bacteria resistant to heavy metals were frequently isolated from environmental sources such as soil and water. Resistance to mercury compounds being a common property of both Gram-positive and Gram-negative bacteria. Staphylococcus aureus a Gram-positive organism was intermediate in heavy metal sensitivity between E. coli and Pseudomonas aeruginosa which are Gram-negative organisms.
The other bacterial genera such as Bacillus cereus, Mycobacterium scrofulaceum, Streptococcus agalactiae, Streptomyces lividans, Thiobacillus ferrooxidans, Pseudomonas aeruginosa, Yersinia enterocolitica, Staphylococcus aureus, etc. were reported to tolerate both cadmium and mercurry.
Bacterial species of Arthrobacter, Bacillus, Brevibacterium, Corynebacterium, Nocardia, Serratia, etc. absorb mercury and lead along with other heavy metals in the solution. Alcaligenes faecalis tolerate zinc and cadmium when grown in nutrient broth.
3. Actinomycetes:
Actinomyces flavoviridis and several species of Streptomyces exhibited high ability to absorb mercury and lead alongwith the 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 a large amount of uranium from aqueous systems.
4. Fungi:
Heavy metal tolerance is a regular phenomenon exhibited by a number of fungal species. Studies have revealed that fungi accumulate heavy metals from dilute background concentrations.
Lead and copper were more readily accumulated by fungi and actinomycetes in comparison to zinc, managenese, 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 viride, Aspergillus niger and A. giganteus tolerate nickel concentration but showed prolonged growth and inhibited spore formation and spore germination.
Nakajima and Sakaguchi (1986) reported the selective absorption of mercury and lead from a mixed metal solution alongwith the other heavy metals by A. niger, A. oryzae, Chaetomium globosum, Fusarium oxysporum, Giberrella fujikuroi, Mucor hiemalis, Neurospora sitophila, Penicillium chrysogenum, P. lilacinum and Rhizopns oryzae besides yeast species of the genera Candida, Hensenula, Saccharomyces and Torulopsis.
Dubey and Dwivedi (1985) reported Macrophomina phaseolina tolerant to 500 ppm of cadmium in vitro. However, toxicity of this metal was influenced by kaolinite, pH and the presence of zinc and manganese in the medium. Cd toxicity increased with increase in pH, and decreased with increase in concentration of Zn and Mn.
5. Higher Plants:
Decontamination of soils polluted with heavy metals is possibly one of the most intractable of environmental problems. Soils become contaminated in this way either naturally due to proximity to metal, outcrops, or as a result of mining, industry and the dumping of waste.
There are some reports about the potential of certain plants capable of supplying a ‘green’ solution to this problem. Plants capable of accumulating high concentrations of metals such as zinc, nickel, cadmium, lead, copper and cobalt could provide an effective and practical method of cleaning-up heavily polluted soils.
There is an excellent potential for using hyper accumulator plants to remove metals through this process. Certain types (e.g. Thlaspi caerulescens of family Brassicaceae) has been found to be a strong hyper-accumulator of zinc and cadmium. Further research will help to identify the faster growing and most strongly metal-accumulating genotypes; explore the possibility of genetic engineering to improve metal uptake characteristics.
Certain plant species of family Brassicaceae such as Brassica oleracea cv Greyhound, cabbage, Raphanus sativus cv French Breakfast, radish, Thlaspi caerulescens, have been reported as a strong Zn accumulator, Alyssum lesbiacum and A. murale, both are known as Ni hyper accumulators from serpentine soils in Greece, and also from Arabidopsis thahania, a widespread species.
One aspect needed further study is the disposal of the potentially hazardous biomass produced by an effective hyper accumulator crop. It is suggested that one option could be the controlled ashing of harvested material to yield a residue in which metals such as Zn and Ni may be concentrated by more than 10 per cent.
6. Cyanobacteria:
Several species of microalgae including the green alga, Chlorella, blue green alga, Anabaena, marine algae, bacteria, mosses, and macrophytes have been used for heavy metal removal.
However, monospecificity and good operational conditions are some of the prerequisites, difficult to maintain, that limit the practical application of these organisms. Recently, Rai (1998) studied bio sorption of Cd2+ and Ni2+ by a capsulated nuisance cyanobacterium, Microcystis both from field and laboratory. The naturally occurring cells showed higher efficiency for Ni2+ and Cd2+ bio sorption as compared to laboratory cells.