The cycling of various naturally occurring elements including toxic pollutants and heavy metals is a well-regulated and balanced process. But rapid industrialization and immobilization have resulted in elevated emissions of toxic chemicals into the biosphere. The release of heavy metals in biologically available forms by human activity, may damage or alter both natural and man-made ecosystem. Heavy metal ions such as Cu²⁺, Zn²⁺, Fe²⁺ and Ni²⁺ etc. are essential micronutrients for plants and microbial metabolism but when present in excess can become extremely toxic.

Metals discharged from various sources severely pollute aquatic as well as terrestrial habitats. Metals being conservative tend to be bio-accumulated and biomagnified to alarming proportions in organism higher in the tropic chain.

Recently there is a considerable in developing cost effective and environment friendly technology for the remediation of soil and wastewater polluted with toxic trace elements. Cyanobacteria have the ability to accumulate heavy metals such as nickel and chromium and this ability could be harnessed to remove pollutant metals from the environment. The cyanobacteria based bioremediation technologies have received recent attention as strategies to clean up contaminated soil and water. Aulosira fertilissima, which is present as a natural flora in the rice fields in the form of thick mats covering the entire paddy field soils, is particularly useful in the abatement and monitoring of heavy metals.

Immobilization can be defined as “the imprisonment of cells in a distinct phase that allows exchange with, but is separated from the bulk phase in which substrate effectors or inhibitor molecules are dispersed and monitored and which is one of the thrust area in the field of biotechnology”. Various methods are available for immobilizing cells like entrapment, adsorption, covalent linkage, cross linkage or flocculation and encapsulation. Immobilization by entrapment is probably the most extensively used method. Immobilization of cyanobacteria improves functional longevity, higher yields of nitrogen fixation, hydrogen evolution and ammonia production.

Cyanobacteria, which are commonly known as blue-green algae are considered as the simplest, living microscopic autotrophs that are organisms capable of building up food materials from inorganic matter. Blue-green algae contribute significantly in maintaining soil fertility of the rice fields by their nitrogen fixing capacity. That is why, they are renewable bio-fertilizers and represent self-supporting biological systems. Any deterioration of these naturally occurring cyanobacteria due to leaching of heavy metals in the rice fields via irrigation can greatly affect the productivity of the paddy crop, which is a staple food for nearly two-third population of the Indian sub-continent. Barren alkaline lands in India have also been reclaimed and made productive by inducing the proper growth of certain cyanobacteria.

The two heavy metals nickel and chromium chosen for this study are the ones which are found abundantly in water and soil ecosystems from leaching of factories related to paint and idol immersion and prove to be quite hazardous if present above the normal level for both humans and other flora and fauna.

This research paper points to the positive role of immobilized cyanobacterium Aulosira fertilissima in reducing toxicity of nickel and chromium. In natural ecosystems like the rice fields Aulosira that is present as an immobilized sheath on the soil surface may be playing a very similar role in mitigating heavy metal toxicity.

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