Characterization of an exopolysaccharide mutant of Nostoc spongiaeforme: Zn2+-sorption and uptake |
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Authors: | Singh Neetu Asthana RK Singh SP |
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Institution: | (1) Algal Research Laboratory, Centre of Advanced Study in Botany, Department of Botany, Banaras Hindu University, Varanasi, 221 005, India |
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Abstract: | Exposure of the exopolysaccharide (EPS)-synthesizing cyanobacterium Nostoc spongiaeforme to Zn2+ (20 M) transformed the biomass into white debris. However, a few blue–green pin-heads emerged after 2 weeks in the same Zn2+-containing medium and formed less mucoid microcolonies (1–2 mm) relative to the protruding colonies (2–4 mm) of the parent strain on nutrient agar. One of such survivors (designated as Zn20) that was stable through 10 successive transfers in Zn2+-lacking medium has been adopted for further characterization. The parent strain retained almost 88% of the total EPS synthesized, the rest being released into the ambient medium, while for Zn20, the EPS retained approximated to 74%. Although the Zn2+-sensitivity of the mutant was comparable with that of the parent (LD50, 7 M), Zn2+ uptake was still 5-fold higher in the former (2 g mg–1 biomass dry wt., 20 M, external concentration). Also, both the strains showed insignificant difference in Zn2+-sorption onto their isolated EPS. The mutant was characterized by having higher cell carbohydrate content (642.8 g mg–1 dry wt.) than its parent (513.6 g). The X-ray diffraction pattern revealed Zn2+ deposition on EPS from the parent mainly as zinc hypophosphite monohydrate Zn(H2PO2)2·H2O], whereas there was a lack of distinct peaks in similar samples from Zn20, thus confirming the amorphous nature. There was participation in Zn2+ binding of only COO–, N=O, NO2, SO2 groups in the parent while participation of P—O and C=O groups in mutant EPS was evident in IR spectra. The observations suggest that the mutant could be deployed to achieve sustained EPS synthesis, its release and metal sorption/desorption in repeated cycles. |
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Keywords: | Exopolysaccharide Nostoc spongiaeforme variant zinc |
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