Characterization of an exopolysaccharide mutant of Nostoc spongiaeforme: Zn2+-sorption and uptake

Abstract

Exposure of the exopolysaccharide (EPS)-synthesizing cyanobacterium Nostoc spongiaeforme to Zn2+ (20 μM) 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 μM), Zn2+ uptake was still 5-fold higher in the former (2 μg mg-1 biomass dry wt., 20 μM, 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 μg mg -1 dry wt.) than its parent (513.6 μg). 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.