In the present study we have made an attempt to develop an eco-friendly, cheap and convenient biological (green) method for the synthesis of silver nanoparticles (AgNPs) using the cell extract of the cyanobacterium Nostoc sp. strain HKAR-2. Their anticancerous, antifungal and antibacterial properties were also studied against MCF-7 cells, two fungal strains (Aspergillus niger and Trichoderma harzianum) and two plant bacterial strains (Ralstonia solanacearum and Xanthomonas campestris), respectively. The structural, morphological and optical properties of green synthesized AgNPs were determined by UV-VIS spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, transmission electron microscopy selected area electron diffraction (TEM-SAED) and scanning electron microscopy (SEM). Spectroscopic analysis showed the peak at 419 nm due to the reduction of AgNO3 into silver ion by cyanobacterial extract indicating surface plasmon resonance (SPR) of the synthesized AgNPs. The XRD pattern of AgNPs showed the characteristic Bragg peaks at (111), ( 200), ( 220) and (311) facets of the face centre cubic (fcc) confirming their crystalline nature. FTIR analysis revealed that proteins and amino acids are responsible for the reduction of AgNO3 into Ag + as well as for the stability of nanoparticles. Zeta potential confirmed that the charge on the nanoparticles is 1.80 mV which indicates the presence of stable nanoparticles. The results of SEM and TEM confirmed the large agglomerated shape of AgNPs with size ranging between 51-100 nm. The AgNPs showed a dose-dependent cytotoxic activity against human breast cancer MCF-7 cells with IC50 of 27.5 µg/ml. They also exhibited excellent antibacterial and antifungal activities.

In the present investigation, biological crusts from the surface of eight historical monuments of Varanasi, India, were examined for the presence of scytonemin (a cyanobacterial photoprotective pigment) containing cyanobacterial species. Lyngbya sp. and Scytonema sp. were the dominant cyanobacteria present in all crust samples. The absorption spectroscopic data of chlorophyll, carotenoids and scytonemin showed that scytonemin was more abundant than the carotene and chlorophyll in all the crusts. Identification of these compounds was done using UV-Vis spectroscopy and High Performance Liquid Chromatography (HPLC) analysis. HPLC analysis revealed the presence of scytonemin in seven out of eight samples and peaks of scytonemin with retention time ranging from 1.4-1.9 min with corresponding absorbance maxima at 386, 300 and 252±2 nm. As per our knowledge this is the first report of its kind from monuments of Varanasi. From this study, it can be concluded that synthesis of photoprotective compounds like scytonemin and its derivatives counteract the damaging effects of solar radiation which enable cyanobacteria to colonize and inhabit almost all kinds of habitats, including extreme lithic habitats, such as rocks and walls of monuments which face prolonged high intensity solar radiation.

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Cyanobacteria are cosmopolitan in distribution and have adapted to diverse habitats. Adaptation of cyanobacteria is one of the key factors to withstand harsh environmental conditions. We have investigated the effects of photosynthetically active radiation (PAR; 400-700 nm), ultraviolet-B (UV-B; 280-315 nm) radiation and PAR+UV-B radiations on phycobiliproteins (PBPs) of a hot-spring cyanobacterium Nostoc sp. HKAR-2. There was a continuous induction of both phycoerythrin (PE) and phycocyanin (PC) after exposure of PAR up to 300 min. However, there was an induction in the synthesis of both PE and PC up to 240 min exposure of UV-B and PAR+UV-B radiations. Further exposure showed decline in the synthesis due to rapid uncoupling, bleaching and degradation of PBPs. Similarly, emission fluorescence also showed an induction with a shift towards longer wavelengths after 240 min of UV-B and PAR+UV-B exposure. These results indicate that short duration of UV radiation may promote the synthesis of PBPs that can be utilized in various biotechnological and biomedical applications.