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Aim: To investigate the ability of Haloferax alexandrinus GUSF-1 (KF796625) to biosynthesize non-toxic elemental selenium (Se 0 ) and check their capacity in in vitro crystal structure modulation of calcium oxalate, which are implicated in the development of renal calculi. Methods and Results:Haloferax alexandrinus GUSF-1 (KF796625) during growth in the presence of 5 mmol L −1 of selenite formed insoluble brick-red particles. Se 0 formed was monitored spectrophotometrically using a combination of two assays; the ascorbic acid reduction and sodium sulphide solubilization assay. After 168 h of growth, 2.89 mmol L −1 of Se 0 was formed from 4.9 mmol L −1 of selenite. Absorption bands at 1.5, 11.2 and 12.5 keV in EDX spectroscopy confirmed that the brick-red particulate matter was Se 0 . Furthermore, these selenium nanoparticles (SeNPs) were pentagonal in shape in transmission electron microscopy imaging. The peak positions in X-ray diffractogram at 2θ values of 23.40°, 29.66°, 41.26°, 43.68°, 45.24°, 51.62°, 55.93° and 61.47° and the relative intensities further confirmed the formation of Se 0 . In vitro addition of 50 and 100 µg ml −1 of these SeNPs to the mixture of sodium chloride, calcium chloride and sodium oxalate affected and modulated the shape and size of rectangular-shaped calcium oxalate crystals (average area of 1.23 ± 0.2 µm 2 ) to smaller rectangular-shaped crystals (average area of 0.54 ± 0.2 µm 2 ) and spherical-shaped crystals (average area 0.13 ± 0.005 µm 2 ). Conclusion: Haloferax alexandrinus GUSF-1 (KF796625) transformed selenite toSe 0 pentagonal nanoforms that modulated in vitro the formation of crystal shape and size of calcium oxalate. Significance and Impact of Study:There are no reports on conversion of selenite to Se 0 among the Haloferax genera, and this study involving the formation of pentagonal SeNPs with capacity to modulate the formation of calcium oxalate crystals in haloarchaea is recorded as the first report and of significance in pharmaceutical research related to formulations abetting urinary calculi.

This is the first account of the kinetics of free radical scavenging by bacterioruberin obtained from cells of Haloferax alexandrinus GUSF-1(KF796625), grown at optimum conditions of 25% NaCl, pH 7, 42 °C, 150 rpm in NaCl Tryptone yeast extract medium and light. Bacterioruberin separated from methanolic extract displayed characteristics absorption peaks at 368, 386, 463, 492 and 525 nm and gave an m/z value of 740.4 (C50H76O4) in Liquid Chromatography-Mass Spectroscopy validating its purity. Bacterioruberin (13 µM) decolorized and decayed 0.2 mM 1,1-diphenyl-2-picrylhydrazyl radicals (DPPH•) monitored at 517 nm and reached a steady state within 30 min. An EC50 of 6.50 µM ± 0.27 (4.81 µg/mL ± 0.2) was deduced for the 0.2 mM DPPH•-bacterioruberin reaction using the GraphPad Prism 9 statistical software and employing the right-angled triangle technique. The study also revealed a comprehensive information of the total kinetic activity of bacterioruberin with DPPH•: the antioxidant activity index was 16.38 ± 0.67; time needed to reach the steady state with the added EC50—30 min; the antiradical power 30.77 ± 1.27 and the antiradical efficiency of 54.7 × 10–3 ± 2.24, thus reflecting the strong antioxidant nature of bacterioruberin. Scavenging of DPPH• by bacterioruberin was a pseudo-first-order reaction with a rate constant k2 of 2.76 × 10–5 ± 0.001 µM−1 s−1 calculated at t = 0 or initial time and t = 30 min. The knowledge of the kinetics of bacterioruberin to scavenge DPPH• adds to its effective application as an antioxidant in medicinal use, pharmaceutical products and others. Additionally, the use of simple conventional method of DPPH• free radical scavenging, monitored using  an easily available laboratory spectrophotometer, will certainly help in the effective use of any antioxidant compound.

Manganese oxide nanocomposites attract huge attention in various biotechnological fields due to their extensive catalytic properties. This study reports an easy, rapid, and cost‐effective method of using the cell lysate of haloarchaeon, Haloferax alexandrinus GUSF‐1 for the synthesis of manganese oxide nanoparticles. The reaction between the cell lysate and manganese sulfate resulted in the formation of a dark brown precipitate within 48 h at room temperature. The X‐ray diffraction pattern showed the existence of Mn3O4 and MnO2 phases consistent with the JCPDS card no. (01‐075‐1560 and 00‐050‐0866). The dark brown colloidal suspension of MnO3‐MnO2 in methanol showed maximum absorption between 220 and 260 nm. The EDX spectrum confirmed the presence of manganese and oxygen. The Transmission electron microscopy revealed the spherical morphology with an average particle size between 30 and 60 nm. The magnetic moment versus magnetic field (MH) curve, at room temperature (300 K) did not saturate even at a high magnetic field (±3T) indicating the paramagnetic nature of the prepared nanocomposite. The Atomic Emission Spectroscopic analysis showed a negligible amount of soluble manganese (0.03 ppm in 50 ppm) in the Mn3O4‐MnO2 suspension suggesting the maximum stability of the material in the solvent over time. Interstingly, Mn3O4‐MnO2 nanocomposites evidenced antimicrobial activity in the order of Pseudomonas aeruginosa > Salmonella typhi > Escherichia coli > Proteus vulgaris > Candida albicans > Staphylococcus aureus. Conclusively, this is the first report on the formation of Mn3O4‐MnO2 nanocomposites using cell lysate of salt pan haloarcheon Haloferax alexandrinus GUSF‐1 with antimicrobial potential.