Haloarchaea are the predominant microflora of hypersaline econiches such as solar salterns, soda lakes, and estuaries where the salinity ranges from 35 to 400 ppt. Econiches like estuaries and solar crystallizer ponds may contain high concentrations of metals since they serve as ecological sinks for metal pollution and also as effective traps for river borne metals. The availability of metals in these econiches is determined by the type of metal complexes formed and the solubility of the metal species at such high salinity. Haloarchaea have developed specialized mechanisms for the uptake of metals required for various key physiological processes and are not readily available at high salinity, beside evolving resistance mechanisms for metals with high solubility. The present paper seeks to give an overview of the main molecular mechanisms involved in metal tolerance in haloarchaea and focuses on factors such as salinity and metal speciation that affect the bioavailability of metals to haloarchaea. Global transcriptomic analysis during metal stress in these organisms will help in determining the various factors differentially regulated and essential for metal physiology.

Nanoparticles synthesis by bacteria and yeasts has been widely reported, however, synthesis using halophilic archaea is still in a nascent stage. This study aimed at the intracellular synthesis of selenium nanoparticles (SeNPs) by the haloarchaeon Halococcus salifodinae BK18 when grown in the presence of sodium selenite. Crystallographic characterization of SeNPs by X-ray diffraction, Selected area electron diffraction, and transmission electron microscopy exhibited rod shaped nanoparticles with hexagonal crystal lattice, a crystallite domain size of 28 nm and an aspect ratio (length:diameter) of 13:1. Energy disruptive analysis of X-ray analysis confirmed the presence of selenium in the nano-preparation. The nitrate reductase enzyme assay and the inhibitor studies indicated the involvement of NADH-dependent nitrate reductase in SeNPs synthesis and metal tolerance. The SeNPs exhibited good anti-proliferative properties against HeLa cell lines while being non-cytotoxic to normal cell line model HaCat, suggesting the use of these SeNPs as cancer chemotherapeutic agent. This is the first study on selenium nanoparticles synthesis by haloarchaea.

Numerous bacteria, fungi, yeasts and viruses have been exploited for biosynthesis of highly structured metal sulfide and metallic nanoparticles. Haloarchaea (salt-loving archaea) of the third domain of life Archaea, on the other hand have not yet been explored for nanoparticle synthesis. In this study, we report the intracellular synthesis of stable, mostly spherical silver nanoparticles (AgNPs) by the haloarchaeal isolate Halococcus salifodinae BK3. The culture on adaptation to silver nitrate exhibited growth kinetics similar to that of the control. NADH-dependent nitrate reductase was involved in silver tolerance, reduction, synthesis of AgNPs, and exhibited metal-dependent increase in enzyme activity. The AgNPs preparation was characterized using UV-visible spectroscopy, XRD, TEM and EDAX. The XRD analysis of the nanoparticles showed the characteristic Bragg peaks of face-centered cubic silver with crystallite domain size of 22 and 12 nm for AgNPs synthesized in NTYE and halophilic nitrate broth (HNB), respectively. The average particle size obtained from TEM analysis was 50.3 and 12 nm for AgNPs synthesized in NTYE and HNB, respectively. This is the first report on the synthesis of silver nanoparticles by haloarchaea.

No abstract