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.

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Microbial synthesis of highly structured metal sulfide and metallic nanoparticles is a benign approach of nanomaterial synthesis. Various microbes have been exploited for nanoparticle synthesis, but nanofabrication using haloarchaea is still in nascent stages. Here, we report the intracellular synthesis of hexagonal needle-shaped tellurium nanoparticles with an aspect ratio of 1:4.4, by the haloarcheon Halococcus salifodinae BK3. The isolate was able to tolerate up to 5.5 mM K2TeO3. The yield of tellurium nanoparticles was highest when the culture was exposed to 3 mM K2TeO3, even though the isolate exhibited slightly decreased growth rate as compared to the culture growing in the absence of K2TeO3. The enzyme tellurite reductase was responsible for tellurite resistance and nanoparticle synthesis in H. salifodinae BK3. These tellurium nanoparticles exhibited anti-bacterial activities against both Gram-positive and Gram-negative bacteria, with higher antibacterial activity towards Gram-negative bacteria. This is the first report on the synthesis of tellurium nanoparticles by Halophilic archaea.

Nanobiotechnology is a multidisciplinary branch of nanotechnology which includes fabrication of nanosized materials using biological approaches. Highly structured metallic and metal sulfide nanoparticles have been reported to be synthesized by numerous bacteria, fungi, yeasts and viruses. However, biosynthesis of nanoparticles by Haloarchaea (salt-loving archaea) of the third domain of life, Archaea, is in its nascent stages. In this study, we report the intracellular synthesis of stable, mostly spherical silver nanoparticles (SNPs) by the haloarchaeal isolateHalococcus salifodinaeBK6. The isolate adapted to silver nitrate was found to exhibit growth kinetics similar to that of cells unexposed to silver nitrate. The nitrate reductase enzyme assay and the enzyme inhibitor studies showed the involvement of NADH dependent nitrate reductase in silver tolerance, reduction, and synthesis of SNPs. UV visible spectroscopy, XRD, TEM and EDAX were used for characterization of SNPs. The XRD exhibited characteristic Bragg peaks of face centered cubic silver with crystallite domain size of 26 nm and 12 nm for SNPs synthesized in NTYE and halophilic nitrate broth, respectively. TEM analysis exhibited an average particle size of 50.3 nm and 12 nm for SNPs synthesized in NTYE and halophilic nitrate broth (HNB), respectively. The as synthesized SNPs exhibited antimicrobial activity against both Gram positive and Gram negative organisms.

The fabrication of nanoparticles by microorganisms presents a "green" method for generating biocompatible nanomaterials. We discovered the intracellular biosynthesis of fluorescent lead(IV) sulfide nanoparticles by the moderate halophile, sp. strain PR58-8. The bacterium tolerated up to 8 mM Pb(NO) during growth. Non-protein thiols dose-dependently increased in response to metal exposure, which suggests they are involved in the growth of PbS crystals and lead detoxification. Using X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, and energy dispersive analysis of X-rays, the nanoparticles were characterized as spherical β-PbS nanoparticles (PbSNPs) with a tetragonal crystal lattice, a crystallite domain size of 2.38 nm, and an interplanar distance of 0.318 nm. A narrow symmetric emission spectrum with a Gaussian distribution and an emission maximum at 386 nm was obtained when the particles were excited at 570 nm. The PbSNPs exhibited a large Stokes' shift (8,362 cm) and a relatively high quantum yield (67%). These properties, along with fluorescence that was maintained in various microenvironments and their biocompatibility, make these nanoparticles excellent candidates for bioimaging. The particles were internalized by HeLa cells and evenly distributed within the cytoplasm, exhibiting their potential for bioimaging applications. The "as-synthesized" lead(IV) sulfide nanoparticles may provide expanded opportunities for targeted bioimaging via modifying the surface of the particles. This article reports the intracellular synthesis of fluorescent lead(IV) sulfide nanoparticles (PbSNPs) by a microorganism. All previous reports on the microbial synthesis of lead-based nanoparticles are on lead(II) sulfide that exhibits near-infrared fluorescence, requiring expensive instrumentation for bioimaging. Bioimaging using PbSNPs can be achieved using routine epifluorescence microscopes, as it fluoresces in the visible range. The research on PbS nanoparticles to date is on their chemical synthesis employing toxic precursors, extreme pH, pressure, and temperature, resulting in cytotoxic products. In this context, the synthesis of PbS nanoparticles by sp. strain PR58-8, described in this work, occurs at ambient temperature and pressure and results in the generation of biocompatible nanoparticles with no hazardous by-products. The excellent fluorescence properties that these particles exhibit, as well as their abilities to easily penetrate the cells and evenly distribute within the cytoplasm, make them exceptional candidates for bioimaging applications. This study demonstrated the synthesis and fluorescence bioimaging application of microbially synthesized PbS nanoparticles.

Nanobiotechnology is a multidisciplinary branch of nanotechnology which includes fabrication of nanomaterials using biological approaches. Many bacteria, yeast, fungi, algae and viruses have been used for synthesis of various metallic, metal sulfi de, metal oxide and alloy nanoparticles , since the fi rst report on biosynthesis of cadmium sulfi de quantum dots by Candida glabrata and Schizosaccharomyces pombe in 1989. These nanofactories offer a better size control through compartmentalization in the periplasmic space and vesicles, and are usually capped by stabilizing cellular metabolites. Halophiles depending on their salt requirements may be classifi ed as slight, moderate and extreme halophiles. They are found in marine and/or hypersaline environments. These organisms are known to encounter metals in their environment as the econiches they inhabit serve as ecological sinks for metals. Metal based nanoparticle synthesis by halophilic organisms is in its infancy and has only been reported in few organisms. This chapter aims to shed light on the various halophilic organisms and their by-products that have been exploited for nanomaterial synthesis, the mechanisms that may be involved in the nanomaterial fabrication and the possible applications of the fabricated nanoparticles. A special section would be dedicated for the bioavailability of metals to halophiles under varying salinity conditions.