Extremophile environments are important for understanding the limits of life, with applications in biotechnology, and for informing astrobiology and space exploration. There are different types of extremophilic ecosystems, such as hypersaline ecosystems, which are spread worldwide. Hypersaline environments include ecosystems showing salt concentrations higher than those of seawater (3.5% w/v in seawater vs. up to 35% w/v in brines). Some of the most representative hypersaline ecosystems that occupy vast geographical extensions include salterns, inland lakes, lagoons, marshes, and coastal/inland salty ponds (also termed salt flats or salt pans). In brines or soils of these ecosystems characterised by the presence of very high salt concentrations (between 2 and 4 M NaCl or equivalents), moderate or extreme halophilic microorganisms of the archaea domain (haloarchaea, families Halobacteriaceae and Haloferacaceae) constitute the predominant microbial communities, all together with the bacteria of the genus Salinibacter (which is the most abundant) and the unicellular green algal genus Dunaliella.

As a general feature, extreme halophilic microorganisms have evolved to adapt and optimise their molecular machineries to the different biotic and abiotic factors characterising hypersaline ecosystems, most of which cause cellular and molecular stresses in most living beings (high UV radiation, high ionic strength, low water and oxygen availability, nutrient scarcity, etc.). To be isosmotic with their surroundings, these microorganisms can follow one of the following two strategies: the “salt-in” strategy, where they accumulate high concentrations of intracellular potassium ions to match external salinity, and the “osmolyte” strategy, where they accumulate organic compounds like ectoine and glycine betaine to balance the osmotic pressure. Because of these molecular and physiological adaptations, most biomolecules of halophilic microorganisms exhibit unique biochemical parameters and behaviours that distinguish them from their non-halophilic counterparts; examples include the following: (i) higher content of acidic amino acids in protein surface (aspartate, glutamate) and short, polar amino acids that enhance solubility in high salt environments. They have a lower proportion of bulky, hydrophobic amino acids; (ii) a higher number of G-C pairs in DNA molecules, which increases their thermal and chemical stability; and (iii) specialised ion pumps for salt regulation.

Consequently, thanks to these molecular differences, biomolecules from halophilic microorganisms, like enzymes, proteins, exopolysaccharides, lipids, and carotenoids, are promising targets from a biotechnological perspective because they offer some advantages or benefits compared to their non-halophilic homologues. In this context, many studies recently reported that using haloarchaea as model microorganisms for biotechnology has confirmed that they can be easily used as cell factories to upscale processes aiming at the production of marketed biomolecules like biopolymers (mainly polyhydroxyalkanoates), enzymes (that show the highest catalytic activity at high ionic strength and high temperatures), antimicrobials (most of them small peptides, like halocins), carotenoids (natural pigments), compatible solutes (like ectoine), lipids, antiadhesives, and biofuels. Related to potential applications in pharma industries and in biomedicines of biomolecules synthesised by haloarchaea, C₅₀ carotenoids called bacterioruberin (BR), bisanhydrobacterioruberin (BABR), and monoanhydrobacterioruberin (MABR) stand out for their biological activities, from which positive effects on human health have been recently reported.

Most of the works described here correspond to those reported by using haloarchaea as a natural source of BR, BABR, and MABR, but research conducted with BR from bacterial strains has also been integrated in this review. Overall, these carotenoids are promising natural biomolecules with potential applications in the design and development of new approaches and/or pharmaceutical formulations to combat various human diseases or metabolic disorders.

As the studies on the potential uses of BR in biomedicine progress promisingly, a new innovative research question arises as a challenge in connection to BR-based formulations and their delivery to support therapeutic applications. In this context, the use of nanomaterials (mainly nanovesicles, micelles, or liposomes, but also green metallic nanoparticles) is revealed as the best strategy to mobilise and deliver BR in human cells and tissues. A recent study has developed an inhalable polymeric-lipid nanocapsule formulation with mucus-penetrating properties to co-encapsulate Pirfenidone (Pfd) and BR, aiming to enhance anti-inflammatory, antioxidant, and antifibrotic effects by modulating pulmonary macrophage activity. The nanocapsules with immobilised Pfd and BR (pNC-BR-Pfd) preserved their physicochemical properties after nebulisation using a vibrating mesh nebuliser. The results demonstrated that pNC-BR-Pfd capsules reduced intracellular reactive oxygen species and suppressed IL-6 and TNF-α levels by approximately 3-fold at 10 μg/mL Pfd and 0.64 μg/mL BR, whilst free Pfd only partially inhibited IL-6 and did not affect TNF-α.

Other studies have successfully achieved the synthesis of nanovesicles embedding BR, demonstrating that the biological activity and stability of BR improve substantially. Regarding these formulations, a type of nanovesicles termed TA-nanoarchaeosomes (which are made of polar archaeolipids) and Tween 80 (TA: polar archaeolipids (PAs): Tween 80, 5:5:4 w:w:w, TA-nanoARC) was used as carriers of BR to explore the effect of those complexes on commercial cells that are representative of human lung epithelial cells (A549 cell line) and human monocytic cells (THP-1 cell line). The results demonstrated that the nanovesicles were stable to storage and nebulization. In addition, TA-nanoARC showed cytotoxic effect on A549 cells after 48 h, with an IC₅₀ of 0.15 μg/mL (~0.20 µM). Such cytotoxicity was exerted at a concentration harmless to mTHP-1 cells. In addition, the conditioned medium from TA-nanoARC nebulised on A549 cells reduced the expression of the CD204/SRA-1, an M2 phenotype marker, and induced pro-inflammatory activity, comparable or even greater than that induced by lipopolysaccharide, including IL-6 and TNF-α, in mTHP-1 as a model of tumour-associated macrophages.

The main conclusion from this work pointed out the internalisation of the highly viscous and ordered TA-nanoARC rich in neutral archaeolipids and subsequent lysosomal dysfunction (and not its antioxidant activity), as responsible for the negative impact on A549 cells. These nanostructures based on archaeolipids as carriers have also been used for topical administration of vitamin D3 and BR in the treatment of psoriasis. The results demonstrated anti-inflammatory and antioxidant activity of the nanostructures (which were stable over time and showed good occlusion capacity and spreadability) on bi-cellular spheroids, consisting of a fibroblast core surrounded by a ring of keratinocytes activated with imiquimod (IMQ) as a psoriasis model. The nanostructures did not decrease the viability of spheroids but significantly reduced the release of pro-inflammatory cytokines (IL-8, IL-6, and TNF-α), ROS, and matrix metalloproteinases from IMQ-induced spheroids to levels similar to the uninduced spheroids. In a more recent work, the same authors have improved these nanostructures (macrophage-targeted nanostructured archaeolipid carriers made of a compritol, C₅₀dipolar carotenoid bacterioruberin (BR), and vitamin D3 (VD3) core, covered by sn 2,3 ether-linked polar archaeolipids extracted from the haloarchaeon Halorubrum tebenquichense and Tween 80) in an effort to reduce the use of topical corticosteroids in psoriasis and atopic dermatitis. Results obtained clearly justify the potential use of these nanocomplexes as a promising topical anti-inflammatory agent with wound healing and antimicrobial activities that deserve future in vivo exploration.

Metal oxide nanoparticles (NPs) have also been described as nanomaterials useful to carry carotenoids, including BR. In 2015, the immobilisation of BR on nanomaterials (together with the immobilisation of the protein bacteriorhodopsin) was described for the first time. On this occasion, these two biomolecules were used as sensitisers by immobilising them on nanoporous titanium dioxide films successfully and employing them as molecular sensitisers in dye-sensitised solar cells with efficient photocurrent generation as an alternative to silicon crystalline solar cells. A few years later, this research line reported new results on the optimisation of green synthesis of TiO₂ NPs in combination with the immobilisation of BR and other carotenoids to improve photoelectric conversion efficiently. The results revealed the highest photoelectric conversion efficiency (η) of 0.44%, which was almost as good as natural dye-sensitised solar cells. Insofar as these nanoparticles with immobilised BR have proven stable and with solar sensitised activity, it would be feasible to think of NPs as carriers of BR so that their administration (perhaps by topical injection and/or dermal applications) could be more efficiently localised and controlled.