The production of pigments by halophilic archaea has been analysed during the last half a century. The main reasons that sustains this research are: (i) many haloarchaeal species possess high carotenoids production availability; (ii) downstream processes related to carotenoid isolation from haloarchaea is relatively quick, easy and cheap; (iii) carotenoids production by haloarchaea can be improved by genetic modification or even by modifying several cultivation aspects such as nutrition, growth pH, temperature, etc.; (iv) carotenoids are needed to support plant and animal life and human well-being; and (v) carotenoids are compounds highly demanded by pharmaceutical, cosmetic and food markets. Several studies about carotenoid production by haloarchaea have been reported so far, most of them focused on pigments isolation or carotenoids production under different culture conditions. However, the understanding of carotenoid metabolism, regulation, and roles of carotenoid derivatives in this group of extreme microorganisms remains mostly unrevealed. The uses of those haloarchaeal pigments have also been poorly explored. This work summarises what has been described so far about carotenoids production by haloarchaea and their potential uses in biotechnology and biomedicine. In particular, new scientific evidence of improved carotenoid production by one of the better known haloarchaeon (Haloferax mediterranei) is also discussed.

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Carotenoids from Haloarchaea and their potential in biotechnology

Dye-sensitized solar cells based on bacterial-based photosensitizers (bio-sensitized DSSCs) are promising bio-photoelectronic molecular devices exhibiting enhanced electron excitation, injection, and dye regeneration for efficient photon-to-electron quantum-conversion. Achieving high DSSCs performance via environmentally sustainable, cost-effective, and naturally-sensitized plant-based or bacterial-based biomolecules remains a challenge. Here, we provide a comprehensive study on the mechanisms involved in the utilization of biomolecular bacterial-based pigments (e.g. proteins and carotenoids) for an improved bio-sensitized DSSCs performance. Protein complexes and chlorophyll a/carotenoids are among many bio-photosensitizers demonstrating high incident photon-to-current efficiency (IPCE). Pigments molecular structure, donor-π-acceptor conjugation, and anchoring groups have been discussed and attributed to theoretical dye HOMO–LUMO bandgaps and their corresponding bio-sensitized DSSCs IPCE. This review provides critical understanding of advancements towards natural photosensitization: (i) carboxyl/hydroxyl groups attached to acceptor segments provide firm attachment and rapid electron injection, (ii) proteins/carotenoids hybrid dyes induce visible-light photosensitivity and broaden absorbance, (iii) increased conjugated π -bonds (n > 13) develop pigment visible-NIR absorption with intensified photoactivity, (iv) chromatophores integrated with bio-electrolyte provide a unidirectional flow of electrons, (v) reaction center (RC)-sensitized DSSCs have better optoelectronic properties than light-harvesting complex (LH2) due to its efficient charge separation, (vi) antioxidants hinder degradation of pigmented-photoanodes from UV radiation, (vii) solid-state redox improves device stability and dye neutralization; which all together would boost the dye sensitization performance in bio-sensitized DSSCs. The highest recorded IPCEs are found for TiO2-based DSSCs using plant-based coumarin (9%) and from [A. amentacea + P. pterocarpum] pigments (8.22%). Futuristically, we anticipate that these biologically-derived photosensitizers can be integrated into photoanodes for photoelectronic applications including DSSCs, multi-junction cells, photodiodes, phototransistors, photodetectors, flexible bioelectronic films and clothes, bio-LEDs, and photo-tunneling junctions.

Biomolecular photosensitizers for dye-sensitized solar cells: Recent developments and critical insights

Halophilic archaea push the limits of life at several extremes. In particular, they are noted for their biochemical strategies in dealing with osmotic stress, low water activity and cycles of desiccation in their hypersaline environments. Another feature endemic to their habitats is intense ultraviolet (UV) radiation, which is a challenge that microorganisms must overcome. The consequences of high UV exposure include DNA lesions arising directly from bond rearrangement of adjacent bipyrimidines, or indirectly from oxidative damage, which may ultimately result in mutation and cell death. As such, these microorganisms have evolved a number of strategies to navigate the threat of DNA damage, which we differentiate into two categories, DNA repair and photoprotection. Photoprotection encompasses damage avoidance strategies that serve as a “first line of defense,” and in halophilic archaea include pigmentation by carotenoids, mechanisms of oxidative damage avoidance, polyploidy, and genomic signatures that make DNA less susceptible to photodamage. Photolesions that do arise are addressed by a number of DNA repair mechanisms that halophilic archaea efficiently utilize, which include photoreactivation, nucleotide excision repair, base excision repair, and homologous recombination. This review seeks to place DNA damage, repair, and photoprotection in the context of halophilic archaea and the solar radiation of their hypersaline environments. We also provide new insight into the breadth of strategies and how they may work together to produce remarkable UV-resistance for these microorganisms.

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DNA Repair and Photoprotection: Mechanisms of Overcoming Environmental Ultraviolet Radiation Exposure in Halophilic Archaea.

Carotenoids are essential for photosynthesis and photoprotection in photosynthetic organisms and beneficial for human health. Apocarotenoids derived from carotenoid degradation can serve critical functions including hormones, volatiles, and signals. They have been used commercially as food colorants, animal feed supplements, and nutraceuticals for cosmetic and pharmaceutical purposes. This review focuses on the molecular evolution of carotenogenic enzymes and carotenoid cleavage oxygenases (CCOs) from bacteria, fungi, cyanobacteria, algae, and plants. The diversity of carotenoids and apocarotenoids as well as their complicated biosynthetic pathway in different species can shed light on the history of early molecular evolution. Some carotenogenic genes (such as phytoene synthases) have high protein sequence similarity from bacteria to land plants, but some (such as phytoene desaturases, lycopene cyclases, carotenoid hydroxylases, and CCOs) have low similarity. The broad diversity of apocarotenoid volatile compounds can be attributed to large numbers of carotenoid precursors and the various cleavage sites catalyzed by CCOs enzymes. A variety of carotenogenic enzymes and CCOs indicate the functional diversification of carotenoids and apocrotenoids in different species. New carotenoids, new apocarotenoids, new carotenogenic enzymes, new CCOs, and new pathways still need to be explored.

Carotenoids biosynthesis and cleavage related genes from bacteria to plants.

Carotenoids are pigments that may be used as colorants and antioxidants in food, pharmaceutical, and cosmetic industries. Since they also benefit human health, great efforts have been undertaken to search for natural sources of carotenoids, including microbial ones. The optimization of culture conditions to increase carotenoid yield is one of the strategies used to minimize the high cost of carotenoid production by microorganisms. Halophilic archaea are capable of producing carotenoids according to culture conditions. Their main carotenoid is bacterioruberin with 50 carbon atoms. In fact, the carotenoid has important biological functions since it acts as cell membrane reinforcement and it protects the microorganism against DNA damaging agents. Moreover, carotenoid extracts from halophilic archaea have shown high antioxidant capacity. Therefore, current review summarizes the effect of different culture conditions such as salt and carbon source concentrations in the medium, light incidence, and oxygen tension on carotenoid production by halophilic archaea and the strategies such as optimization methodology and two-stage cultivation already used to increase the carotenoid yield of these microorganisms.

Carotenoid Production by Halophilic Archaea Under Different Culture Conditions

Haloarcula japonica, an extremely halophilic archaeon that requires high concentrations of NaCl for growth, accumulates the C50 carotenoid bacterioruberin (BR). By homology analysis, a gene cluster, including c0507, c0506, and c0505, was found and predicted to be involved in the synthesis of bacterioruberin. To elucidate the function of the encoded enzymes, we constructed Ha. japonica mutants of these genes and analyzed carotenoids produced by the mutants. Our research showed that c0507, c0506, and c0505 encoded a carotenoid 3,4-desaturase (CrtD), a bifunctional lycopene elongase and 1,2-hydratase (LyeJ), and a C50 carotenoid 2″,3″-hydratase (CruF), respectively. The above three carotenoid biosynthetic enzymes catalyze the reactions that convert lycopene to bacterioruberin in Ha. japonica. This is the first identification of functional CrtD and CruF in archaea and elucidation of the complete biosynthetic pathway of bacterioruberin from lycopene. IMPORTANCEHaloarcula japonica, an extremely halophilic archaeon, accumulates the C50 carotenoid bacterioruberin (BR). In this study, we have identified three BR biosynthetic enzymes and have elucidated their functions. Among them, two enzymes were found in an archaeon for the first time. Our results revealed the biosynthetic pathway responsible for production of BR in Ha. japonica and provide a basis for investigating carotenoid biosynthetic pathways in other extremely halophilic archaea. Elucidation of the carotenoid biosynthetic pathway in Ha. japonica may also prove useful for producing the C50 carotenoid BR efficiently by employing genetically modified haloarchaeal strains.

Nobuhiro Miyoko

Papers

Complete Biosynthetic Pathway of the C50 Carotenoid Bacterioruberin from Lycopene in the Extremely Halophilic Archaeon Haloarcula japonica