Desiccated soil exposed to photoradiation generates ROS species in the desert environment. ROS is also produced inside a cell in response to radiation and desiccation. These selective environmental stresses influence essential components and cellular processes of microbial cells directly or indirectly through ROS derivatives. These ROS are reactive, damage biological molecules with strong affinity, and interfere with cellular processes. ROS interferes with the genome, nucleotides, mitochondrial DNA, proteins, lipids, and the redox cycle of the target cell (Krisko and Radman, 2010; Daly, 2012; Imlay, 2013, 2015; Schieber and Chandel, 2014). Despite their toxicity on cellular contents, a high abundance of microbial communities have evolved ROS scavenging mechanisms with the ability to resist desiccation, radiation, and their derivatives. It is postulated that similar mechanisms work in both cell corrosion and cell protection through ROS intermediates (Figure 1).

ROS are produced in the cell and are also found in the surrounding desert environment. These ROS drive the abundance, diversity, and origin of the Rr and Dr. bacteria. ROS are generated by solar radiation and desiccation. Normally, photodesiccated soils accumulate peroxides and superoxide at higher levels than non-desert soils. A study highlighted evidence of superoxide (O₂^¯) and hydroxyl (-OH) production in desert soil (Georgiou et al., 2015). Nitrogen oxide (NOx) is released from the desiccated soil of the Mojave Desert at rates comparable to wet soil if the soil is irradiated by solar radiation. This shows the importance of radiation and desiccated soil in regulating ROS and reactive nitrogen species (RNS) (Austin and Vivanco, 2006; McCalley and Sparks, 2009). However, due to the short lifespan of reactive species and the lack of suitable techniques for their detection in the soil, these photochemical oxidative reactions and their associated mechanisms are not fully understood (Ito et al., 1985; Schneider et al., 2014; Halliwell and Gutteridge, 2015).

The most accepted and main driving force for the origin of Rr bacteria is desiccation. Rr bacteria are naturally resistant to desiccation. It is known that desiccation and radiation share a common target in a bacterial cell by causing the disintegration of the DNA molecules (Singh, 2018), while in response, Dr. and Rr bacteria also effectively recover the lesions of DNA caused by either desiccation or radiation. The similar target of radiation and desiccation in the cell and the similar mechanism of the cell to evade damaging effects in both conditions give a clear understanding that the desiccated environment of deserts is one of the important aspects of the rich diversity of Rr bacteria (Rainey et al., 2005). The hypothesis that Rr Bacterial community is a consequence of desiccation was previously studied. It was shown that resistance to a high radiation dose and desiccation is regulated through ROS scavenging system and efficient DNA repair mechanisms. It was shown that such mechanisms compensate for desiccation and also evolved to resist radiation (Makarova et al., 2001; Cox and Battista, 2005; Yu et al., 2015). To correlate Rr bacteria with desiccation, a phylogenetically diversified Rr bacteria belonging to Bacteroidetes, Proteobacteria, Deinococcus-Thermus, Firmicutes, and Actinobacteria were studied from the Taklimakan Desert (Yu et al., 2015). Interestingly, all Rr bacteria have the potentiality of carotenoid-like molecules, an efficient scavenger of ROS and reactive nitrogen species (RNS) (Asker et al., 2012). These ecologically distributed Rr bacteria and their associated ROS scavengers possess higher activities against desiccation and oxidation. Research on the desiccated environment of Taklimakan supports the hypothesis that the resistant phenotype is a consequence of the evolution of the ROS scavenging system, which defends cells against oxidative damage caused by desiccation (Yu et al., 2015).

The Rr bacteria at desiccated habitat balance radicals generated by radiation at the atomic level. This mechanism is based on the availability of free electrons with ROS species and their immediate balancing. These ROS species and their unstable revolving electron are hit by incoming radiation. These electrons are excited after they gain energy from the incoming photon; these electrons circulate freely in the cytoplasm and damage cellular structures, commonly DNA, lipids, and proteins. Due to available free electrons, ROS species are very reactive and damage biological and cellular structures (Atri et al., 2022). To balance charge particles caused by radiation, these ROS species have to lose or gain more charge ions either to build a new molecule or convert the toxic to a less toxic form. In simple words, this redox cycle is imbalanced by photon–electron interaction through ROS intermediates and balanced again by an antioxidant system of Rr bacteria. Due to this evolved feature, Rr bacteria are more resistant than radiosensitive cells (Tiquia-Arashiro and Rodrigues, 2016).

Photosynthetic cells also have a photo-induced electron cycle at the atomic level. The electrons derive excitation as soon as the incoming photons collide with the electrons in the antenna complex of the photosystem. These excited electrons are consumed in photophosphorylation (Kramer and Evans, 2011; Maróti et al., 2020). Photon energy from electromagnetic radiation transforms visible light into electrical energy and chemical energy (Chandra et al., 2018). Bacteria inhibiting in desiccated soil are naturally pigmented in response to radiation. Biologists assume that the Rr bacterial community of deserts drives this useful phenomenon at the cellular level in response to radiation. The radiation (photon–electrons) interaction or ionic interaction mediates the flow of charge radicals in the cell. Thus, it is concluded that radiation is a source of energy for physiological function and also the transformation of energy in the cell from one form to another (Shukla and Subba Rao, 2017), while a considerable amount of radiation converts to heat, as an answer is why the Rr bacterial community evades from a dose higher than the natural radiation.

The extreme Rr bacteria and rich diversity of desert-borne bacteria also have temperature dependency. At freezing temperature, the ROS produced due to radiation is restricted to diffuse, whereas, at room temperature, the ROS produced freely moves and disintegrates DNA molecules higher than ROS at freezing conditions. The gamma radiation resistance of D. radiodurans and isolated strains from the extreme cold desert of Antarctica Dry Valley was compared under varying temperatures. D. radiodurans withstand a high dose of gamma radiation at −79^°C when frozen on dry ice compared to room temperature (Dartnell et al., 2010). The IR resistance of microbes is also studied with correlation to salinity. There is no correlation between high Rr and Dr. bacteria with salt concentration (Shukla et al., 2007).

IR resistance in bacteria has focused on explaining how cells cope with DNA and protein damage and detoxify ROS (Villa et al., 2021; Rai and Dutta, 2024). DNA and protein damage are believed to have the most significant impact on cell survival. Increased DNA copy number has been positively correlated to increased IR resistance in E. coli (Krasin and Hutchinson, 1977) and Saccharomyces cerevisiae (Mortimer, 1958). Increased numbers of DNA copies provide enough genetic information that DNA repair systems can use to correct the damages caused by IR. Recombination, a process that bacteria frequently use to repair DNA double-strand breaks (DSBs), requires more than one copy of the DNA being repaired (Cox and Battista, 2005). The nucleoid of IR-resistant bacteria, including Rubrobacter radiotolerans and Deinococcus spp., appears to be more condensed than the genome of IR-sensitive species such as Thermus aquaticus and E. coli (Zimmerman and Battista, 2005). The condensation prevents DNA fragment diffusion and limits the post-irradiation activity of intracellular DNases, thus facilitating cell survival.

The oxidative stress caused by IR is prevented through enzymatic processes. The activity of the following proteins protecting against oxidative stress (peroxidases, catalases, and superoxide dismutases) relates to ROS accessibility and may be a contributive factor for IR resistance (Wang and Schellhorn, 1995; Tian et al., 2004). As for non-enzymatic processes, antioxidant molecules, including carotenoid pigments and intracellular salts, are involved in protection against oxidative stress. Novel proteins in D. radiodurans have been linked to IR resistance. Five novel protein genes are highly expressed and encoded proteins of unknown function in D. radiodurans following exposure to IR, such as DdrA, DdrB, DdrC, DdrD, and PprA. These proteins mediate Rec A-independent processes related to IR (Harris et al., 2004; Tanaka et al., 2004).

DNA damage caused by UV radiation is dependent on wavelength. UV-A (320 to 400 nm) induced only indirect damage to DNA, proteins, and lipids through ROS species intermediates. UV-B radiation (280 to 320 nm) and UV-C (100 to 280 nm) cause direct and indirect damage. The most common products formed by UV-B irradiation are cyclobutane pyrimidine dimers (CPD) (Mitchell and Karentz, 1993). Rr bacteria have numerous repair mechanisms in response to damage caused by UVR. These mechanisms are divided into photoreactivation and dark repair (DR). There are a total of three different dark repair mechanisms (i) nucleotide excision repair, (ii) error-prone repair, and (iii) postreplication recombinational repair (Fernández Zenoff et al., 2006).

The antioxidant system of Rr bacteria efficiently protects cells from internal and external stresses. The antioxidant system of Rr bacteria has both enzymatic and non-enzymatic pathways. The homeostasis of metal ions (Peana et al., 2018) further strengthens the antioxidant system of bacteria. Metal ions such as Cu²⁺, Mn²⁺, and Zn²⁺ are important to balance free radicals in bacterial cells exposed to UV-B radiation (Santos et al., 2013). Rr bacteria regulate intracellular Cu²⁺, Mn²⁺, and Zn²⁺. These ions reduce peroxide stress by blocking the Fenton and Haber–Weiss reactions (Bagwell et al., 2008; Daly et al., 2010). The cellular metabolism of Rr bacteria is least affected by environmental stresses with evolved antioxidant systems (McLean and McLean, 2010; Etemadifar et al., 2016). Rr bacteria with efficient antioxidant systems are less, vulnerable to protein oxidation, lipid peroxidation, and nucleic acid breaks with radiation, desiccation, and oxidation than radiosensitive bacteria (Sajjad et al., 2018).

Carotenoids are non-enzymatic antioxidants (Krinsky and Johnson, 2005; Bing Tian et al., 2007). Carotenoids are natural pigments, tetra-terpenoids including C40 hydrocarbon backbones (Carotenes) and their oxygenated derivatives xanthophylls. Carotenoids have a major role in radioresistance due to their efficient ROS scavenging ability (Yang, 2021). These ROS include hydrogen peroxide (H₂O₂) singlet molecular oxygen (¹O₂), -OH, O₂^¯, and RNS, such as 2,2-diphenyl-1-picrylhydrazyl (DPPH^•) and nitric oxide (NO) (Tian et al., 2009). Microorganisms and plants manufacture carotenoids and have wide applications (Figure 3). D. radiodurans and other bacterial species synthesize different types of carotenoids (Krinsky and Johnson, 2005; Asker et al., 2012). The 13 genes of D. radiodurans are involved in the biosynthesis of red color carotenoids (Saito, 2011). However, bacteria produce carotenoids of different colors. The pink, yellow, and red color appearance of Rr isolates from the Taklamakan Desert is due to the production of carotenoid-like molecules (Yu et al., 2015). A unique carotenoid deinoxanthin efficiently scavenges O₂^¯and H₂O₂. Deionxanthin prevents protein oxidation at lower concentrations and has potent scavenging ability than lycopene and alpha-carotene (Zhang L. et al., 2007). These pigments and metabolites are responsible for the photoprotection of cells against UV light in deserts (Yu et al., 2015). Carotenoid pigments are significant to human health. They are efficient quenchers of reactive O₂ and are linked to several diseases in humans, such as life-threatening cancer and other chronic disorders. The epidemiological data on carotenoid supplementation showed that ROS-mediated disorders are largely controlled by their consumption in a normal diet. Carotenoids regulate the balance of free radicals in a cell and are potential supplements for aging control, cancer prevention, and other beneficial aspects of human health (Burda, 2014; Rivera-Madrid et al., 2020).

Rr bacteria produce various enzymes, such as catalase (CAT) and superoxide dismutase (SOD), and some species also possess oxidases. These enzymes scavenge free radicals and ions. ROS detoxification is vital for the survival of the irradiated cell. Therefore, bacterial cells evolved to yield various enzymes to maintain the balance of ROS production and their removal. These enzymes are the first line of defense to protect microbial cells from radiation. D. radiodurans enzymatic antioxidants CAT and SOD are involved in the chemical conversion of ROS to less toxic substances (Markillie et al., 1999; Makarova et al., 2007).

Rr bacteria evolved with such efficient enzymatic pathways could, fortunately, serve as a supplement in the injured lung tissue. These enzymes can potentially detoxify harmful radicals caused by smoking or any other inhaled chemical inducer. A balance of oxidant and the antioxidant system is important in the cellular hemostasis of normal lung tissue, but physiological disturbance occurs when the antioxidant system fails to neutralize a high amount of ROS. These lung complications include chronic obstructive pulmonary disease (COPD) and asthma (Leibel and Post, 2016). Cigarette smoking causes nearly 90% of all lung cancer deaths and 80% of COPD, such as emphysema and chronic bronchitis. An innovative approach involving these enzymes is crucial to overcome lung disorders caused and worsened by a high amount of ROS. A competent Rr bacteria evolved with an enzymatic antioxidant system is important to control ROS toxicity in lung disorders, which might overcome radicals and prevent their entry into blood through air sacs and other body tissue. ROS induces oxidative damage to DNA, carbohydrates, lipids, and proteins, initiating an array of downstream processes promoting the progression of COPD in affected lungs. The genetic ability of Rr bacteria against oxidative damage could be engineered as probiotics supplementing to treat smoking lung disorders.

The extremolytes of Rr bacteria include mycosporine-like amino acids (MAAs), systonemin, bacterioruberin, ectoine, sphaerophorin, melanin, pannarin, and polyhydroxyalkanoates (PHA). These extremolytes reduce photodamage associated with UV radiation. The enhanced potential sunscreen ability of MAA is already commercialized and effectively utilized for skin care industries (De la Coba et al., 2009; Slaninova et al., 2018; He et al., 2022). Extremolytes such as asterina, palythine, palythene, and palythinol indicate anticancer and skin care products (Llewellyn and Airs, 2010). Scytonemin may act as an anti-inflammatory and antiproliferative drug to offer a novel pharmacophore for protein kinase inhibitors. Scytonemin has been shown as a competitive ATP inhibitor of Polo-like kinases (PLKs) (Murugan et al., 2011). PLKs have been the cancer target for many years because PLKs control many oncogenes. Previous studies have shown that scytonemin has a major role in hyperproliferative disorder because of its ability to inhibit PLK1 (Stevenson et al., 2002). Another research showed PLK1 inhibition mediated by scytonemin-induced apoptosis of cancer cell types such as osteosarcoma (Strebhardt and Ullrich, 2006; de Cárcer et al., 2007; Duan et al., 2010), suggesting that scytonemin is a promising anticancer agent. Similarly, ectoine also has therapeutic potential; for example, the human kidney keratinocytes irradiated with UV-A were effectively treated with ectoine. Ectoine has also been studied for skincare applications against desiccation, water loss, and UV damage. Due to water loss, ectoine is a prophylactic agent for dry skin (Buenger and Driller, 2004). Ectoine was also observed to prevent the damage associated with bacterial lipopolysaccharide (Buommino et al., 2005). Rubrobacter radiotolerans produce another compound, bacterioruberin, which prevents cancer and repairs the DNA breaks caused by IR in human cells.

Oxidative stress is linked with cancer, a physiological disorder. Rr bacteria scavenge imbalanced radicals and potentially prevent protein oxidation and DNA damage. D. radiodurans could be a model bacteria for cancer control because of their resistance to extreme radiation levels, desiccation, and other oxidative stress conditions (Slade and Radman, 2011; Krisko and Radman, 2013; Qi et al., 2020). High levels of ROS promote disease pathological conditions. These ROS stimulate the apoptosis of cancer cells and are efficiently utilized as a chemotherapeutic tool (Li et al., 2011; Redza-Dutordoir and Averill-Bates, 2016). Recently, a study highlighted the role of crude secondary metabolite extract (CSME) of D. radiodurans in triple-negative human breast carcinoma MDA-MB-231 cells. CSME-induced ROS production encourages nuclear membrane alterations with apoptotic destruction of MDA-MB-231 cells. D. radiodurans CSME upregulates apoptotic marker expression in breast cancer chemotherapy (Maqbool et al., 2020). Further study is required to explore the role of CSME as a bioactive compound for cancer control on a molecular level.

The Rr microbes that are utilized for the production of different useful compounds, for example, use the recombinant D. radiodurans for the fermentation of organic waste, particularly lignocellulosic biomass (Jiang et al., 2017) to produce value-added compounds. Using its extremophilic properties and resistance to many stresses, like heavy metal resistance, pollutants resistance, osmotic and acid stress, temperature, and the ability to resist DNA-damaging agents of this host will help in survival in fermentation along with a high expression of the desired pathway and high production. The engineering of this host for the pretreatment of lignocellulosic biomass can be coupled with physicochemical pre-treatments such as radiation and acid–alkali under different conditions (temperature, suspension pH, and nutrient availability).

Rr bacteria could also be utilized as an expression system to clone desired products. The deep genome study of D. radiodurans has provided insight into the genetic makeup for tolerating multiple stress conditions. The presence of different pathways and some unique genomics ability to uptake genetic material from outside has made this bacterium suitable for use in a different application through genetic engineering (Makarova et al., 2001). The introduction of novel bioremediation capabilities into D. radiodurans has been successfully achieved. Chromosomal integration and vector-based expression of foreign genes in trans have proved effective (Misra et al., 2012). Using such multiresistant microbes will help to overcome the limitations of the enzymatic processes, such as the high costs of the pure enzyme, the fragile nature of the enzyme, very narrow reaction conditions, the low half-life of the enzyme, and the need for exogenous cofactors. Instead of using regulators and genes from D. radiodurans (Pan et al., 2009), introducing the whole pathway in D. radiodurans can help increase the desired product’s expression in extreme conditions. Due to the remarkable DNA repair mechanism, engineering this host will help to design a process where other microbes are unable to survive due to the presence of DNA-damaging agents.

Rr bacteria have distinctive morphological pigmentation. Rr bacteria abundantly display red, yellow, pink, orange, or simultaneous contrast such as red-orange or yellow-green characteristics colors. These features and colors are inclined from one trait to another (Kreusch and Duarte, 2021; Liu et al., 2022). Most of the reflected colors match the wavelength specified for the respective range of the visible light spectrum. This raises an interesting scientific query to investigate whether color rich diversity of the arid habitats evolved through the influence of different wavelengths of electromagnetic radiation (Figure 4). The characteristic color of bacteria is due to the reflected spectrum of radiation, which cannot be absorbed by a cell. Meanwhile, the absorbed radiation is a driving force for cellular processes, energy generation, and consumption from raised ROS species. A considerable amount of radiation also dissipates as thermal energy in the form of heat (Castillo et al., 2015; Makhneva et al., 2020). Bacteriochlorophylls (B) Chls and carotenoids are bound non-covalently to specific apoproteins, as the main light-harvesting and energy-transforming pigments of photosynthetic organisms. B Chls are vital components of the photochemical reaction centers and account for most of the antenna pigments in anoxygenic photosynthetic bacteria. Due to electromagnetic exposure, the excitation reaching the photochemical reaction centers of B Chls at the lowest singlet excited state causes initial charge separation. Thus, the conversion of light energy into electrochemical energy is initiated and ultimately provides a driving force for all essential processes in photosynthetic and heterotrophic organisms (Renger, 2007).

The world is concerned about discharging radioactive waste from anthropogenic sources before it is converted to less hazardous or fully non-hazardous form. Recently, alarming news about Fukushima tritium discharge in the marine ecosystem was circulated. This raised responsiveness about the risk to aquatic life and the food chain it disturbs. Reconsidering and global management of both natural and anthropogenic reservoirs of tritium discharge in the ocean and its impact on aquatic life is necessary. The insight of earlier studies has revealed the effects of the tritium on natural biota and the potential risk to human health it poses through the food chain (Galeriu et al., 2008; Melintescu and Galeriu, 2011; Zhao et al., 2021). Tritiated water has 99% of tritium, rapidly reaching the ocean through precipitation, river runoff, and evaporation (Liger et al., 2018; Oms et al., 2019). Three main reactors are located on the shore of Canada’s Great Lakes with a maximum level of tritium (8.4 Bq L − 1) greater than twofold as offshore waters (3.5 Bq L − 1). Non-nuclearized coastal rivers in France have a tritium concentration of 3 to 4 Bq L − 1, but in Rhone River, this concentration varied from 2.50 to 12.85 Bq L − 1 with a mean of 6.31 Bq L − 1, and this is because of high nuclear reactors (Jean-Baptiste et al., 2018). The prevalence of tritium in oceans and other contaminated sites is important to conserve aquatic biota and to reduce its potential risk to human health. This would not been possible without innovative ways. Tritium releases negative charge beta particles from radioactive decay and causes cellular damage if inhaled or ingested (Roch-Lefèvre et al., 2018). Humans acquire a considerable amount of organically bound tritium from aquatic biota, mainly through fishes that rely on zooplankton and phytoplankton for their food cycle. The evolved features of bacterial cells that receive and transform harmful charge radicals into a thermal spectrum are fascinating. This could be utilized to balance charge radicals produced through radioactive decay by introducing its consortium into radioactive sites. Rr bacteria could transform radioactive contents into a non-hazardous form or possibly energy exchange (Figure 5). Another approach is E. coli, and Saccharomyces cerevisiae are genetic engineering platforms of biotechnological applications. However, due to the specific ability to grow and express novel engineered functions in recent years, research has begun using D. radiodurans in biotechnologies (Basu, 2022) and bioremediation (Manobala et al., 2019). Toxic waste management is achieved by successful gene transfer, and expression is reported in D. radiodurans for bioremediation of nuclear radioactive and heavy metal-polluted environments (Shukla et al., 2017; Khan et al., 2021). Using D. radiodurans regulators in yeast and bacteria enhances its activity, and production is successful in fermentation. These IR-resistant bacteria have achieved their target for radioactive waste degradation and could be utilized for radioactive-contaminated sites (Pan et al., 2009; Ma et al., 2011).

This review article has mainly emphasized the origin, applications, and geographical distribution of Rr bacteria in extreme desert environments. Their by-products, such as enzymes and non-enzymatic antioxidants, were linked to disease pathological conditions. Desert environmental traits such as the photochemical production of ROS in desiccated soil and their impact on radiation resistance have been highlighted. The review article also summarized Rr bacteria from different geographical locations, and data were collected on their survival potential in varying intensities of electromagnetic radiation. Furthermore, the importance of Rr bacteria was evaluated in several sectors, of which microbial biotechnology, bioremediations, fermentation, the pharmaceutical industry, and biological therapy are mainly described.

Extensive research is further required to evaluate the intermediate role of ROS, desiccation, and radiation in influencing the abundance, diversity, and evolved capabilities of Rr bacteria. An antioxidant system of Rr bacteria must be carefully studied to investigate its role in cancer therapy, lung disorders, and the importance of bioremediation of radioactive waste.