Radiation-resistant bacteria in desiccated soil and their potentiality in applied sciences

A rich diversity of radiation-resistant (Rr) and desiccation-resistant (Dr) bacteria has been found in arid habitats of the world. Evidence from scientific research has linked their origin to reactive oxygen species (ROS) intermediates. Rr and Dr. bacteria of arid regions have the potential to regulate imbalance radicals and evade a higher dose of radiation and oxidation than bacterial species of non-arid regions. Photochemical-activated ROS in Rr bacteria is run through photo-induction of electron transfer. A hypothetical model of the biogeochemical cycle based on solar radiation and desiccation. These selective stresses generate oxidative radicals for a short span with strong reactivity and toxic effects. Desert-inhibiting Rr bacteria efficiently evade ROS toxicity with an evolved antioxidant system and other defensive pathways. The imbalanced radicals in physiological disorders, cancer, and lung diseases could be neutralized by a self-sustaining evolved Rr bacteria antioxidant system. The direct link of evolved antioxidant system with intermediate ROS and indirect influence of radiation and desiccation provide useful insight into richness, ecological diversity, and origin of Rr bacteria capabilities. The distinguishing features of Rr bacteria in deserts present a fertile research area with promising applications in the pharmaceutical industry, genetic engineering, biological therapy, biological transformation, bioremediation, industrial biotechnology, and astrobiology.


Introduction
Radiation-resistant (Rr) and desiccation-resistant (Dr) bacteria can survive with exposure to radiation and desiccation cycles in natural habitats.Their resistance capabilities were confirmed during laboratory experimentation (Singh and Gabani, 2011;Paulino-Lima et al., 2013), while the radiosensitive bacterial cells immediately reduced in number with radiation exposure (Sghaier, 2011).Based on the wavelength, electromagnetic radiation comprises ionizing radiation (IR) and non-IR.The wavelength of IR falls below 100 nm and includes X-rays and gamma rays and is extremely hazardous because it produces ions in the cell.Other IR includes alpha and beta particles with undetectable wavelengths and high penetrating ability (Griffiths, 2020;Chaudhary and Kumar, 2023).Non-IR does not result in the formation of ions but includes ultraviolet radiation (UVR), whose wavelengths fall between 100 and 400 nm (Mba et al., 2012;Leszczynski, 2014).These Rr bacteria of desiccated soil evolved in response to photo-irradiation and can survive with exposure to radiation environments.Due to selective stresses that prevail in deserts, they are regarded as a unique environment for studying ecological diversity and function potentiality of Rr bacteria.In a desiccated environment, the photochemical production of ROS interlinks defensive pathways against radiation and desiccation, and bacterial communities can survive multiple stress conditions.
Desiccation is a typical feature of the desert's environment and an important factor that helps microbial cells evolve with resistant features.Microbial communities of deserts have resistant deoxyribonucleic acid (DNA), efficient proteomic systems, richness in metabolites, enzymes, and dissimilar pigmentation (Castillo and Smith, 2017;Orellana et al., 2020;Kanekar and Kanekar, 2022).Desiccated regions such as the Taklamakan Desert, Sonoran Desert, Sahara Desert, and Atacama Desert are extensively studied for their rich bacterial diversity with extreme resistance to radiation, desiccation, and oxidation (Sajjad et al., 2017;Belov et al., 2018;Guesmi et al., 2021;Liu et al., 2022).Radiation and desiccation are the main ecological factors affecting the microbial diversity of the desert ecosystem.However, researchers endeavor to reveal the actual factor affecting the evolved abilities of desert-borne bacteria.Scientists contemplate that the radiation tolerance found in bacterial species of desiccated regions is unusual.This is because the intensity of radiation survived by bacteria inhabiting deserts does not even reach the surface of the Earth.In particular, D. radiodurans exhibits an ability to resist radiation that is four times greater in magnitude, with no loss in cell numbers, than the highest recorded radiation on earth (Battista, 1997).Rr bacteria of the desert environment, especially D. radiodurans, resist 15,000 Gy of gamma, which is explained by its ability to scavenge ROS (Daly, 2006).ROS plays an intermediate role in interlinking resistance mechanisms against radiation and desiccation.
The evolved abilities of Rr bacteria have wide applications in applied sciences.Rr bacteria utilize the imbalanced charge radicals for their energy generation and consumption.As the redox cycle of Rr bacteria evolved and operated with the intensity of radiation, it might be with desiccation.These bacteria managed intracellular induction of electrons by their counter-response to these environmental stresses.The by-products of radiations and desiccation of bacterial cells in the form of charge radicals and their counter-response help in the evolved abilities (Daly, 2006).Rr bacteria also produce distinct pigmentation with absorbance and reflection of incoming radiation.These pigments are important for scavenging radicals produced by radiation and desiccation.Rr bacteria have an efficient antioxidant system, highlighting their importance in chemotherapy and curing lung diseases, the main obstacle of which is overcoming imbalanced radicals.Rr bacteria efficiently balance elevated radicals that are induced by radiation, desiccation, or any other stress factor (Latifi et al., 2009;Yoboue et al., 2014;Fagliarone et al., 2017;Han et al., 2020).Rr bacteria resist osmotic and acid stress temperature variation and have a pool of enzymes, making them a suitable candidate in fermentation biotechnology.In addition, Rr bacteria exhibit tolerance to heavy metals, playing a crucial role in the bioremediation of pollutants and the biodegradation of radioactive and radionucleotide waste (Luo et al., 2014;Beblo-Vranesevic et al., 2017;Chen et al., 2021;Nayak et al., 2021;Kumar et al., 2022).Furthermore, Rr bacteria produce various extremolytes, and their potential sunscreen ability is already commercialized and effectively utilized by skincare industries (Mendes-Silva et al., 2020).
Therefore, it is evident that the interaction of Rr and Dr. bacteria with various stresses is interesting to study further.Thus, the current review summarized the most recent knowledge about the origin, abundance, and diversity of Rr bacteria in response to radiation and desiccation.This article also covers the following topics: (i) desert environmental traits, (ii) a common antioxidant system by which Rr bacteria sustain in radiation and desiccated environment, and (iii) the potential application of Rr bacteria in the field of biotechnology, bioremediation, biological therapy, fermentation, energy, and pharmaceutical industry.
2 Environmental traits of deserts and their influence

ROS is elicited through radiation and desiccation
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, 2013Imlay, , 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).

Photochemical redox reactions
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 (O 2 ¯) 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).
3 Hypothetical models of Rr bacteria origin in deserts

A naturally occurring desiccation
The most accepted and main driving force for the origin of Rr bacteria is desiccation.Rr bacteria are naturally resistant to  (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).

A naturally occurring radiation
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 Radiation and desiccation initiate the origin of Rr and Dr. capabilities in deserts through the antioxidant system and repair mechanisms: Radiation and desiccation triggered the production of ROS species in bacterial cells.Due to their strong reactivity, ROS causes DNA damage, protein oxidation, and lipid peroxidation.Bacterial cells having no proper antioxidant system lose their viability and cells marked with evolved antioxidant systems survive with exposure to radiation, desiccation, and oxidation.
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.

Temperature, salinity, and Rr bacteria
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).

Geographical distribution of Rr bacteria
Rr bacterial communities are ecologically distributed in hot and cold deserts (Chanal et al., 2006;Shukla et al., 2007;Azua-Bustos et al., 2012;Musilova et al., 2015;Shirsalimian et al., 2018;Guesmi et al., 2022) Deinococcus thermus strains LD4 and LD5 were isolated from the Lut Desert of Iran (the hottest place on earth) and are resistant to a dose of >15 KGy of gamma radiation and > 600 j/m 2 of UV-C (Mohseni et al., 2014).Their extreme radiation resistance is explained by desiccation and temperature fluctuation during the day and night.Similarly, Hymenobacter xinjiangensis X2-1g T has been reported from the Xinjiang Desert, China, which can resist 8 KGy of gamma radiation from a 60 Co source at a dose rate of 10 Gy min −1 (Zhang Q. et al., 2007).Desertibacter roseus strain 2622 T has been reported from the Xinjiang Taklamakan Desert of China, which can survive at 10 KGy of gamma radiation at a dose rate of 300 Gy min −1 at room temperature (Liu et al., 2011).These species are ecologically dispersed, and the genus Deinococcus is widely found in the Deserts.For instance, 60 strains of this genus were isolated from the irradiated arid soil of the Sonoran Desert, including 9 strains identified for the first time (Rainey et al., 2005).In total, 14 Rr strains, such as species of Bacillus cereus/thuringiensis of phylum Firmicutes, have been reported from the Atacama Desert.These strains showed 300 j/m 2 of UV-C tolerance except for Bacillus strain S3.300-2, which showed a 10% survival rate even at the higher dose of 318 j/m 2 (Paulino- Lima et al., 2013).The details of Rr bacteria and their potential to resist radiation are described in Table 1.Due to harsh ecological elements prevailing in deserts and most commonly fluctuations of chemical and physical weathering, Rr strains have evolved to produce antiviral, antibacterial, and antifungal metabolites.As a result of rapid emerging resistance against the preexisting antibiotics, the search for new metabolites obtained from such extreme habitats is crucial to contain human and animal diseases (Gabani and Singh, 2013;Nayak et al., 2021).Rr bacteria are not only resistant to oxidation and radiation but survive with fluctuation of temperature and are categorized into mesophile, thermophile, and hyperthermophile subgroups (Ranawat and Rawat, 2017;Li et al., 2020;Marszalkowski et al., 2021).These bacteria are found in less non-arid habitats than in arid ones, as plenty of photochemical production of ROS in the desert ecosystem facilitates their origin, diversity, and abundance (Rainey et al., 2005).It is therefore concluded that Rr bacteria is found in soil either irradiated or desiccated and eventually grows by following a cycle of oxidative regulation (Schulze-Makuch et al., 2017).
5 Mechanism for ionizing radiation resistance 5.1 Copy number of DNA and nucleoid organization 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.

Enzymatic and non-enzymatic processes and proteins
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).

Radiation resistance mechanism against UV radiation
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).

Potentiality and diversity in applications of Rr bacteria
The Rr bacteria are under consideration in different domains (Figure 2).The extraordinary capabilities of Rr bacteria are defined by known features such as radiation resistance, regulation of ROS, antioxidation, DNA repair mechanism, efficient proteomic system, enzymes, and metabolites such as carotenoid pigments.These features of Rr bacteria are very effective in natural conditions.These natural traits of Rr bacteria can be manipulated and cloned by modern biotechnological approaches.Rr bacteria could be utilized for their importance in the pharmaceutical industry, lung disorders, chemotherapy, bioremediation, fermentation, radioactive waste management, and biodegradation of biomass to yield valued-added Low resistance (LR) refers to 10% survival at 1-5 KGy of gamma radiation and < 300 J m −2 UVC and < 1,000 J m −2 UV-B.Highly resistant (HR) refers to 10% survival at >10 KGy gamma radiation, >500 J m −2 UVC, and > 1,500 J m −2 UV-B.Intermediate resistance (IR) refers to 10% survival at 6-10 KGy of gamma radiation and 300-500 J m −2 UVC and 1,000-1,500 J m −2 UV-B. 10.3389/fmicb.2024.1348758 Frontiers in Microbiology 06 frontiersin.orgcompounds (Kumar et al., 2010;Guan et al., 2017;Nayak et al., 2021;He et al., 2022).

Antioxidant system of Rr bacteria
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 Cu 2+ , Mn 2+ , and Zn 2+ are important to balance free radicals in bacterial cells exposed to UV-B radiation (Santos et al., 2013).Rr bacteria regulate intracellular Cu 2+ , Mn 2+ , and Zn 2+ .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).

Non-enzymatic antioxidants and their applications
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 (H 2 O 2 ) singlet molecular oxygen ( 1 O 2 ), -OH, O 2 ¯, 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 O 2 ¯and H 2 O 2 .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 O 2 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).

Enzymatic antioxidants and curative measures of the lungs
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.

Pharmaceutical products
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 Pololike 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 Pros and cons of natural bacterial pigments and their applications.Khan et al. 10.3389/fmicb.2024.1348758Frontiers in Microbiology frontiersin.orgmediated 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.

Rr bacteria impact on cancer control
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.

Fermentation of organic waste
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).

Efficient expression system of Rr bacteria
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.

Energy conversion by Rr bacteria
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).

Radioactive waste management through Rr bacteria
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 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 radioactivecontaminated sites (Pan et al., 2009;Ma et al., 2011).
7 Gaps and future recommendations ROS regulation and common defensive pathways in Rr bacteria help to understand the origin of resistance against radiation, desiccation, and oxidation.Rr-and Dr-resistant bacteria found in deserts are important for understanding ROS-associated toxicities in human pathological and physiological conditions.The features of the antioxidant system found in the Rr bacterium can be inserted in human disorders, which are known by a disturbance in the regulation of the redox cycle.Rr bacterium regulates a high level of ROS due to Visible spectrum of solar radiation and characteristics of bacteria pigments with a range of wavelengths.Khan et al. 10.3389/fmicb.2024.1348758Frontiers in Microbiology 10 frontiersin.orgradiation exposure.The sensitivity of human cells to radiation underscores the knowledge of the fact that if bacteria can resist radiation, could human cells also evade doses capable of causing damage?Due to its radio-tolerant nature, the Rr bacterium also opens opportunities for degrading radioactive waste.Rr bacterium is a possible solution to generate useful energy in response to incoming photon flux and a platform for producing novel antimicrobial agents and useful metabolites of commercial significance.

Strength of the review
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.

Limitation of the review
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.

Conclusion
The deserts of the world are rich in Rr and Dr. bacteria.Deserts are mostly desiccated and irradiated.These ecological factors are evidence of the rich diversity of radio-tolerant bacteria.The higher resistance against radiation, desiccation, and oxidation is more evident in the link between ROS species.ROS plays a central role in the origin of a common defensive system, such as an antioxidant system of bacteria against radiation and desiccation.Deserts are home to ROS due to photodesiccated soil.These ROS are evident in the adaptation of bacterial communities in deserts.The antioxidant system regulates cellular stresses by scavenging free radicals among the bacterial community, which inhibits them in deserts.A rise of ROS in the cell damages cellular components and breaks DNA molecules, causing protein oxidation, lipid peroxidation, and abnormality in metabolism.ROS species are mediums that link the toxicity of desiccation to radiation.ROS induction through these stresses also activates counterresponses, which include DNA repair enzymes, an efficient proteomic system, distinct pigmentation, and metabolic and anti-scavenging pathways that reduce toxicity to cellular contents.The similar targets of radiation and desiccation through ROS and the common protection mechanism of Rr bacteria to combat these stresses explained their coevolution.The Rr bacteria in deserts tend to possess more enzymes and extremolytes of biotechnological importance.Due to their resistant nature, Rr bacteria could be efficiently utilized as an expression system in gene cloning.E. coli and Saccharomyces cerevisiae are platforms of choice for genetic engineering and biotechnological applications.Radioactive waste degradation at the contaminated site by utilizing the antioxidant potential of Rr bacteria.Khan et al. 10.3389/fmicb.2024.1348758Frontiers in Microbiology 11 frontiersin.org However, in recent years, due to specific ability to grow and express novel engineered functions, research has begun using Rr bacteria in biotechnologies and bioremediation.Rr bacterial species appear to have a distinct pigmented appearance.Most bacterial species bear blue, red, green, and yellow appearances, and researchers have studied them as reliable sources of natural colorants.Furthermore, more microbial extracts of these species are tested for their antioxidant ability in biomedical research (Sajjad et al., 2020).These bacterial species absorb incoming radiations, dissipate a certain amount in the form of heat, and remain transformed into photoelectric, electronic, and chemical based on their consumption and cellular needs.Rr bacteria is worth exploring due to its importance in pharmaceutical companies, radioactive site decontamination, industrial economic interest, genetic engineering, and biotechnological products.
is known that desiccation and radiation share a common target in a bacterial cell by causing the disintegration of the DNA molecules

FIGURE 2
FIGURE 2 Rr bacteria is under different considerations: (A) Rr bacterium in various sectors; (B) cellular factors influencing Rr bacteria; (C) external factors influencing Rr bacteria.

TABLE 1
Rr bacteria and their resistance capabilities: Rr bacteria isolated from various desiccated soil across the world.