Recent advancements of ‘scytonemin’ and its potential to sustainable and green world

Sunita Kumari Singh ¹ , Pratibha Singh²

¹Department of Botany, Jai Prakash University, Chapra, Bihar, India.

²Department of Botany, University of Allahabad, Prayagraj, India.

Corresponding Author Email: sunita251@gmail.com

DOI : https://doi.org/10.51470/ABP.2023.02.03.30

Abstract

Keywords

Download this article as:

Introduction

Cyanobacteria, the earliest Gram-negative prokaryotes, emerged during the Precambrian era and have pivotal role in oxygen evolution, fostering the development of diverse life forms [1]. However, contemporary challenges such as anthropogenic atmospheric pollutants and ozone depletion have intensified UV radiation reaching Earth’s surface, posing significant threats to cyanobacteria. To counteract these challenges, cyanobacteria produce photo-protective compounds, including scytonemin [2-3]. Scytonemin is synthesized within the extracellular polysaccharide sheath of approximately 300 cyanobacterial species was first identified by Nägeli in certain terrestrial cyanobacteria[4]. This matrix comprises heteroglycans, peptides, proteins, DNA, and various secondary metabolites.Dark yellow to brown coloration of the sheathis due to the deposition of thislipid-soluble yellow-brown pigment(Figure 1) [5].

Its protective role against harmful UV irradiation and facilitating adaptation to challenging environments is demonstrated by the experiments conducted in Nostoc flagelliforme, found in the upper layers of microbial mat communities exposed to high solar irradiance [6]. Itis approximately 5 % of the cellular dry weight of the culture [7]. Extraction methods involve the use of 100% acetone, with subsequent re-cultivation of Lyngbya sp. allowing for extraction after three weeks [8-12].

Research suggests that scytonemin synthesis is influenced by various environmental factors, including hydration periods, nitrogen availability, salt stress, UV radiation (particularly UV-B), high light intensity, and temperature [13-15]. For instance, longer hydration periods between desiccation cycles in Nostocpunctiforme promote higher scytonemin synthesis. Conversely, periodic desiccation inhibits scytonemin synthesis in Chroococcidiopsis, with nitrogen restriction leading to increased production [16-19]. While the exact mechanism of scytonemin induction remains unclear, it is evident that multiple environmental signals modulate its levels in different cyanobacterial species. Scytonemin serves as a crucial defense mechanism for cyanobacteria against environmental stressors, highlighting its importance in microbial adaptation and survival strategies [20-22].

MALDI-TOF MS analysis, demonstrated thatscytonemin composed of indolic and phenolic subunits, possesses a molecular mass of 544 Da and its molecular formula is C₃₆H₂₀N₂O₄ [23-25]. This analysis revealed characteristics identical to the oxidized state of scytonemin. Its IUPAC name is (3E,3’E)-3,3’-Bis(4-hydroxybenzylidene)-1,1’-bicyclopenta[b]indole-2,2’(3H,3’H)-dione [26-27]. The linkage between the subunits of scytonemin occurs at an olefinic carbon atom, which is a exclusive feature among natural compounds [28-34], defining a novel ring system termed ‘the scytonemin skeleton’ [35-36].

It exists in both oxidized (fuscochlorin,green) and reduced (fuscorhodin,red) forms. Additionally, Scytonema sp. has yielded derivatives such as dimethoxyscytonemin, tetramethoxyscytonemin, and scytonin (Figure 2). Purified scytonemin exhibits peak absorption at 380 nm and is identifiable in cyanobacteria via MALDI-TOF MS, with its absorption spectrum covering the UVC-UVB-UVA-violet-blue spectral range [37]. Analysis of scytonemin commonly employs UV-absorbance, HPLC, and Raman spectroscopy techniques, facilitating precise characterization and quantification in various biological contexts and enhancing our understanding of its roles and attributes within cyanobacteria [38-39].

Scytonemin serves as a protective shield for cyanobacteria against UV radiation, essentially acting as a sunscreen [25]. This protective function extends to cyanobacterial lichens like Collema, Gonohymenia, Petulla, etc., shielding them from high radiation levels [32]. Its role as a UV shield was studied in the terrestrial cyanobacteriumChlorogloeopsis sp. [33]. Nostoc punctiforme cells, remained intact even after 2 months of constant exposure to UV-A radiation, demonstratedthe remarkable stability of this metabolite [16]. In Lyngbyasp. CU2555, with minimal impact on its absorption properties,it exhibits environmental stability against various stress [34].

Some scientists have reported the preservation of scytonemin in sedimentary lakes [35], while others have noted its abundant preservation in deep sea sediments, indicating its resilience and resistance to degradation during erosion and transport [36].Consequently, scytonemin stands as a significant biomarker in paleoclimatological reconstructions and terrestrial extreme environments [37-39].

Scytonemin biogenesis and its transcriptional regulation

Scytonemin, a heterocyclic indole-alkaloid compound is synthesized from tryptophan and tyrosine derivatives, both of which absorb UV-B radiation. The genes for scytonemin biosynthesis are present as a single operon, comprising Scy genes (core genes), Ebo genes (responsible for transporting an intermediate product into the periplasm for final assembly), and additional genes involved in precursor synthesis from tryptophan. Within the N. punctiforme genome, NpR1276 to NpR1259 genes, have been identified which are associated with scytonemin biosynthesis.Six conserved genes within this cluster, NpR1276 to NpR1271 (ScyA to ScyF), have a substantial role in scytonemin biosynthesis. The process occurs in three modules: Module I (ScyABCDEF) catalyzes the formation and oxidation of the scytonemin monomer, while Module II (NpR1270-NpR1259) translocates the monomer to the periplasm. Module III (EboABCEF) facilitates this translocation process (Figure 3).

ScyA (NpR1276) initiates synthesis by coding for acetolactate synthase, which condenses pyruvate molecules. Oxidative deamination of L-tryptophan, yielding indole-3 pyruvic acidiscatalysed by ScyB (NpR1275). Subsequent steps involve cyclization, decarboxylation, and monomer dimerization to form scytonemin (Figure 4). While ScyD and ScyF may not be essential, they likely contribute to scytonemin synthesis. Moreover, NpR1270 (TryP), a copper monooxygenase, is crucial for tyrosine oxidation, a pivotal step in scytonemin biosynthesis. Additionally, genes NpF5232 to NpF5236 are associated with scytonemin biogenesis and are upregulated under UV-A radiation.

Transcriptional studies in N. punctiforme reveal that UV-A radiation upregulate scytonemin biosynthesis genes leading to the synthesis of tryptophan and p-hydroxyphenylpyruvate monomers, which undergo processing in the cytoplasm before being transported to the periplasm for further enzymatic reactions, resulting in the formation of thescytonemin (reduced form). Once secreted into the extracellular matrix, scytonemin blocks incoming UV-A radiation, thereby regulating gene expression and halting further scytonemin synthesis. It is suggested that a type IV secretion system is involved in secreting scytonemin to the extracellular matrix. These mechanisms underscore the intricate regulation of scytonemin biosynthesis and its pivotal role in protecting cyanobacteria from UV radiation.

Two-component regulatory system (TCRS), composed of a sensor kinase (histidine kinase) (NpRF1277) and a response regulator (NpRF1278), which is highly conservedlies adjacent to the scytonemin biosynthetic gene cluster (Figure 3). Study demonstrated that a mutant lacking NpRF1278 failed to produce scytonemin under UV-A stress compared to the wild type [55]. NpRF1277 is an HKII-type histidine kinase containing the structural domains of HKII + (PAS)2 PAS/PAC. PAS/PAC domains can bind small molecules, thereby signaling responses to stimuli such as light, oxygen, pH, and salinity. The response regulator NpRF1278 is an RRII featuring an AraC output DNA-binding domain [56]. The expression response of TCRS to light and UV radiation differs from that of cells exposed to oxidative stress, indicating its photosensitivity [57].

Role of scytonemin in evolution

During the early Proterozoic era, cyanobacterial photosynthesis markedly increased oxygen levels, while UV radiation remained abundant on Earth’s surface. There likely existed mechanisms to balance UV radiation and photosynthetically active radiation (PAR), crucial for life’s evolution during that period (Figure 5). Early photosynthetic life depended on protective organic molecules in aquatic habitats [58]. UV-screening compounds, evolving during the Precambrian era, provided UV protection, potentially enhancing cyanobacteria’s resilience to high radiation [29]. While the chemistry of the first specific UV-absorbing molecules on Archean Earth remains poorly understood, aromatic-containing reaction centres probably served as some of the earliest UV screens, enabling cyanobacteria to harvest light for photosynthesis [59]. The presence of ancient UV-absorbing pigments in modern cyanobacteria, ranging from UVC-absorbing pigments in the Archean eon to pigments absorbing longer UV wavelengths in the Phanerozoic eon, suggests the evolutionary selection of photon dissipation mechanisms for photo-protection throughout life’s history [24].

Considerable amounts of scytonemin have been observed in the topdeposits of the terrestrial cyanobacterial mats or crusts, offering shield to cells beneath by dissipating UV radiation [19, 60]. The presence of scytonemin in Precambrian era matswith silica, likely provided similar protection in extreme photic environments. Scytonemin’s long-term stability [33] is advantageous for understanding life’s evolutionary history in paleobotanical studies. It holds substantialrole in ecological management, as it often accrues in the upper layers of cyanobacterial mats thriving in intensely sunlight-exposed regions. Scytonemin’s evolutionary functions include UV absorption, antioxidant properties, reduced ROS production and thymine dimers, heat dissipation from absorbed UV radiation, and increased soil surface temperature [4, 34, 61, 62]. This contributes to cyanobacteria’s high tolerance to desiccation [16] and stabilization of the exopolysaccharide matrix [63]. Scytonemin interacts with the WspA protein (in matrix), enables desiccation resistance in cyanobacteria [64]. It also forms iron-complexes thatenable the cyanobacterial survival on rocks [65, 66].

Biotechnological potential of scytonemin

Cyanobacteria offer a promising avenue for biogenesis of fuels, chemicals, medicines, plant secondary metabolites etc. [67-71]. They serve as a valuable source of biofuels [72]. Ultraviolet (UV) radiation poses risks such as sunburn, premature skin aging, and skin cancer, including malignant melanoma, due to repeated exposure to sunlight’s high radiations [73]. To counteract the detrimental effects of high radiations, cyanobacteria synthesize scytonemin, that acts as a natural substitute to synthetic UV filters to safeguard them [74]. Scytonemin’s photoprotective and antioxidant properties give it commercial value in cosmetics and medicine [75]. Moisturizing chemicals have side effects like contact sensitivity and estrogenicity on human skin, with harm to aquatic environments also,cyanobacteria offer superior substitutes to commercially manufactured antioxidants used in pharmaceutical and food industries, providing carotenoids, phycobiliproteins, phenolics, glutathione, scytonemin, MAAs, and vitamins like ascorbate and tocopherol [76].

Scytonemin stands out amongst natural products due to its cellular location, strong UV-A and violet-blue absorption, and high Sun Protection Factor (SPF) value. Extracellular substances with high-water retention capacity, can serve as moisturizers in cosmetic products. The demand for natural ingredients in cosmetics is rising, as synthetic inorganic UV filters like titanium dioxide (TiO₂) and zinc oxide (ZnO) in sunscreen products produce highly oxidizing radicals [77]. Scytonemin, as a natural sunscreen compound, garners interest from dermatologists and cosmetic industries for skin protection [78, 79]. Producing scytonemin sunscreen synthesized by Lyngbya notarisii would be cost-effective [80]. Scytonemin from Antarctic cyanobacterium Nostoc commune exhibits a high SPF value and scavenges free radicals, suggesting its potential as a natural UV sunscreen cream ingredient [81].

Beyond its UV-A shielding, scytonemin finds biomedical applications due to its anti-proliferative and anti-inflammatory activities without chemical toxicity [82-83]. Studies with Nostoc commune demonstrate scytonemin’s antioxidant and radical scavenging activity, potentially preventing UV-induced cellular damage [23, 84, 85]. While scytonemin’s antioxidant activity varies across strains like Lyngbya sp. CU2555, it still shows radical scavenging ability [34]. Scytonemin synthesized by Leptolyngbya mycodia acts as a potent antioxidant, reducing DPPH radicals [20]. Its role in scavenging reactive oxygen species and controlling cancer cell growth is noteworthy [86]. Scytonemin inhibits skin inflammation by down-regulating NF-κB activity [87], regulateshuman fibroblastsand endothelial cells proliferation [83, 84], and inhibits human polo-like kinase 1 (target for anticancer drugs) [88, 89]. It can suppress human T-lymphoid Jurkat cell growth [90], and LPS/IFNc-stimulated NO production in murine macrophage RAW264 cells [91]. Scytonemin can also restraint the activity of other kinases like Myt1, cyclin B, checkpoint kinase1, and protein kinase C [92], making it a promising small-molecule drug. Leptolyngbya holds significant commercial potential in scytonemin synthesis and ecological biotechnology [93]. Various medicinal and agriculturally important bioactive secondary compounds from cyanobacteria have been identified [94], indicating their potential to produce natural substances sustainably.

Conclusions

Scytonemin, a secondary metabolite produced by cyanobacteria, holds significant market potential due to its varied roles as a UV protectant and antioxidant relevant to anhydrobiosis. Its stability against various stresses and its photoprotective abilities suggest its potential application as a sunscreen. Scytonema sp. stands as a promising candidate for bioremediation of saline soils while producing valuable metabolites like scytonemin [95]. However, comprehensive studies on the physiological, biochemical, and molecular aspects, as well as the presence of effective UV-screening/absorbing compounds in these organisms, are still lacking. Therefore, further research is crucial to explore the ecological, industrial, and pharmaceutical applications of scytonemin. Metabolic engineering techniques are expected to play a pivotal role in attaining cost-effective biosynthesis of scytonemin in the future.

Acknowledgements

We extend our gratitude to the Head and the Program Coordinator of the Department of Botany at Jai Prakash University and the University of Allahabad for generously providing laboratory facilities.

Conflict of interest

The authors confirm that this article’s content has no conflict of interest.

References

1. Tomitani, A., Knoll, A. H., Cavanaugh, C. M., & Ohno, T. (2006). The evolutionary diversification of cyanobacteria: molecular-phylogenetic and paleontological perspectives. PNAS,103(14), 5442-5447.

2. Fischer, W. W. (2008). Life before the rise of oxygen. Nature, 455, 1051-1052.

3. Sinha, R. P., Klisch, M., Vaishampayan, A., & Häder, D.-P. (1999). Biochemical and spectroscopic characterization of the cyanobacterium Lyngbya sp. inhabiting mango (Mangifera indica) trees: presence of an ultraviolet-absorbing pigment, scytonemin. Acta Protozoologica, 38, 291-298.

4. Dillon, J. G., & Castenholz, R. W. (2003). The synthesis of the UV-screening pigment, scytonemin, and photosynthetic performance in isolatesfrom closely related natural populations of cyanobacteria (Calothrixsp.). Environmental Microbiology, 5,484-491.

5. Nägeli,C. (1849). Gattungen einzelliger Algen, physiologisch und systematisch bearbeitet. Neue Denkschrift der Allgemeinen SchweizerischenGesellschaft für die Gesammten Naturwissenschaften, 10, 1-138.

6. Geitler, L. (1932). Cyanophyceae (Blaulagen).In:L. Rabenhorst (Ed.), Kryptogamen-Flora von Deutschland Osterreich und der Schweiz  Leipzig (pp. 1-1196), Akademische Verlags Gesellschaft, 14.

7. Desikachary, T. V. (1959). Cyanophyta. New Delhi, India: Indian Council of Agriculture Research. (pp. 1-686).

8. Pereira, S., Zille, A., Micheletti, E., Moradas-Ferreira, P., de Philippis, R., & Tamagnini, P. (2009). Complexity of cyanobacterial exopolysaccharides: Composition, structures, inducing factors and putative genes involved in their biosynthesis and assembly. FEMS Microbiology Reviews, 33, 917-941.

9. Ehling-Schulz, M., Bilger, W., & Scherer, S. (1997). UV-B induced synthesis of photoprotective pigments and extracellular polysaccharides in the terrestrial cyanobacterium Nostoc commune.  Journal of Bacteriology, 179, 1940-1945.

10. Ehling-Schulz, M., & Scherer, S. (1999). UV protection in cyanobacteria.European Journal of Phycology,34, 329-338.

11. Balskus, E. P., Case, R. J., & Walsh, C. T. (2011). The biosynthesis of cyanobacterial sunscreen scytonemin in intertidal microbial mat communities.FEMS Microbiology Ecology, 77(2),322-332.

12. Ferroni, L., Klisch, M., Pancaldi, S.,& Häder, D.-P. (2010). Complementary UV-absorption of mycosporine-like amino acids and scytonemin is responsible for the UV-insensitivity of photosynthesis in Nostocflagelliforme. Marine Drugs, 8,106-121.

13. Castenholz, R. W., & Garcia-Pichel, F. (2000). Cyanobacterial responses to UV-radiation. In: B.A. Whitton & M. Potts (Eds.), Ecology of Cyanobacteria: Their Diversity in Time and Space(pp. 591-611), Kluwer Academic, Dordrecht.

14. Castenholz, R. W. (1997). Multiple strategies for UV tolerance in cyanobacteria. Spectrum, 10, 10-16.

15. Richter, P. R., Sinha, R. P., & Häder, D.-P. (2006).Scytonemin-rich epilithic cyanobacteria survive acetone treatment. Current Trends in Microbiology, 2, 13-19.

16. Fleming, E. D., & Castenholz, R. W. (2007). Effects of periodic desiccation on the synthesis of the UV-screening compound, scytonemin, in cyanobacteria. Environmental Microbiology, 9,1448-1455.

17. Fleming, E. D., &Castenholz, R. W. (2008). Effects of nitrogen source on the synthesis of the UV-screening compound, scytonemin, in the cyanobacterium Nostoc punctiforme PCC 73102. FEMS Microbiology Ecology, 63, 301-308.

18. Dillon, J. G., Tatsumi, C. M., Tandingan, P. G.,& Castenholz, R. W. (2002). Effect of environmental factors on the synthesis of scytonemin, a UV-screening pigment, in a cyanobacterium (Chroococcidiopsis sp.). Archives of Microbiology, 177, 322-331.

19. Garcia-Pichel, F., & Castenholz, R. W. (1991). Characterization and biological implication of scytonemin, a cyanobacterial sheath pigment. Journal of Phycology,27, 395-409.

20. Madrahi, G. S., & Naeimpoor, F. (2022). UV Induced biosynthesis of cyano-sunscreen “scytonemin” by Leptolyngbya mycodia and its effectual antioxidant activity. Iranian Journal of Pharmaceutical Sciences, 18(1),19-33.

21. Rath, J., Mandal, S.,&Adhikary, S. P. (2012). Salinity induced synthesis of UV-screening compound scytonemin in the cyanobacterium Lyngbyaaestuarii. Journal of Photochemistry and Photobiology B: Biology,115, 5-8.

22. Rastogi, R. P., Sonani, R. R., & Madamwar, D. (2015). Cyanobacterial sunscreen scytonemin: role in photoprotection and biomedical research. Applied Biochemistry and Biotechnology,176, 1551-1563.

23. Matsui, K., Nazifi, E., Hirai, Y., Wada, N., Matsugo, S.,& Sakamoto, T. (2012). The cyanobacterial UV-absorbing pigment scytonemin displays radical scavenging activity. Journal of the General and Applied Microbiology, 58, 137-144.

24. Simeonov, A.,& Michaelian, K. (2017). Properties of cyanobacterial UV-absorbing pigments suggest their evolution was driven by optimizing photon dissipation rather than photoprotection. arXiv, arXiv, 1702.03588.

25. Proteau, P., Gerwick, W., Garcia-Pichel, F., & Castenholz, R. W. (1993). The structure of scytonemin, an ultraviolet sunscreen pigment from the sheaths of cyanobacteria. Experientia, 49, 825-829.

26. Kylim, H. (1927). Über die Karotinoide Farbstoffe der Algen. Hoppe-Scyler’s zeitschr. Physiological Chemistry,166, 33-77.

             27. Kylim H. Über die Farbstoffe und die Farbe der cyanophyceen. Fysiogr Sällsk

Förhandl1937;7:131-158.

28. Bultel-Poncé, V., Felix-Theodose, F., Sarthou, C., Ponge, J.-F., & Bodo, B. (2004). New pigment from the terrestrial cyanobacterium Scytonema sp. collected on the Mitraka Inselberg, French Guyana. Journal of Natural Products, 67, 678-681. 

29. Dillon, J. G., & Castenholz, R. W. (1999). Scytonemin, a cyanobacterial sheath pigment, protects against UVC radiation: Implication for early photosynthetic life. Journal of Phycology, 35, 673-681.

30. Sinha, R. P., Klisch, M., Gröniger, A., & Häder, D.-P. (1998). Ultraviolet-absorbing/screening substances in cyanobacteria, phytoplankton and macroalgae. Journal of Photochemistry and Photobiology B: Biology, 47, 83-94.

31. Edward, H. G. M., Garcia-Pichel, F., Newton, E. M., & Wynn-Williams, D. D. (1999). Vibrational Raman spectroscopic study of scytonemin, the UV-protective cyanobacterial pigment. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy,56, 193-200.

32. Büdel, B., Karsten, U.,&Garcia-Pichel, F. (1997). Ultraviolet-absorbing scytonemin and mycosporine-like amino acids derivatives in exposed, rock inhabiting cyanobacterial lichens. Oecologia,112, 165-172.

33. Garcia-Pichel, F., Sherry, N. D., & Castenholz, R. W. (1992). Evidence for a UV sunscreen role of the extracellular pigment scytonemin in the terrestrial cyanobacterium Chlorogloeopsis sp. Photochemistry and Photobiology, 56,17-23.

34. Rastogi, R. P., &Incharoensakdi,A. (2014).Characterization of UV-screening compounds, mycosporine-likeamino acids, and scytonemin in the cyanobacterium Lyngbyasp. CU2555. FEMS Microbiology Ecology, 87, 244-256.

35. Squier, A. H., Hodgson, D. A., & Keely, B. J. (2004). A critical assessment of the analysis and distributions of scytonemin and related UV screening pigments in sediments. Organic Geochemistry,35, 1221-1228.

36. Fulton, J. M., Arthur, M. A., & Freeman, K. H. (2012). Subboreal aridity and scytonemin in the Holocene Black Sea. Organic Geochemistry, 49, 47-55.

37. Varnali, T., & Edwards, H. G. M. (2013). Theoretical study of novel complexed structures for methoxy derivatives of scytonemin: Potentialbiomarkers in iron-rich stressed environments. Astrobiology,13, 861-869.

38. Vítek, P., Jehlička, J., Ascaso, C., Mašek, V., Gómez-Silva, B., Olivares, H.,&Wierzchos, J. (2014). Distribution of scytonemin in endolithicmicrobial communities from halite crusts in the hyperarid zone of the Atacama Desert, Chile. FEMS Microbiology Ecology,90, 351-366.

39. Lalić, D., Meriluoto, J., Zorić, M., Dulić, T., Mirosavljević, M., Župunski, M., & Svirčev, Z. (2020). Potential of cyanobacterial secondarymetabolites as biomarkers for paleoclimate reconstruction. Catena,185, 104283.

40. Mandalka, A. (1999). Studies on scytonemin synthesis in cyanobacteria, Bremen, Germany: University of Bremen.

41. Strickland, E. H., Horwitz, J., & Billups, C. (1970). Near-ultraviolet absorption bands of tryptophan, studies using indole and 3-methylindole as models. Biochemistry, 9(25), 4914-4921.

42. Chen, R. (1972). Measurements of absolute values in biochemical fluorescence spectroscopy. Journal of Research of the National Bureau of Standards,76A(6),593-606.

43. Klicki, K., Ferreira, D., Hamill, D., Dirks, B., Mitchell, N., & Garcia-Pichel, F. (2018). The widely conserved ebo Cluster is involved in precursor transport to the periplasm during scytonemin synthesis in Nostoc punctiforme. mBio,9, e02266-18.

44. Soule, T., Palmer, K., Gao, Q., Potrafka, R. M., Stout, V., &  Garcia-Pichel, F. (2009). A comparative genomics approach to understanding the biosynthesis of the sunscreen scytonemin in cyanobacteria. BMC Genomics,10, 336-347. 

45. Balskus, E. P., & Walsh, C. T. (2008). Investigating the initial steps in the biosynthesis of cyanobacterial sunscreen scytonemin. Journal of the American Chemical Society,130, 15260-15261.

46. Ferreira, D., & Garcia-Pichel, F. (2016). Mutational studies of putative biosynthetic genes for the cyanobacterial sunscreen scytonemin in Nostoc punctiforme ATCC 29133. Frontiers in Microbiology,7,735-745.

47. Chipman, D., Baraka, Z., & Schloss, J. V. (1998). Biosynthesis of 2-aceto-2-hydroxy acids: acetolactate synthases and acetohydroxyacid synthases. Biochimica et Biophysica Acta,1385(2), 401-419.

48. Gao, Q., & Garcia-Pichel, F. (2011). Microbial ultraviolet sunscreens. NatureReviews Microbiology, 9,1-12.

49. Balskus, E. P.,& Walsh, C. T. (2009). An enzymatic cyclopentyl[b]indole formation involved in scytonemin biosynthesis. Journal of the American Chemical Society,131, 14648-14649.

50. Malla, S., & Sommer, M. O. (2014). A sustainable route to produce the scytonemin precursor using Escherichia coli. Green Chemistry, 16, 3255-3265.

51. Soule, T.,Stout, V.,Swingley, W. D., Meeks, J. C.,& Garcia-Pichel, F. (2007). Molecular genetics and genomic analysis of scytonemin biosynthesis in Nostoc punctiforme ATCC 29133. Journal of Bacteriology, 189(12),4465-4472.

52. Sanchez-Ferrer, A., Rodriguez-Lopez, J. N., Garcia-Canovas, F., & Garcia-Carmona, F. (1995). Tyrosinase: a comprehensive review of its mechanism. Biochimica et Biophysica Acta, 1247, 1-11.

53. Sorrels, C. M., Proteau, P. J., & Gerwick, W. H. (2009). Organization, evolution, and expression analysis of the biosynthetic gene cluster for scytonemin, a cyanobacterial UV-absorbing pigment. Applied and Environmental Microbiology, 75, 4861-4869.

54. Christie, P. J., Atmakuri, K., Krishnamoorthy, V., Jakubowski, S.,& Cascales, E. (2005). Biogenesis, architecture, and function of bacterial type IV secretion systems. Annual Review of Microbiology,59, 451-485.

55. Naurin, S., Bennett, J., Videau, P., Philmus, B., & Soule, T. (2016).The response regulator Npun_f1278 is essential for scytonemin biosynthesis in the cyanobacterium Nostoc punctiforme ATCC 291331. Journal of Phycology, 52, 564-571.

56. Ashby, M. K., &Houmard, J. (2006). Cyanobacterial two-component proteins: structure, diversity, distribution, and evolution. Microbiology and Molecular Biology Reviews,70, 472-509.

57. Janssen, J., & Soule, T. (2016). Gene expression of a two-component regulatory system associated with sunscreen biosynthesis in the cyanobacterium Nostocpunctiforme ATCC 29133. FEMS Microbiology Letters,363(2),235-241.

58. Sagan, C. (1973). Ultraviolet radiation selection pressure on the earliest organisms. Journal of Theoretical Biology, 39, 195-200.

59. Mulkidjanian, A. Y., & Junge, W. (1997). On the origin of photosynthesis as inferred from sequence analysis. Photosynthesis  Research, 51, 27-42.

60. Garcia-Pichel, F., & Belnap, J. (1996). Microenvironments and microscale productivity of cyanobacterial desert crusts. Journal of Phycology, 32, 774-782.

61. Rastogi, R. P., Sinha, R. P. &Incharoensakdi, A. (2013). Partial characterization, UV-induction and photoprotective function of sunscreenpigment, scytonemin from Rivularia sp. HKAR-4.Chemosphere,93, 1874-1878.

62. Couradeau, E., Karaoz, U., Lim, H. C., Nunes da Rocha, U., Northen, T., Brodie, E., & Garcia-Pichel, F. (2016). Bacteria increase arid-landsoil surface temperature through the production of sunscreens. Nature Communications,7,10373-10380.

63. Gao, X. (2017). Scytonemin plays a potential role in stabilizing the exopolysaccharidic matrix in terrestrial cyanobacteria. Microbial Ecology, 73, 255-258.

64. Wright, D. J., Smith, S. C., Joardar, V., Scherer, S., Jervis, J., Warren, A., Helm, R. F., & Potts, M. (2005). UV irradiation and desiccationmodulate the three-dimensional extracellular matrix of Nostoc commune (Cyanobacteria). Journal of Biological Chemistry, 280, 40271-40281.

65. Varnali, T.,&Edwards, H. G. M. (2010). Iron-scytonemin complexes: DFT calculations on new UVprotectants for terrestrial cyanobacteriaand astrobiological implications. Astrobiology,10, 711-716.

66. Varnali, T., & Gören, B. (2018). Two distinct structures of the sandwich complex of scytonemin with iron and their relevance to astrobiology.Structural Chemistry29, 1565-1572.

67. Nunnery, J. K., Mevers, E., & Gerwick, W. H. (2010). Biologically active secondary metabolites from marine cyanobacteria. Current Opinion in Biotechnology,21, 787-793. 

68. Xue, Y., & He, Q. (2015). Cyanobacteria as cell factories to produce plant secondary metabolites. Frontiers in Bioengineering and Biotechnology, 3,57-63.

69. Knoot, C. J., Ungerer, J., Wangikar, P. P., & Pakrasi, H. B. (2018). Cyanobacteria: promising biocatalysts for sustainable chemical production. Journal of Biological Chemistry,293, 5044-5052.

70. Miao, R., Xie, H., Liu, X., Lindberg, P., & Lindblad, P. (2020). Current processes and future challenges of photoautotrophic production of acetyl-CoA-derived solar fuels and chemicals in cyanobacteria. Current Opinion in Chemical Biology, 59, 69-76.

71.  Zahra, Z., Choo, D. H., Lee, H., & Parveen, A. (2020). Cyanobacteria: review of current potentials and applications. Environments, 7, 13-30.

72. Parmar, A., Singh, N. K., Pandey, A., Gnansounou, E., &Madamvar, D. (2011). Cyanobacteria and microalgae: a positive prospect for biofuels. Bioresource Technology,102(22), 10163-10172.

73. Lisby, S., Gniadecki, R., & Wulf, H. C. (2005). UV-induced DNA damage in human keratinocytes: quantitation and correlation with long-term survival. Experimental Dermatology, 14, 349-355.

74. Bennett, J.,&. Soule, T. (2022). Expression of scytonemin biosynthesis genes under alternative stress conditions in the cyanobacterium Nostocpunctiforme. Microorganisms,10, 427-434.

75. Gao, X., Jing, X., Liu, X., & Lindblad, P. (2021). Biotechnological production of the sunscreen pigment scytonemin in cyanobacteria: progress and strategy. Marine Drugs, 19, 129-144.

76. Md Hatha, A. A., & Sumayya, N. S. (2023). Antioxidants from marine cyanobacteria. In: Marine Antioxidants,(pp. 119-131), Academic Press.

77. Serpone, N., Dondi, D.,& Albini, A. (2007). Inorganic and organic UV filters:their role and efficacy in sunscreens and suncare products. InorganicaChimica Acta, 360, 794-802.

78. Siezen, R. J. (2011). Microbial sunscreens. Microbial Biotechnology, 4, 1-7.

79. Derikvand, P., Llewellyn, C. A., & Purton, S. (2017). Cyanobacterial metabolites as a source of sunscreens and moisturizers: a comparison with current synthetic compounds. European Journal of Phycology,52(1),43-56.

80. Aziz, A. (2018). “Scytonemin” Pigment in Lyngbya notarisii (Meneghini) Wille and possibility of using it in preparing skinprotecting cream. Journal of Drug Research and Development,4(2),1-5.

81. Ručová, D., Vilková, M., Sovová, S., Vargová, Z., Kostecká, Z., Frenák, R., Routray, D., & Bačkor, M. (2023). Photoprotective and antioxidant properties of scytonemin isolatedfrom Antarctic cyanobacterium Nostoc commune Vaucher ex Bornet & Flahault and its potential as sunscreen ingredient. Journal of Applied Phycology,35, 2839-2850.

82. Stevenson, C. S., Capper, E. A., Roshak, A. K., Marquez, B., Eichman, C., Jackson, J. R., Mattern, M., Gerwick, W. H., Jacobs, R. S., &Marshall, L. A.(2002). The identification and characterization of the marine natural product scytonemin as a novel antiproliferative pharmacophore. Journal of Pharmacoogy and Experimental Therapeutics,303, 858-866.

83. Stevenson, C. S., Capper, E. A., Roshak, A. K., Marquez, B., Grace, K.,Gerwick, W. H., Jacobs, R. S.,& Marshall, L. A. (2002). Scytonemin, a marine natural product inhibitor of kinases key in hyperproliferative inflammatory diseases. Inflammation Research,51, 112-114.

84. Takamatsu, S., Hodges, T. W., Rajbhandari, I., Gerwick, W. H., Hamann, M. T.,&Nagle, D. G. (2003). Marine natural products as novel antioxidant prototypes. Journal of Natural Products, 66, 605-608.

85. De la Coba, F., Aguilera, J., Figueroa, F. L., de Galvez, M. V., & Herrera, E. (2009). Antioxidant activity of mycosporine-like amino acids isolated from three red macroalgae and one marine lichen. Journal of Applied Phycology,21, 161-169.

86. Pathak, J., Pandey, A., Maurya, P. K., Rajneesh, Sinha, R. P., &Singh, S. P. (2020). Cyanobacterial secondary metabolite scytonemin: a potential photoprotective and pharmaceutical compound. Proceedings of the National Academy of Sciences, India Section B: Biological Sciences,90, 467-481.

87. Kang, M. R., Jo, S. A., Lee, H., Yoon, Y. D., Kwon, J.-H., Yang, J.-W., Choi, B. J., Park, K. H., Lee, M. Y., Lee, C. W., Lee, K.-R., & Kang, J. S.(2020). Inhibition of skin inflammation by scytonemin, an ultraviolet sunscreen pigment. Marine Drugs 18, 300-308.

88. Strebhardt, K., & Ullrich, A. (2006). Targeting polo-like kinase 1 for cancer therapy. Nature Reviews Cancer, 6, 321-330.

89. Zhang, G., Zhang, Z., & Liu, Z. (2013). Scytonemin inhibits cell proliferation and arrests cell cycle through downregulating Plk1 activity in multiple myeloma cells. Tumor Biology, 34, 2241-2247.

90. Itoh, T., Tsuzuki, R., Tanaka, T., Ninomiya, M., Yamaguchi, Y., Takenaka, H., Ando, M., Tsukamasa, Y., & Koketsu, M. (2013). Reduced scytonemin isolated from Nostoc commune induces autophagic cell death in human T-lymphoid cell line Jurkat cells. Food and Chemical Toxicology, 60,76-82.

91. Itoh, T., Koketsu, M., Yokota, N., Touho, S., Ando, M., & Tsukamasa, Y. (2014). Reduced scytonemin isolated from Nostoc commune suppresses LPS/IFNγ-induced NO production in murine macrophage RAW264 cells by inducing hemeoxygenase-1 expression via the Nrf2/ARE pathway. Food and Chemical Toxicology,69, 330-338.

92. Pathak, J., Mondal, S., & Ahmed, H. (2019). In silico study on interaction between human polo-like kinase 1 and cyanobacterial sheath pigment scytonemin by molecular docking approach. Biointerface Research in Applied Chemistry9, 4374-4378.

93. Madrahi, G. S., & Naeimpoor, F. (2023). Overproduction of cyano-sunscreen scytonemin by Leptolyngbya mycodia in two stage illuminated photobioreactor: from submerged to attached cultivation. Algal Research, 74, 103169.

94. Rastogi, R. P., & Sinha, R. P. (2009). Biotechnological and industrial significance of cyanobacterial secondary metabolites. Biotechnology Advances,27, 521-539.

95. Pathak, J., Kumar, D., Singh, D. K., Ahmed, H., Kannaujiya, V. K.,& Sinha, R. P. (2022). Ultraviolet radiation and salinity-induced physiological changes and scytonemin induction in cyanobacteria isolated from diverse habitats. Biointerface Research in Applied Chemistry,12(3),3590-3606.