Evidence Regarding the UV Sunscreen Role of a Mycosporine-Like Compound in the Cyanobacterium Gloeocapsa sp

The UV sunscreen role commonly ascribed to mycosporine-like amino acids (MAAs) was investigated with an isolate of the terrestrial cyanobacterium Gloeocapsa sp. strain C-90-Cal-G.(2), which accumulates intracellularly an MAA with absorbance maximum at 326 nm but produces no extracellular sunscreen compound (i.e., scytonemin). The intracellular concentrations of MAA achieved were directly related to the intensity of the UV radiation (maximum at 320 nm) received by the cells. However, the presence of high concentrations of MAA was not necessary for the physiological acclimation of the cultures to UV radiation. The measured sunscreen factor due to MAA in single cells was 0.3 (the MAA prevented 3 out of 10 photons from hitting potential cytoplasmic targets). High contents of MAA in the cells correlated with increased resistance to UV radiation. However, when resistance was gauged under conditions of desiccation, with inoperative physiological photoprotective and repair mechanisms, cells with high MAA specific contents were only 20 to 25% more resistant. Although UV radiation centered around both 320 and 365 nm resulted in chlorophyll a photobleaching and photoinhibition of photosynthesis, the difference in sensitivity correlated with MAA accumulation occurred only at 320 nm (absorbed by MAA) and not at 365 nm (not absorbed by MAA). This difference represents the maximal protection ascribable to the presence of MAA for single cells, i.e., if one does not consider the enhancing effects of colony formation on protection by sunscreens.     

Ultraviolet and osmotic stresses induce and regulate the synthesis of mycosporines in the cyanobacterium Chlorogloeopsis PCC 6912

The cyanobacterium Chlorogloeopsis PCC 6912 was found to synthesize and accumulate two putative UV sunscreen compounds of the mycosporine (mycosporine-like amino acid; MAA) type: mycosporine-glycine and shinorine. These MAAs were not constitutively present in the cells; their synthesis could be induced specifically either by exposure to UVB radiation (280–320 nm) or by osmotic stress, but not by other stress factors such as heat or cold shock, nutrient limitation, or photooxidative stress. A significant synergistic enhancement of MAA synthesis was observed when both stress factors were applied in combination. Although osmotic stress could induce MAA synthesis, comparison of the intracellular contents of MAAs with those of sugar osmolytes (glucose and trehalose) indicated that MAAs play no significant role in attaining osmotic homeostasis. UVB strongly enhanced the accumulation of shinorine, whereas osmotic stress had a more pronounced effect on mycosporine-glycine. This differential effect on the steady-state contents of each MAA could be explained either by differential regulation of biosynthesis or by differential loss rates of MAAs (leakage) under each condition. A preferential leakage of mycosporine-glycine from the cells after a hypoosmotic shock was detected. The results are interpreted in terms of an adaptive necessity for a combined regulatory control responding to both UV and external osmotic conditions in organisms that accumulate water-soluble sunscreens intracellularly. Received: 26 March 1999 / Accepted: 13 July 1999     

The response regulator Npun_F1278 is essential for scytonemin biosynthesis in the cyanobacterium Nostoc punctiforme ATCC 29133

Following exposure to long‐wavelength ultraviolet radiation (UVA), some cyanobacteria produce the indole‐alkaloid sunscreen scytonemin. The genomic region associated with scytonemin biosynthesis in the cyanobacterium Nostoc punctiforme includes 18 cotranscribed genes. A two‐component regulatory system (Npun_F1277/Npun_F1278) directly upstream from the biosynthetic genes was identified through comparative genomics and is likely involved in scytonemin regulation. In this study, the response regulator (RR), Npun_F1278, was evaluated for its ability to regulate scytonemin biosynthesis using a mutant strain of N. punctiforme deficient in this gene, hereafter strain Δ1278. Following UVA radiation, the typical stimulus to initiate scytonemin biosynthesis, Δ1278 was incapable of producing scytonemin. A phenotypic characterization of Δ1278 suggests that aside from the ability to produce scytonemin, the deletion of the Npun_F1278 gene does not affect the cellular morphology, cellular differentiation capability, or lipid‐soluble pigment complement of Δ1278 compared to the wildtype. The mutant, however, had a slower specific growth rate under white light and produced ~2.5‐fold more phycocyanin per cell under UVA than the wildtype. Since Δ1278 does not produce scytonemin, this study demonstrates that the RR gene, Npun_F1278, is essential for scytonemin biosynthesis in N. punctiforme. While most of the evaluated effects of this gene appear to be specific for scytonemin, this regulator may also influence the overall health of the cell and phycobiliprotein synthesis, directly or indirectly. This is the first study to identify a regulatory gene involved in the biosynthesis of the sunscreen scytonemin and posits a link between cell growth, pigment synthesis, and sunscreen production.     

Bioinformatic,phylogenetic and chemical analysis of the UV-absorbing compounds scytonemin and mycosporine-like amino acids from the microbial mat communities of Shark Bay,Australia

Shark Bay, Western Australia is a World Heritage area with extensive microbial mats and stromatolites. Microbial communities that comprise these mats have developed a range of mitigation strategies against changing levels of photosynthetically active and ultraviolet radiation, including the ability to biosynthesise the UV-absorbing natural products scytonemin and mycosporine-like amino acids (MAAs). To this end, the distribution of photoprotective pigments within Shark Bay microbial mats was delineated in the present study. This involved amplicon sequencing of bacterial 16S rDNA from communities at the surface and subsurface in three distinct mat types (smooth, pustular and tufted), and correlating this data with the chemical and molecular distribution of scytonemin and MAAs. Employing UV spectroscopy and MS/MS fragmentation, mycosporine-glycine, asterina and an unknown MAA were identified based on typical fragmentation patterns. Marker genes for scytonemin and MAA production (scyC and mysC) were amplified from microbial mat DNA and placed into phylogenetic context against a broad screen throughout 363 cyanobacterial genomes. Results indicate that occurrence of UV screening compounds is associated with the upper layer of Shark Bay microbial mats, and the occurrence of scytonemin is closely dependent on the abundance of cyanobacteria.     

The cyanobacterial UV-absorbing pigment scytonemin displays radical-scavenging activity

Scytonemin is a 544-Da hydrophobic pigment that can absorb UV-A radiation. It is present in cyanobacterial sheaths and is thought to function as a UV protectant. In this study, scytonemin was purified from the terrestrial cyanobacterium Nostoc commune, and its radical-scavenging activity was characterized. The purified scytonemin quenched an organic radical in vitro and accounted for up to 10% of the total activity of an ethanol extract of N. commune. These results suggest that the extracellular UV-absorbing pigment scytonemin has multiple roles, functioning as a UV sunscreen and an antioxidant relevant to anhydrobiosis in N. commune.     

UV-B-induced synthesis of photoprotective pigments and extracellular polysaccharides in the terrestrial cyanobacterium Nostoc commune.

Liquid cultures of the terrestrial cyanobacterium Nostoc commune derived from field material were treated with artificial UV-B and UV-A irradiation. We studied the induction of various pigments which are though to provide protection against damaging UV-B irradiation. First, UV-B irradiation induced an increase in carotenoids, especially echinenone and myxoxanthophyll, but did not influence production of chlorophyll a. Second, an increase of an extracellular, water-soluble UV-A/B-absorbing mycosporine occurred, which was associated with extracellular glycan synthesis. Finally, synthesis of scytonemin, a lipid-soluble, extracellular pigment known to function as a UV-A sunscreen, was observed. After long-time exposure, the UV-B effect on carotenoid and scytonemin synthesis ceased whereas the mycosporine content remained constantly high. The UV-B sunscreen mycosporine is exclusively induced by UV-B (< 315 nm). The UV-A sunscreen scytonemin is induced only slightly by UV-B (< 315 nm), very strongly by near UV-A (350 to 400 nm), and not at all by far UV-A (320 to 350 nm). These results may indicate that the syntheses of these UV sunscreens are triggered by different UV photoreceptors.     

A survey of the distribution of UV-absorbing substances in tropical macroalgae

UV-absorbing mycosporine-like amino acid compounds (MAA) were identified and quantified in 13 macroalgal Chlorophyceae, six Phaeophyceae and 28 Rhodophy-ceae collected in the intertidal zone from the tropical island Hainan, People's Republic of China, as well as from tropical mangrove locations in America, Africa, Australia and Japan. All of these habitats receive naturally high solar ultraviolet (UV) irradiances. The study revealed that all Rhodophyceae contained several MAA, which are assumed to function as natural UV sunscreens. Within all species investigated eight distinct compounds were found, seven of which were identified as mycosporine-glycine, shinorine, porphyra-334, pal-ythine, asterina-330, palythinol and palythene. The unknown substance had an absorption spectrum with a maximum at 357 nm. This compound was restricted to two red algal species from Hainan. In contrast, the Chlorophyceae and Phaeophyceae did not contain MAA or exhibited only trace concentrations. Compared with data from the literature, the amount of all MAA in the tropical Rhodophyceae seemed to be higher than in temperate organisms, probably reflecting acclimation to the stronger solar radiation which is typical for lower latitudes. The data suggest that accumulation of MAA may represent a natural defence system against exposure to biologically harmful UV radiation.     

UV-Induced Effects on Growth,Photosynthetic Performance and Sunscreen Contents in Different Populations of the Green Alga Klebsormidium fluitans (Streptophyta) from Alpine Soil Crusts

Members of the green algal genus Klebsormidium (Klebsormidiales, Streptophyta) are typical components of biological soil crust communities worldwide, which exert important ecological functions. Klebsormidium fluitans (F. Gay) Lokhorst was isolated from an aeroterrestrial biofilm as well as from four different biological soil crusts along an elevational gradient between 600 and 2350 m in the Tyrolean and South Tyrolean Alps (Austria, Italy), which are characterised by seasonally high solar radiation. Since the UV tolerance of Klebsormidium has not been studied in detail, an ecophysiological and biochemical study was applied. The effects of controlled artificial ultraviolet radiation (UVR; <9 W m^–2 UV-A, <0.5 W m^–2 UV-B) on growth, photosynthetic performance and the capability to synthesise mycosporine-like amino acids (MAAs) as potential sunscreen compounds were comparatively investigated to evaluate physiological plasticity and possible ecotypic differentiation within this Klebsormidium species. Already under control conditions, the isolates showed significantly different growth rates ranging from 0.42 to 0.74 μm day^?1. The UVR effects on growth were isolate specific, with only two strains affected by the UV treatments. Although all photosynthetic and respiratory data indicated strain-specific differences under control conditions, UV-A and UV-B treatment led only to rather minor effects. All physiological results clearly point to a high UV tolerance in the K. fluitans strains studied, which can be explained by their biochemical capability to synthesize and accumulate a putative MAA after exposure to UV-A and UV-B. Using HPLC, a UV-absorbing compound with an absorption maximum at 324 nm could be identified in all strains. The steady-state concentrations of this Klebsormidium MAA under control conditions ranged from 0.09 to 0.93 mg g^?1 dry weight (DW). While UV-A led to a slight stimulation of MAA accumulation, exposure to UV-B was accompanied by a strong but strain-specific increase of this compound (5.34–12.02 mg^?1 DW), thus supporting its function as UV sunscreen. Although ecotypic differences in the UVR response patterns of the five K. fluitans strains occurred, this did not correlate with the altitude of the respective sampling location. All data indicate a generally high UV tolerance which surely contributes to the aeroterrestrial lifestyle of K. fluitans in soil crusts of the alpine regions of the European Alps.     

Effects of nitrogen source on the synthesis of the UV-screening compound, scytonemin, in the cyanobacterium Nostoc punctiforme PCC 73102

The effects of nitrogen source (N(2), NO(3)(-) and NH(4)(+)) on scytonemin synthesis were investigated in the heterocystous cyanobacterium Nostoc punctiforme PCC 73102. With the required UVA radiation included, Nostoc synthesized three to seven times more scytonemin while fixing nitrogen than when utilizing nitrate or ammonium. A similar increase in scytonemin synthesis occurred when nitrate or ammonium became depleted by growth and Nostoc switched to diazotrophic metabolism with the differentiation of heterocysts. In addition, UVA-exposed cultures grown in medium with both NO(3)(-) and NH(4)(+) synthesized some scytonemin but synthesis increased when NH(4)(+) was depleted and growth had become dependent on NO(3)(-) reduction. Although the mechanism is unclear, these results suggest that the greater the restriction in nitrogen accessibility, the greater the production of scytonemin. Perhaps the entire response may be an interaction between this restriction and a resultant sensitivity to UV radiation that acts as a cue for determining the level of scytonemin synthesis. Scytonemin is a stable UVR screening compound and appears to be synthesized by cyanobacteria as a long-term solution for reducing UVR exposure and damage, but mainly or solely, when metabolic activity is absent. It is likely that during metabolic resurgence, the presence of a dense scytonemin sheath would facilitate the recovery process without the need for active defenses against UV radiation.     

Polyols and UV‐sunscreens in the Prasiola‐clade (Trebouxiophyceae,Chlorophyta) as metabolites for stress response and chemotaxonomy

In many regions of the world, aeroterrestrial green algae of the Trebouxiophyceae (Chlorophyta) represent very abundant soil microorganisms, and hence their taxonomy is crucial to investigate their physiological performance and ecological importance. Due to a lack in morphological features, taxonomic and phylogenetic studies of Trebouxiophycean algae can be a challenging task. Since chemotaxonomic markers could be a great assistance in this regard, 22 strains of aeroterrestrial Trebouxiophyceae were chemically screened for their polyol‐patterns as well as for mycosporine‐like amino acids (MAAs) in their aqueous extracts using RP‐HPLC and LC‐MS. d ‐sorbitol was exclusively detected in members of the Prasiolaceae family. The novel MAA prasiolin and a related compound (“prasiolin‐like”) were present in all investigated members of the Prasiola‐clade, but missing in all other tested Trebouxiophyceae. While prasiolin could only be detected in field material directly after extraction, the “prasiolin‐like” compound present in the other algae was fully converted into prasiolin after 24 h. These findings suggest d ‐sorbitol and prasiolin‐like compounds are suitable chemotaxonomic markers for the Prasiolaceae and Prasiola‐clade, respectively. Additional UV‐exposure experiments with selected strains show that MAA formation and accumulation can be induced, supporting their role as UV‐sunscreen.     

Molecular genetics and genomic analysis of scytonemin biosynthesis in Nostoc punctiforme ATCC 29133

The indole-alkaloid scytonemin is the most common and widespread sunscreen among cyanobacteria. Previous research has focused on its nature, distribution, ecology, physiology, and biochemistry, but its molecular genetics have not been explored. In this study, a scytonemin-deficient mutant of the cyanobacterium Nostoc punctiforme ATCC 29133 was obtained by random transposon insertion into open reading frame NpR1273. The absence of scytonemin under conditions of induction by UV irradiation was the single phenotypic difference detected in a comparative analysis of the wild type and the mutant. A cause-effect relationship between the phenotype and the mutation in NpR1273 was demonstrated by constructing a second scytoneminless mutant through directed mutagenesis of that gene. The genomic region flanking the mutation revealed an 18-gene cluster (NpR1276 to NpR1259). Four putative genes in the cluster, NpR1274 to NpR1271, with no previously known functions, are likely to be involved in the assembly of scytonemin. Also in this cluster, there is a redundant set of genes coding for shikimic acid and aromatic amino acid biosynthesis enzymes, leading to the production of tryptophan and tyrosine, which are likely to be biosynthetic precursors of the sunscreen.     

MYCOSPORINE‐LIKE AMINO ACIDS AND PHYLOGENIES IN GREEN ALGAE: PRASIOLA AND ITS RELATIVES FROM THE TREBOUXIOPHYCEAE (CHLOROPHYTA)1

A UV‐absorbing mycosporine‐like amino acid (324 nm‐MAA), so far only known from the green macroalgal genus Prasiola (Trebouxiophyceae), was also identified in other morphologically diverse green algae closely related to Prasiola spp. in 18S rDNA phylogenies. Using HPLC, a second UV‐absorbing compound was found only in Myrmecia incisa Reisigal among all studied strains. This substance showed an absorption maximum at 322 nm and hence was designated as putative 322 nm‐MAA. Preliminary UV‐exposure experiments indicated that all species containing one or the other MAA showed a strong accumulation of the respective compound, thus supporting their function as putative UV sunscreen. Both UV‐absorbing substances were only identified in the studied members of the Trebouxiophyceae but were absent in members of the Ulvophyceae and Chlorophyceae. When mapped on an 18S rDNA phylogeny, the distribution of 324 nm‐MAA was found to be scattered within the Trebouxiophyceae but was consistent with a distribution that follows phylogenetic patterns rather than ecological adaptations. The 324 nm‐MAA was also detected in two phylogenetically related species from freshwater as well as from subaerial habitats, Watanabea reniformis Hanagata et al. and isolate UR7/5, which were phylogenetically independent of Prasiola and its closer allies. MAAs were absent in another Trebouxiophyceae clade comprising lichen photobionts (Coccomyxa pringsheimii Jaag) as well as freshwater picoplanktonic algae (Choricystis minor (Skuja) Fott). The data presented suggest a chemotaxonomic value of the 324 nm‐MAA in green algal taxonomy. To address the paraphyly of the genus Myrmecia Printz as presently circumscribed, we propose the new combination Lobosphaera incisa.     

Multiple Strategies of Bloom-Forming Microcystis to Minimize Damage by Solar Ultraviolet Radiation in Surface Waters

The occurrence of bloom-forming cyanobacteria is one of the most obvious sign of eutrophication in freshwaters. Although in eutrophic lakes water transparency in the ultraviolet (UV) region is strongly reduced, bloom-forming cyanobacteria are exposed to high solar UV radiation at the surface. Here, we show that, in a natural phytoplankton community from a very eutrophic lake, Microcystis synthesizes UV sunscreen compounds identified as mycosporine-like amino acids (MAAs). The biomass-specific MAA concentration was significantly correlated with the occurrence of Microcystis but not with other algal groups, even though they were dominant in terms of biomass. Based on a photo-optical model, we estimated that the maximum MAA concentration per cell observed (2.5% dry weight) will confer only ~40% of internal screening to a single layer of Microcystis cells. Thus, the formation of a colony with several layers of cells is important to afford an efficient UV screening by internal self-shading. Overall, we propose that Microcystis uses a combination of photoprotective strategies (MAAs, carotenoids) to cope with high solar UV radiation at the water surface. These strategies include also the screening of UV radiation by d-galacturonic acid, one of the main chemical components of the slime layer in Microcystis.     

Field relationships between scytonemin density,growth, and irradiance in cyanobacteria occurring in low illumination regimes

In situ measurements of ultraviolet (UV) irradiance, carbon fixation, and scytonemin pigmentation were made on Scytonema populations from contrasting localities in England. Significant negative correlations were obtained between the following variate pairs: pigmentation and UV irradiance; pigmentation and carbon fixation rate. A significant positive correlation was found between pigmentation and sheath thickness.The negative correlation between pigmentation and UV irradiance was unexpected and appeared contrary to the results of previous studies, which indicated a positive correlation between the variates and the recognition of scytonemin as a radiation shield. However, by considering how radiation damage is related to cell division and the water relations of the sites investigated, it was shown that scytonemin is still functioning as a radiation shield, even in shaded sites. Rivularia colonies produced scytonemin only upon their upper, sun-exposed surfaces and were positively correlated with UV irradiance. This paper also describes the successful use of some new and inexpensive techniques to measure pigments in cyanobacterium sheaths, and integrated in situ UV-irradiance.     

Photoprotective compounds in weakly and strongly pigmented copepods and co-occurring cladocerans

1. The prevalence of mycosporine‐like amino acids (MAAs) – a group of potential ultraviolet (UV)‐photoprotective compounds – was surveyed across 11 species of freshwater copepods from 20 lakes of varying ultraviolet radiation (UVR) transparency in North America, New Zealand and Argentina. Co‐occurring cladocerans were also analysed (seven species from 12 lakes). Many of the calanoid copepod populations were red with carotenoid pigmentation, allowing comparison of MAA and carotenoid accumulation as photoprotective strategies. 2. In two Pennsylvania (U.S.A.) lakes, MAA and carotenoid contents were followed during the early spring to mid‐summer period of lake warming. A pronounced seasonal pattern of higher carotenoid/low MAA content in spring, shifting to low carotenoid/higher MAA content in summer, was observed in calanoids from the more UV transparent lake. 3. All copepod samples contained MAAs. Visibly red calanoids, especially southern Hemisphere Boeckella, often had moderate to high concentrations (2.5–11 μg MAA mg^?1 dw), but low concentrations (0.04–1 μg MAA mg^?1 dw) in some N. American red calanoids show that high carotenoid pigmentation (e.g. 5–10 μg carotenoid mg^?1 dw) does not necessarily imply high MAA content. 4. No cladoceran sample had more than trace amounts of MAAs (<0.05 μg mg^?1 dw). Therefore, MAA accumulation does not seem to be a photoprotective strategy utilized by Daphnia (five species from nine lakes) or other cladocerans. 5. Seven identifiable MAAs were widely distributed among both calanoids and cyclopoids. Shinorine was ubiquitous and was usually the most abundant MAA in N. American samples. In contrast, porphyra‐334 was the predominant MAA in the southern Hemisphere Boeckella. 6. Copepods from higher UVR lakes tended to have a higher MAA content, but this relationship was statistically weak overall and taxon‐specific when found.     

Effects of salinity and ultraviolet radiation on the concentration of mycosporine-like amino acids in various isolates of the benthic cyanobacterium Microcoleus chthonoplastes

The effects of salinity and ultraviolet B (UV‐B) treatment on the intracellular mycosporine‐like amino acid (MAA) concentration in three isolates of the benthic cyanobacterium Microcoleus chthonoplastes from the Baltic Sea (WIS), Spain (EBD) and Australia (TOW) were compared. All strains contained shinorine and, in addition, both EBD and TOW exhibited the unknown MAA‐332, and WIS exhibited the unknown MAA‐346. Salinity treatment led to MAA accumulation in TOW and WIS, but not in EBD. Whereas UV‐B exposure was accompanied by a strong increase in MAA in EBD and TOW, WIS did not survive the treatment. All data indicate isolate‐specific MAA accumulation patterns under different environmental conditions and can be explained by ecotypic differentiation. A double function of MAAs as organic osmolytes and photoprotect‐ants seems possible.     

The synthesis of the UV-screening pigment,scytonemin, and photosynthetic performance in isolates from closely related natural populations of cyanobacteria (Calothrix sp.)

Two populations of the cyanobacterium Calothrix sp. found in Yellowstone thermal spring outflows differ greatly in their contents of scytonemin, a UV-screening pigment, and in their photosynthetic carbon assimilation rates. Clonal isolates from both populations were used to investigate these phenotypic differences. Identical partial 16S rDNA sequences ( approximately 900 bp) suggest a very close relationship between the two Calothrix populations and indicate that environmental differences may, in part, explain the field observations. The effects of native spring water on scytonemin synthesis and photosynthesis were tested during experiments using plated cells. Results show differences in the spring water environment were at least partly responsible for the differences in scytonemin content observed in the field. Furthermore, spring water effects on photosynthetic performance suggest adaptation in these strains to their spring of origin. Controlled experiments performed using cultures grown in artificial liquid medium showed no significant difference in photosynthetic carbon uptake between strains. However, significant differences were detected in their ability to synthesize scytonemin indicating genetic differences between populations. These findings suggest that both genetic and environmental differences are responsible for the naturally occurring variation in scytonemin content and photosynthetic ability in these two closely related populations.     

The biosynthesis of cyanobacterial sunscreen scytonemin in intertidal microbial mat communities

We have examined the biosynthesis and accumulation of cyanobacterial sunscreening pigment scytonemin within intertidal microbial mat communities using a combination of chemical, molecular, and phylogenetic approaches. Both laminated (layered) and nonlaminated mats contained scytonemin, with morphologically distinct mats having different cyanobacterial community compositions. Within laminated microbial mats, regions with and without scytonemin had different dominant oxygenic phototrophs, with scytonemin-producing areas consisting primarily of Lyngbya aestuarii and scytonemin-deficient areas dominated by a eukaryotic alga. The nonlaminated mat was populated by a diverse group of cyanobacteria and did not contain algae. The amplification and phylogenetic assignment of scytonemin biosynthetic gene scyC from laminated mat samples confirmed that the dominant cyanobacterium in these areas, L. aestuarii, is likely responsible for sunscreen production. This study is the first to utilize an understanding of the molecular basis of scytonemin assembly to explore its synthesis and function within natural microbial communities.     

Molecular characterization of hot spring cyanobacteria and evaluation of their photoprotective compounds

Phylogenetic analysis of 4 cyanobacterial strains isolated from hot springs in Rajgir, India, was carried out using the 16S rRNA gene (1400 bp). These strains were identified as members of Chroococcales ( Cyanothece sp. strain HKAR-1) and Nostocales ( Nostoc sp. strain HKAR-2, Scytonema sp. strain HKAR-3, and Rivularia sp. strain HKAR-4). Furthermore, we evaluated the presence of ultraviolet-screening and (or) photoprotective compounds, such as mycosporine-like amino acids (MAAs) and scytonemin, in these cyanobacteria by using high-performance liquid chromatography. Well-characterized MAAs, including the critical and highly polar compounds shinorine, porphyra-334, and mycosporine-glycine, as well as several unknown MAAs, were found in these hot-spring-inhabiting microorganisms. The presence of scytonemin was detected only in Scytonema sp. strain HKAR-3 and Rivularia sp. strain HKAR-4. The results indicate that hot spring cyanobacteria, namely Cyanothece, Nostoc, Scytonema, and Rivularia, belonging to different groups possess various photoprotective compounds to cope up with the negative impacts of damaging radiations.