Mycosporine-like amino acids (MAAs) are water-soluble UV-absorbing pigments, and structurally different MAAs have been identified in eukaryotic algae and cyanobacteria. In this study novel glycosylated MAAs were found in the terrestrial cyanobacterium Nostoc commune (N. commune). An MAA with an absorption maximum at 334 nm was identified as a hexose-bound porphyra-334 derivative with a molecular mass of 508 Da. Another MAA with an absorption maximum at 322 nm was identified as a two hexose-bound palythine-threonine derivative with a molecular mass of 612 Da. These purified MAAs have radical scavenging activities in vitro, which suggests multifunctional roles as sunscreens and antioxidants. The 612-Da MAA accounted for approximately 60% of the total MAAs and contributed approximately 20% of the total radical scavenging activities in a water extract, indicating that it is the major water-soluble UV-protectant and radical scavenger component. The hexose-bound porphyra-334 derivative and the glycosylated palythine-threonine derivatives were found in a specific genotype of N. commune, suggesting that glycosylated MAA patterns could be a chemotaxonomic marker for the characterization of the morphologically indistinguishable N. commune. The glycosylation of porphyra-334 and palythine-threonine in N. commune suggests a unique adaptation for terrestrial environments that are drastically fluctuating in comparison to stable aquatic environments.

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SUMMARY The cyanobacterium Nostoc commune is adapted to terrestrial environments and has a cosmopolitan distribution. Four genotypes of N. commune can be identified based on differences in their 16S rRNA genes, and these genotypes are distributed throughout Japan without regional specificity. Mycosporine‐like amino acids (MAAs) are UV‐absorbing pigments, and novel glycosylated MAA derivatives with radical scavenging activities have been identified in N. commune. In this study, we investigated the consistency of the relationship between MAA compositions and N. commune genotypes. The MAA compositions were different in a genotype‐specific manner, suggesting that the types of MAA derivatives can feasibly be used as chemotaxonomic markers to characterize N. commune. The novel 756‐Da MAA, which was identified as an aglycone of the 1050‐Da MAA and named nostoc‐756, occurred in genotype C of N. commune. Nostoc‐756 functioned as a radical scavenger in vitro. In conclusion, N. commune is classified into four groups representing genetically different chemotypes, namely, the arabinose‐bound porphyra‐334 producer (chemotype A), the glycosylated nostoc‐756 producer (chemotype B), the nostoc‐756 producer (chemotype C) and the glycosylated palythine‐threonine producer (chemotype D). Either the molecular taxonomical method or chemical analysis of a characteristic secondary metabolite is sufficient to identify the types of N. commune; however, there are no obvious ecophysiological differences that allow us to distinguish them.

The terrestrial cyanobacterium Nostoc commune forms macroscopic colonies in its natural habitats, and these colonies consist of both cellular filaments and massive extracellular matrixes. In this study, the biochemical features of the extracellular matrix components were investigated. Naturally growing N. commune was tolerant to desiccation, and produced massive extracellular polysaccharides that contained both neutral sugars and glucuronic acid as constituent monosaccharides. The extracellular polysaccharide contents and desiccation tolerance were compared in laboratory culture strains of Nostoc species. The laboratory culture of N. commune strain KU002 was sensitive to desiccation and produced smaller amounts of extracellular polysaccharides, unlike the field-isolated naturally growing colonies. Nostoc punctiforme strain M-15, which is genetically closed to N. commune, was able to tolerate desiccation, although the other Nostoc strains were desiccation-sensitive. A laboratory culture strain of the aquatic cyanobacterium Nostoc sphaericum produced massive extracellular polysaccharides but was sensitive to desiccation, suggesting that extracellular matrix production is not enough to make this strain tolerant to desiccation. WspA (water stress protein) and SodF (superoxide dismutase) were found to be characteristic protein components of the extracellular matrix of N. commune. Because the WspA proteins were heterogeneous, the wspA genes were highly diverse among the different genotypes of N. commune, although the sodF gene was rather conservative. The heterogeneity of the WspA proteins suggests their complex roles in the environmental adaptation mechanism in N. commune.