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 population dynamics of the capitellid polychaete Capitella aff. teleta were studied in Gamo Lagoon, located in northeast Japan, for the subsequent 2 years from 2016, when a series of restoration works was conducted following the 2011 Great East Japan Earthquake and tsunami. Capitella aff. teleta was found to be widely distributed from the estuary side, where the levee was located, to the innermost part but was more abundant in the innermost part, which is rich in organic matter. In the lagoon, the daily maximum water level dropped from 2017 to 2018 during the reconstruction of a flow-conducting levee, which blocked water flow and isolated the inner part of the lagoon. Although the density decreased drastically for approximately 11 months under diurnal hypoxia and strongly reducing conditions, small-sized new recruits were observed and the population recovered quickly after the daily maximum water level increased. In Gamo Lagoon, C. aff. teleta inhabiting the innermost part and estuary side of the lagoon contributed to maintaining the population by dispersing planktonic larvae between them. Thus, the maximum water level had a significant effect on the maintenance of the C. aff. teleta population in the lagoon, and sufficiently high water levels enable the dispersion of planktonic larvae to help recover the population quickly, suggesting that it is important to keep the water area connected.

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