The taxonomic diversity of actinomycetes from soils in a cool-temperate area (74 samples from Rishiri Island) and a subtropical area (94 samples from Iriomote Island) of Japan was compared. Soil samples were treated with five different pre-treatment methods and subsequently cultured on humic acid-vitamin agar, resulting in the selective isolation of 1,234 actinomycete strains. These isolates were classified using 16S rDNA sequence analysis. 16S rDNA sequence similarity values of ≥97% and ≥98.7% were tentatively used to identify isolates at the genus and species ranks, respectively. The actinomycete assemblages isolated comprised diverse species. Specifically, 668 of the Rishiri isolates fell into 17 families, 40 genera, and 178 species, and 566 of the Iriomote isolates belonged to 21 families, 59 genera, and 194 species. Diversity analyses based on the Shannon index revealed that there was no significant difference between the species diversity of the Rishiri isolates and that of the Iriomote isolates. However, there was little taxonomic overlap between the Rishiri isolates and the Iriomote isolates, as only 66 of the species occurred in both groups. Several species that were reportedly isolated from other tropical areas were found among the Iriomote isolates but not among the Rishiri isolates. This result suggests that climate may influence the species types of soil-inhabiting actinomycetes.

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Actinomycetes isolated from marine sediments in neritic zone in the sea around Japan were compared with terrestrial isolates. Among actinomycetes that are macroscopically Streptomyces and Micromonospora, marine isolates of both genera were found to have higher NaCl tolerance than terrestrial ones. Around 37% of Streptomyces and 26% of Micromonospora that were isolated from the marine environment could tolerate up to 12% and 5% NaCl, respectively. However, no terrestrial isolates could tolerate NaCl at these concentrations. The results of the 16S rRNA gene sequence analysis of 10 strains with high NaCl tolerance among these marine isolates showed that they had a close phylogenetic relationship with terrestrial strains isolated previously. The isolation of actinomycetes from marine sediments and terrestrial soils in the presence or absence of 6% NaCl in media revealed that the selective pressure of NaCl demonstrated no clear difference between both isolates. Terrestrial strains isolated by using a medium supplemented with 6% NaCl exhibited a high frequency (95%) of NaCl tolerance up to 9%. The highly NaCl tolerant strains isolated from terrestrial soils were frequently found to produce antimicrobial substances in the presence of seawater. Almost 75% of strains tested showed antimicrobial activities against Bacillus subtilis PCI 219 and 40% against Candida albicans 3147. It was suggested that the frequency of microorganisms with antimicrobial properties was increased in the medium supplemented with seawater.

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Terrestrial Actinobacteria have served as a primary source of bioactive compounds. However, a rapid reduction in the discovery of new compounds strongly necessitates new investigational approaches, including screening Actinobacteria from marine habitats. We therefore studied and compared the diversity of culturable Actinobacteria associated with 18 marine sponge samples. A total of 462 strains of Actinobacteria belonging to 19 genera were isolated; 24 of these strains represented new species in six genera. Only 28 strains showed seawater-dependent growth. Eleven strains belonging to the genera Streptomyces (8 strains), Rhodococcus (2), and Nocardiopsis (1) required ≥50% (v/v) seawater for their growth, whereas an additional 17 strains showed better growth in the presence of 50% seawater (v/v). Screening of strains for antimicrobial activity showed that 144, 24, and 77 strains exhibited activity against Micrococcus luteus, Escherichia coli, and Candida albicans, respectively.

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