| Summary: | Cyanobacteria are a diverse group of photosynthetic Gram-negative bacteria that produce an array of secondary compounds with selective bioactivity against a broad spectrum of organisms and cell lines. In this study, 29 strains isolated from freshwaters in Greece were classified using a polyphasic approach and assigned to Chroococcales, Synechococcales, and Nostocales, representing 11 genera and 17 taxa. There were good agreements between 16S ribosomal RNA (rRNA)−<i>cpcBA</i>−internal genetic spacer (IGS) characterization and morphological features, except for the <i>Jaaginema</i>−<i>Limnothrix</i> group which appears intermixed and needs further elucidation. Methanol extracts of the strains were analyzed for cyanotoxin production and tested against pathogenic bacteria species and several cancer cell lines. We report for the first time a <i>Nostoc oryzae</i> strain isolated from rice fields capable of producing microcystins (MCs) and a <i>Chlorogloeopsis fritschii</i> strain isolated from the plankton of a lake, suggesting that this species may also occur in freshwater temperate habitats. Strains with very high or identical 16S rRNA gene sequences displayed different antibacterial and cytotoxic activities. Extracts from <i>Synechococcus</i> cf. <i>nidulans</i> showed the most potent antibacterial activity against <i>Staphylococcus aureus</i>, whereas <i>Jaaginema</i> sp. strains exhibited potent cytotoxic activities against human colorectal adenocarcinoma and hepatocellular carcinoma cells. <i>Jaaginema</i> Thessaloniki Aristotle University Microalgae and Cyanobacteria (TAU-MAC) 0110 and 0210 strains caused pronounced changes in the actin network and triggered the formation of numerous lipid droplets in hepatocellular carcinoma and green monkey kidney cells, suggesting oxidative stress and/or mitochondrial damage leading to apoptosis.
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