In biotechnological applications, cyanobacteria are employed for conversion of CO2 into bioproducts with sunlight as sole energy source. We describe the isolation of motile filamentous cyanobacteria from rockpools of the North Sea or the Mediterranean Sea and their characterization by physiological assays and genome sequencing. The five isolated lines are genetically highly similar, we regard them as strains of the same species. Phylogenetic studies placed the strains in the genus Phormidium; the species is termed Phormidium lacuna. Under liquid media growth conditions or in photobioreactors, Phormidium growth rates were comparable with the single celled model cyanobacterium Synechocystis PCC6803. However, Phormidium strains tolerate different media that can contain up to 3.7 x the salt concentration of seawater and grows at temperatures up to 50 degrees C. Growth in medium free of NH3 or NO3- suggests that Phormidium can fix atmospheric dinitrogen by nitrogenase even in the presence of light. Genome data confirmed the presence of nitrogenase and revealed its evolutionary position close to anoxygenic delta-proteobacteria. Genes for photosynthesis, photoreceptors, nitrogen metabolism, hydrogenases, tryptophan synthesis, glucose uptake, and fermentative pathways are discussed in the context of biotechnological applications.