​The phenomenon of cyanobacteria multiplying unlimitedly and accumulating on the water surface of lakes and pond is called freshwater bloom. I am studying the formation of cyanobacteria freshwater blooms particularly the mechanisms of freshwater bloom formation and toxin-producing cyanobacteria. Some cyanobacteria produce a variety of toxins. Recently, we clarified that neurotoxin anatoxins producing cyanobacterium Cuspidothrix issatschenkoi is widely distributed throughout Japan. Previously, toxin-producing strains of this species had been not reported in the Asian region. This species consists of nitrogen fixing ability lacking and/or toxin-producing ability lacking strains. The ability to transfer molecular nitrogen to ammonia is called nitrogen fixation ability. In addition to cyanobacteria, some species of eubacteria, streptomycetes and archaea have this ability. Recent studies indicate that ancestor species of cyanobacteria possessed the ability, but most lineages have lost the ability. In lakes and ponds, ammonia and nitrate which are nitrogen resources for algae are easily decreased. So, nitrogen –fixing ability is a very important attribute for cyanobacteria but they have been evolving toward loosing the ability. Likewise, toxin-producing abilities are also known as abilities acquired by ancestral species and lost repeatedly. I'm very interested in the reason and process of losing these functional genes.