Adaptation of the Freshwater Bloom-Forming Cyanobacterium to Brackish Water Is Driven by Recent Horizontal Transfer of Sucrose Genes.

is a bloom-forming cyanobacterium found in eutrophic water bodies worldwide. blooms usually occur in freshwater; however, they have also been reported to occur in brackish water. Because often produces the cyanotoxin microcystin, they are a major concern to public health and environment. Despite this, the ecology, genomic basis, and evolutionary process underlying the bloom invasion from fresh to brackish water have been poorly investigated. Hence, in the present study, we have sequenced and characterized genomes of two newly discovered salt-tolerant strains obtained from Japanese brackish water lakes (Lakes Shinji and Tofutsu). Both genomes contain a set of genes for the synthesis of osmolyte sucrose (, and ), hitherto identified in only one strain (PCC 7806) of . Chemical and gene expression analyses confirmed sucrose accumulation induced by salt. A comprehensive genetic survey of >200 strains indicated that sucrose genes are extremely rare in . Most surprisingly, comparative genome analyses of the three strains indicated extremely low genetic diversity in the sucrose genes compared with other core genome genes, suggesting very recent acquisitions via horizontal transfer. Invasion of blooms into brackish water may be a recent event triggered by anthropogenic eutrophication of brackish water.