Abstract
Microcystis aeruginosa is a bloom-forming cyanobacterium found in eutrophic water bodies worldwide. M. aeruginosa blooms usually occur in freshwater; however, they have also been reported to occur in brackish water. Because M. aeruginosa 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 M. aeruginosa 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 M. aeruginosa strains obtained from Japanese brackish water lakes (Lakes Shinji and Tofutsu). Both genomes contain a set of genes for the synthesis of osmolyte sucrose (sppA, spsA, and susA), hitherto identified in only one strain (PCC 7806) of M. aeruginosa. 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 M. aeruginosa. 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 M. aeruginosa blooms into brackish water may be a recent event triggered by anthropogenic eutrophication of brackish water.
Highlights
Water blooms formed by phytoplankton species are often observed in eutrophic bodies of water
Genetic composition analyses of M. aeruginosa on the basis of ftsZ indicated that the M. aeruginosa blooms in 2010 and 2011 were genetically highly homogenous, comprising almost entirely of a single genotype, which was the same as that of Sj (Figure 1)
The genes actM and pfnM, which were previously suggested to have a possible involvement in salt tolerance (Guljamow et al, 2007), are absent in Sj and NIES-1211. These results suggest that sucrose genes are responsible for the salt tolerance of M. aeruginosa, and allow for M. aeruginosa blooms in brackish water up to 10 psu
Summary
Water blooms formed by phytoplankton species are often observed in eutrophic bodies of water. Microcystis aeruginosa is the most common and widespread cyanobacterial species found in freshwater environments extending from tropical to subfrigid zones (Harke et al, 2016). M. aeruginosa blooms have been reported in brackish water such as lagoons and estuaries (Preece et al, 2017). Some of these cases can be explained by movement from a freshwater origin (e.g., blooms drifting down a river from a freshwater reservoir to a coastal area) rather than local growth (Miller et al, 2010a). The possibility of M. aeruginosa blooms in brackish water arising due to genetic acquisition of salt tolerance has not been explicitly studied
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