Salinity-dependent copy number increase of a marine cyanobacterial endogenous plasmid

A marine cyanobacterium Synechococcus sp. NKBG 042902 grown in the presence of various NaCl concentrations showed different plasmid profiles. This strain possesses a 2.7-kb endogenous plasmid pSY10, whose copy number increased five-fold when the salinity of the growth medium was increased from 0 to 3% NaCl. Cells were grown initially in a fresh water medium and then transferred to a NaCl supplemented medium. The copy number of pSY10 began to increase after 7 h of incubation in 3% NaCl BG11 medium. This phenomenon was specific to NaCl and did not occur when other osmotica such as KCl and sorbitol were used. This is the first report of a cyanobacterial plasmid with a salinity-dependent copy number.