The salt tolerance of the freshwater cyanobacterium Microcystis depends on nitrogen availability

In the past two decades, there has been an apparent increase in the frequency, intensity, and geographical distribution of Microcystis blooms in brackish waters, which migrates through rivers rather than growing in situ. It is unclear how Microcystis maintains its salt tolerance capacity and acclimates in brackish water. In this study, laboratory and field experiments were conducted to investigate the relationship between nitrogen availability and salt tolerance of Microcystis. Results showed that low nitrogen availability significantly enhanced the salt tolerance of Microcystis. Salt tolerance of HN-Microcystis (Microcystis collected under conditions of high nitrogen availability) and LN-Microcystis (Microcystis collected under conditions of low nitrogen availability) was 4.5‰ and 11.3‰, respectively. Field LN-Microcystis (Microcystis collected under low nitrogen availability on the 7th day) performed similar variation and salt tolerance reached up to 17.4‰. Physiological processes of laboratory-cultured Microcystis confirmed the phenomenon that low nitrogen availability alleviated the damage to cells caused by salt stress. Moreover, low nitrogen availability significantly induced carbohydrate accumulation, which was approximately 1.55 and 2.22 times greater than that accumulating in HN-Microcystis and field HN-Microcystis (Microcystis collected under high nitrogen availability on the 2nd day), respectively. Sucrose, trehalose, and glucose were approximately 2, 2, and 31 times greater than HN-Microcystis. This study explored a novel underlying mechanism for enhancing the salt tolerance of Microcystis. We demonstrated that carbohydrate accumulation induced by low nitrogen availability alleviated the disorder of intracellular physiological processes caused by salt stress and thus enhanced the salt tolerance of Microcystis. While studying the migration of Microcystis across the freshwater-to-marine continuum, the physiological characteristics of Microcystis obtained in low nitrogen availability conditions should be considered.