<i>Synechococcus</i> Under Stress: Contrasting Physiological and Transcriptional Responses to Salinity and Temperature in Marine Versus Euryhaline Strains

Temperature and salinity are key environmental drivers that constrain growth and distribution of marine cyanobacteria, yet their combined physiological effects remain unexplored. We analysed the physiological and transcriptional responses of two Synechococcus strains, the marine RS9907 and euryhaline WH5701, across a salinity gradient (18-50 PSU) under optimal (28°C-30°C) and low temperature conditions (15°C-20°C). Growth and photosynthetic efficiency (F_V/F_M) declined under salinity stress (18 PSU for RS9907 and 50 PSU for WH5701), relative to typical marine conditions (36 PSU). RS9907 maintained the photosynthetic electron transport rate under salt stress and the F_V/F_M under cold conditions more effectively than WH5701. Salinity induced a stronger regulatory response in WH5701 (71% genes differentially expressed, compared to only 6% in RS9907). Both strains shared a core response, upregulating carbon fixation genes under cold stress, and glycogen degradation and osmolyte synthesis genes at high salinity (42-50 PSU). Conversely, some photosynthetic genes (psbCD, psaC) showed increased expression at low salinity, but temperature-dependent regulatory differences were observed. WH5701 uniquely upregulated genes related to membrane transporters, fatty acid desaturases and the pentose phosphate pathway within salinity, potentially contributing to their broader tolerance to salt fluctuations. Collectively, our results reveal contrasting strategies of thermohaline acclimation in Synechococcus strains adapted to different salinities.