Abstract. The marine coccolithophore species Emiliania huxleyi tolerates a broad range of salinity conditions over its near-global distribution, including the relatively stable physiochemical conditions of open-ocean environments and nearshore environments with dynamic and extreme short-term salinity fluctuations. Previous studies show that salinity impacts the physiology and morphology of E. huxleyi, suggesting that salinity stress influences the calcification of this globally important species. However, it remains unclear how rapidly E. huxleyi responds to salinity changes and therefore whether E. huxleyi morphology is sensitive to short-term transient salinity events (such as occur on meteorological timescales) in addition to longer-duration salinity changes. Here, we investigate the real-time growth and calcification response of two E. huxleyi strains isolated from shelf sea environments to the abrupt onset of hyposaline and hypersaline conditions over a time period of 156 h (6.5 d). Morphological responses in the size of the cell covering (coccosphere) and the calcium carbonate plates (coccoliths) that form the coccosphere occurred as rapidly as 24–48 h following the abrupt onset of salinity 25 (hyposaline) and salinity 45 (hypersaline) conditions. Generally, cells tended towards smaller coccospheres (−24 %) with smaller coccoliths (−7 % to −11 %) and reduced calcification under hyposaline conditions, whereas cells growing under hypersaline conditions had either relatively stable coccosphere and coccolith sizes (Mediterranean strain RCC1232) or larger coccospheres (+35 %) with larger coccoliths (+13 %) and increased calcification (Norwegian strain PLYB11). This short-term response is consistent with reported coccolith size trends with salinity over longer durations of low- and high-salinity exposure in culture and under natural-salinity gradients. The coccosphere size response of PLYB11 to salinity stress was greater in magnitude than was observed in RCC1232 but occurred after a longer duration of exposure to the new salinity conditions (96–128 h) compared to RCC1232. In both strains, coccosphere size changes were larger and occurred more rapidly than changes in coccolith size, which tended to occur more gradually over the course of the experiments. Variability in the magnitude and timing of rapid morphological responses to short-term salinity stress between these two strains supports previous suggestions that the response of E. huxleyi to salinity stress is strain specific. At the start of the experiments, the light condition was also switched from a light : dark cycle to continuous light, with the aim of desynchronising cell division. As cell density and mean cell size data sampled every 4 h showed regular periodicity under all salinity conditions, the cell division cycle retained its entrainment to pre-experiment light : dark conditions for the entire experiment duration. Extended acclimation periods to continuous light are therefore advisable for E. huxleyi to ensure successful desynchronisation of the cell division cycle. When working with phased or synchronised populations, data should be compared between samples taken from the same phase of the cell division cycle to avoid artificially distorting the magnitude or even direction of physiological or biogeochemical response to the environmental stressor.
