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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. [ABSTRACT FROM AUTHOR]

The future physiology of marine phytoplankton will be impacted by a range of changes in global ocean conditions, including salinity regimes that vary spatially and on a range of short- to geological timescales. Coccolithophores have global ecological and biogeochemical significance as the most important calcifying marine phytoplankton group. Previous research has shown that the morphology of their exoskeletal calcified plates (coccoliths) responds to changing salinity in the most abundant coccolithophore species, Emiliania huxleyi. However, the extent to which these responses may be strain-specific is not well established. Here we investigated the growth response of six strains of E. huxleyi under low (ca. 25) and high (ca. 45) salinity batch culture conditions and found substantial variability in the magnitude and direction of response to salinity change across strains. Growth rates declined under low and high salinity conditions in four of the six strains but increased under both low and high salinity in strain RCC1232 and were higher under low salinity and lower under high salinity in strain PLYB11. When detailed changes in coccolith and coccosphere size were quantified in two of these strains that were isolated from contrasting salinity regimes (coastal Norwegian low salinity of ca. 30 and Mediterranean high salinity of ca. 37), the Norwegian strain showed an average 26% larger mean coccolith size at high salinities compared to low salinities. In contrast, coccolith size in the Mediterranean strain showed a smaller size trend (11% increase) but severely impeded coccolith formation in the low salinity treatment. Coccosphere size similarly increased with salinity in the Norwegian strain but this trend was not observed in the Mediterranean strain. Coccolith size changes with salinity compiled for other strains also show variability, strongly suggesting that the effect of salinity change on coccolithophore morphology is likely to be strain specific. We propose that physiological adaptation to local conditions, in particular strategies for plasticity under stress, has an important role in determining ecotype responses to salinity. [ABSTRACT FROM AUTHOR]

Coccolith mass is an important parameter for estimating coccolithophore contribution to carbonate sedimentation, organic carbon ballasting and coccolithophore calcification. Single coccolith mass is often estimated based on the ks model, which assumes that length and thickness increase proportionally. To evaluate this assumption, this study compared coccolith length, thickness, and mass of seven Emiliania huxleyi strains and one Gephyrocapsa oceanica strain grown in 25, 34, and 44 salinity artificial seawater. While coccolith length increased with salinity in four E. huxleyi strains, thickness did not increase significantly with salinity in three of these strains. Only G. oceanica showed a consistent increase in length with salinity that was accompanied by an increase in thickness. Coccolith length and thickness was also not correlated in 14 of 24 individual experiments, and in the experiments in which there was a positive relationship r2 was low (<0.4). Because thickness did not increase with length in E. huxleyi, the increase in mass was less than expected from the ks model, and thus, mass can not be accurately estimated from coccolith length alone. [ABSTRACT FROM AUTHOR]

Abstract: We analysed the morphology of Emiliania huxleyi from globally distributed plankton samples and demonstrate that the size of E. huxleyi placoliths is highly correlated to in-situ sea surface water salinity. We used multiple linear regression analysis to link morphological parameters of E. huxleyi to in-situ salinity and in-situ temperature. The best multiple regression model yielded an R 2 = 0.84 with a standard residual error of 0.65 for in-situ salinity over a gradient from 32.6 to 38.8. No significant correlation existed with temperature. Our results suggest that the morphology of E. huxleyi placoliths of recent and ancient sediments may provide a robust method to reconstruct sea surface salinities. One caveat is that the plankton-derived multiple regression model for in-situ salinities is different from that reported from previous work on Holocene sediments. This discrepancy is most likely caused by taphonomic processes or from the biogeographically biased data sets which were used (open ocean versus near shore). The cause of the tight relation between salinity and morphological response is not well understood but may be related to cell turgor regulation that affects the size of the cell and thus the size of a single coccolith. [Copyright &y& Elsevier]

Abstract: Morphometric analysis of coccoliths of various coccolithophore species from core top and culture samples has revealed that the morphology of single placoliths can be affected by environmental parameters such as temperature, salinity, and productivity. In this study, we have tested the morphological variations of placoliths of Emiliania huxleyi, the most common coccolithophore species in the modern ocean, with respect to environmental gradients. We have investigated especially the potential effect of salinity on the morphology, as it is known from culture studies that the growth of E. huxleyi placoliths is strongly influenced by varying salinities. Morphometric analysis of about 2150 placoliths from 37 globally distributed core top and plankton samples revealed that the morphology of E. huxleyi varies significantly along a sea surface salinity gradient from 33 to 38. Morphological parameters such as the mean size of placoliths are used as explaining variables in multiple regression analysis to reconstruct sea surface salinities (SSS). The best model for mean SSS yields an R 2 of 0.88 with a standard error of 0.49 whereas statistical models describing surface water temperature or productivity are less reliable and accurate. Application of the best model to plankton samples allows the prediction of in-situ salinities within given error estimates. The application of the best model to fossil assemblages from the LGM, however, is more puzzling. Paleosalinity reconstructions in the South Atlantic, the Caribbean Sea, and the subtropical Atlantic are in good agreement with the published values. [Copyright &y& Elsevier]

Culture experiments were used to assess the applicability of Emiliania huxleyi coccolith morphology as a palaeo-sea-surface salinity (SSS) proxy. Coccolith morphology was dependent on salinity over a range reflecting present day marine conditions; both coccolith size and the number of coccolith elements increased linearly with increasing salinity. Using regression analysis, the effect of salinity on coccolith morphology was compared to those previously observed in sediment core-top and plankton data. No significant differences were found between the slopes of these data, suggesting that salinity is the primary control on E. huxleyi coccolith size and element number in the ocean. However, the intercepts of the culture data were significantly higher. A combination of experimental and literature analysis indicated that temperature and nutrients were unlikely to be the causes of this discrepancy. Literature analysis also highlighted that coccolith size data from marginal environments displayed different intercepts to those from the open-ocean data. This suggests that discrete morphotypes exist in these marginal locations. We, therefore, recommend that the original E. huxleyi coccolith morphology palaeo-SSS transfer function requires further evaluation before being routinely applied. [ABSTRACT FROM AUTHOR]