Abstract
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.
Highlights
Changes in global climate are altering many of the fundamental physical and chemical properties of the oceans that regulate the productivity and ecology of marine phytoplankton groups
We investigated the morphological response to salinity in two of these six strains, which were selected to represent a contrast between a low salinity environment origin and high salinity environment origin (Table 1) that are representative of the end-members of the typical range in modern openocean sea-surface salinity: PLYB11 was isolated from coastal North Sea waters near Bergen, Norway (60 ̊ 18’N, 05 ̊ 15’E) with a mean annual salinity of 30–33 [29]; RCC1232 was isolated in the Mediterranean Sea (43 ̊ 41’N, 07 ̊ 19’E) with a mean annual salinity of ~37 [29]
It is known that the coccolith morphology of the key calcifying phytoplankton species E. huxleyi responds to increasing salinity by increasing coccolith size in natural populations and in laboratory settings
Summary
Changes in global climate are altering many of the fundamental physical and chemical properties of the oceans that regulate the productivity and ecology of marine phytoplankton groups. The response of the single-celled marine phytoplankton group coccolithophores to changes in surface ocean conditions is of particular interest: as major pelagic producers of both biomass and inorganic carbon (calcium carbonate) [1,2] they are key components of marine primary production, ocean biogeochemical cycles, and biogenic-climate feedbacks [3]. The export of their mineralized exoskeletons from the photic zone to ocean sediments forms a remarkably complete fossil record [4] that archives the integrated physiological and ecological effects of changes in marine conditions over geological timescales (last ~200 million years) and their biogeochemical consequences. Records of changes in coccolith morphology make a valuable contribution to biogenic archives of past oceanographic conditions
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