Coccolithophores form a layer of calcium carbonate (CaCO3) platelets (coccoliths) around their cells. Coccoliths are of biogeochemical importance due to ballasting of organic matter with CaCO3, a phenomenon which is thought to promote the transport of organic carbon to the deep ocean (Klaas and Archer, 2002; Rost and Riebesell, 2004). The coccolithophore Emiliania huxleyi forms extensive blooms under favorable light intensity, temperature, and nutrient conditions with different morphotypes in certain regions (Cook et al., 2011; Henderiks et al., 2012; Smith et al., 2012; Balch et al., 2014; Krumhardt et al., 2017).

Variable responses of growth, photosynthetic carbon fixation, and calcification rates of different E. huxleyi strains to rising CO2 levels have been reported (Langer et al., 2009; Hoppe et al., 2011; Müller et al., 2015; Hattich et al., 2017) and are likely a result of intra-specific variability of genotypes (Langer et al., 2009). Several recent studies observed optimum curve responses in physiological rates of a single E. huxleyi strain to a broad pCO2 range from about 20 to 5000 µatm, and linked them to inorganic carbon substrate limitation at low pCO2 and inhibiting H+ concentrations at high pCO2 (Bach et al., 2011, 2015; Kottmeier et al., 2016). Until now, studies on the physiological responses of E. huxleyi to rising CO2 are mostly based on a few genotypes and little is known about the potential variability in CO2 and H+ sensitivity between and within populations. Recently, several studies found substantial variations in CO2 responses for N2 fixation rates between Trichodesmium strains, as well as for growth rates between strains of Gephyrocapsa oceanica, Ostreococcus tauri, and Fragilariopsis cylindrus (Hutchins et al., 2013; Schaum et al., 2013; Pančić et al., 2015; Hattich et al., 2017). Hence, multiple strains, ideally from geographically distinct regions, should be considered for investigating phytoplankton responses to climate change (Zhang et al., 2014; Blanco-Ameijeiras et al., 2016; Krumhardt et al., 2017).

Oceanographic boundaries formed by both ocean currents and environmental factors such as temperature, can limit dispersal of marine phytoplankton, reduce gene flow between geographic populations, and give rise to differentiated populations (Palumbi, 1994). Different populations were found to show different growth rates for E. huxleyi, G. oceanica, and Skeletonema marinoi at the same temperatures, and for Ditylum brightwellii at the same light intensities (Brand, 1982; Rynearson and Armbrust, 2004; Kremp et al., 2012; Zhang et al., 2014). Phenotypic plasticity describes the ability of a strain to change its morphology or physiology in response to changing environmental conditions (Bradshaw, 1965). Plasticity can be assessed by analyzing the reaction norm of one trait and a plastic response may allow a strain to acclimate across an environmental gradient and widen its bio-geographical distribution (Reusch, 2014; Levis and Pfennig, 2016).

In order to better understand how local adaptation affects the physiological response of E. huxleyi to rising CO2 conditions, we isolated 17 strains from three regions in the Atlantic Ocean, and assessed growth, carbon fixation, and calcification responses of the population over a pCO2 range from 120 to 2630 µatm.

We investigated growth, POC, and PIC production rates of 17 E. huxleyi strains from three populations to a broad pCO2 range (120–2630 µatm). The three populations differed significantly in growth and POC production rates at the investigated pCO2 levels. The reaction norms of the individual strains and populations equaled an optimum curve for all physiological rates (Figs. 1 and 3). However, we detected distinct pCO2 optima for growth, POC, and PIC production rates, and different H+ sensitivities for growth and POC production rates among them (Fig. 2). These results indicate the existence of distinct populations in the cosmopolitan coccolithophore E. huxleyi.

In comparison to the Azores and Canary Islands populations, variability in growth rates between strains of the Bergen population was smaller even though they had higher growth rates at all pCO2 levels (Fig. 3). Furthermore, the Bergen population showed significantly higher pCO2 optima and lower H+ sensitivity for growth and POC production rates (Fig. 2). These findings indicate that the Bergen population may be more tolerant to changing carbonate chemistry in terms of its growth and photosynthetic carbon fixation rates. The Bergen strains were isolated from coastal waters, while the Azores and Canary Islands strains were isolated from a more oceanic environment. Seawater carbonate chemistry of coastal waters is usually more dynamic than in the open ocean (Cai, 2011). In fact, previous studies have reported that CO2 and pH variability of the seawater off Bergen was larger than off the Azores and Canary Islands (Table 1). In addition, due to riverine input, seawater upwelling, and metabolic activity of plankton communities, environmental variability in coastal waters are larger than in open-ocean ecosystems (Duarte and Cerbrian, 1996). Doblin and van Sebille (2016) suggested that phytoplankton populations should be constantly under selection when experiencing changing environmental conditions. In this case, the Bergen population, exposed to larger CO2 or pH fluctuations, may have acquired a higher capacity to acclimate to changing carbonate chemistry resulting in a higher tolerance (or lower sensitivity) to rising CO2 levels. In contrast, the Azores and Canary Islands populations experience similar, less variable seawater carbonate chemistry conditions in their natural environment, which could explain why they also show similar pCO2 optima and H+ sensitivity for physiological rates (Fig. 2).

In an earlier study (Zhang et al., 2014), growth rates of the same Azores and Bergen strains as used here were measured at 8–28 ∘C. While at 26–28 ∘C the Bergen strains grew slower than the Azores strains, at 8 ∘C the Azores strains grew slower than the Bergen strains. This illustrates how adaptation to local temperature can significantly affect growth of E. huxleyi strains in laboratory experiments. Considering these findings and the temperature ranges of the three isolation locations (Table S1), the incubation temperature of 16 ∘C used in the present study was lower than the minimum sea surface temperature (SST) commonly recorded at the Canary Islands. In contrast, SSTs of 16 ∘C and lower have been reported for Azores and Bergen waters (Table S1). When exposed to 16 ∘C, growth rate of the Canary Islands population might have already been below their optimum and hence significantly reduced in comparison to the other populations (Fig. 2d).

Furthermore, compared to the Canary Islands population, the Azores population had higher maximum growth and POC production rates, and similar optimum CO2 for these physiological rates. Again, this might be related to sub-optimal incubation conditions as temperature has been found to significantly modulate CO2 responses in coccolithophores in terms of maximum rates, CO2 optima and half-saturation, and H+ sensitivity (Sett et al., 2014; Gafar et al., 2018; Gafar and Schulz, 2018). In a similar fashion, light can also modulate CO2 responses, hence different requirements by strains adapted to different light availabilities could also explain our observations (Zhang et al., 2015; Gafar et al., 2018; Gafar and Schulz, 2018). Thus, with rising CO2, growth, photosynthetic carbon fixation and calcification rates of the Canary Islands population cannot increase as much as in the Azores and Bergen populations. In addition, the Canary Islands population showed smallest variability in optimum pCO2 and maximum values for growth and POC production rates (Fig. 2). The reason may be that low incubation temperature predominantly limited growth and POC production rates of the Canary Islands population, and decreased the sensitivities of these physiological rates to rising pCO2.

Before we started this experiment, strains isolated from the Azores, Bergen, and Canary Islands grew as stock cultures at 15 ∘C and 400 µatm for 4 years, 5 years and 3 months, respectively. Schaum et al. (2015) provide evidence that long-term laboratory incubation affects responses of phytoplankton to different pCO2 levels. Thus, it is conceivable that the same selection history in the laboratory incubation may contribute to a more similar response of growth, POC, and PIC production rates between the Azores and Bergen populations at low pCO2 levels (Fig. 1).

Our results indicate that E. hulxyei populations are adapted to the specific environmental conditions of their origin, resulting in different responses to increasing pCO2 levels. The ability to adapt to diverse environmental conditions is supposed to be one reason for the global distribution of E. huxleyi (Paasche, 2002), spanning a temperature range of about 30 ∘C. In addition, these results will improve our understanding on variation in physiological responses of different E. huxleyi populations to climate change, and variation in production of different areas in future oceans. The optimum temperature for growth of the Bergen population was about 22 ∘C and was 5 ∘C higher than the maximum SST in Bergen waters (Zhang et al., 2014). Furthermore, in comparison to the Azores and Canary Islands populations, larger optimum pCO2 of growth rate indicates that the Bergen population may benefit more from the rising CO2 levels. PIC : POC ratios of the Azores and Bergen populations declined with rising pCO2, whereas PIC : POC ratios of the Canary Islands population were rather constant (Figs. S6, S7). As changes in PIC : POC ratios of coccolithophore blooms were suggested to impact on biological carbon pump (Rost and Riebesell, 2004), variation in PIC : POC ratios of different populations indicates that different regions might have different changes in marine carbon cycle in the future ocean. In natural seawater, due to ocean currents and gene flow, populations at any given location may get replaced by immigrant genotypes transported there from other locations (Doblin and van Sebille, 2016). In addition, E. huxleyi is thought to utilize HCO3- for calcification which generates protons, and increase in proton concentration may mitigate the potential of the ocean to absorb atmospheric CO2 and then give a positive feedback to rising atmosphere CO2 levels (Paasche, 2002).

Within a population, individual strains showed different growth, POC, and PIC production rates at different pCO2 levels, indicating phenotypic plasticity of individual strains (Reusch, 2014). Phenotypic plasticity constitutes an advantage for individual strains to acclimate and adapt to elevated pCO2 by changing fitness-relevant traits and potentially to attenuate the short-term effects of changing environments on fitness-relevant traits (Schaum et al., 2013).

The strain-specific CO2-response curves revealed considerable physiological diversity in co-occurring strains (Fig. 3). Physiological variability makes a population more resilient, increases its ability to persist in variable environments and potentially forms the basis for selection (Gsell et al., 2012; Hattich et al., 2017). It is clear that other environmental factors such as light intensity, temperature, and nutrient concentration affect the responses of physiological rates of individual E. huxleyi strains to changing carbonate chemistry, and thus change the physiological variability within populations (Zhang et al., 2015; Feng et al., 2017). However, different sensitivities and requirements of each strain to the variable environments can allow strains to co-exist within a population in the natural environment (Hutchinson, 1961; Reed et al., 2010; Krueger-Hadfield et al., 2014). In a changing ocean, strain succession is likely to occur and shift the population composition (Blanco-Ameijeiras et al., 2016; Hattich et al., 2017). Strains with higher growth rates or other competitive abilities may out compete others (Schaum et al., 2013). Further, a significant positive correlation between growth and POC production rate or POC quota (Fig. S5) indicates that higher growth rate means larger populations and then greater production.

In the present study, we found population-specific responses in physiological rates of E. huxleyi to a broad pCO2 range, which may have arisen from local adaptation to environmental conditions at their origins. The existence of distinct E. huxleyi populations and phenotypic plasticity of individual strains may both be important for E. huxleyi when adapting to natural environmental variability and to ongoing climate changes. Our results suggest that when assessing phytoplankton responses to changing environments on a global scale, variability in population and strain responses need to be considered. In this study, we only studied the effects of rising CO2 but future studies should take into account simultaneous effects from other interacting factors such as light and temperature variability.