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Independence of nutrient limitation and carbon dioxide impacts on the Southern Ocean coccolithophore Emiliania huxleyi

Future oceanic conditions induced by anthropogenic greenhouse gas emissions include warming, acidification and reduced nutrient supply due to increased stratification. Some parts of the Southern Ocean are expected to show rapid changes, especially for carbonate mineral saturation. Here we compare the physiological response of the model coccolithophore Emiliania huxleyi (strain EHSO 5.14, originating from 50oS, 149oE) with pH/CO2 gradients (mimicking ocean acidification ranging from 1 to 4 × current pCO2 levels) under nutrient-limited (nitrogen and phosphorus) and -replete conditions. Both nutrient limitations decreased per cell photosynthesis (particulate organic carbon (POC) production) and calcification (particulate inorganic carbon (PIC) production) rates for all pCO2 levels, with more than 50% reductions under nitrogen limitation. These impacts, however, became indistinguishable from nutrient-replete conditions when normalized to cell volume. Calcification decreased three-fold and linearly with increasing pCO2 under all nutrient conditions, and was accompanied by a smaller ~30% nonlinear reduction in POC production, manifested mainly above 3 × current pCO2. Our results suggest that normalization to cell volume allows the major impacts of nutrient limitation (changed cell sizes and reduced PIC and POC production rates) to be treated independently of the major impacts of increasing pCO2 and, additionally, stresses the importance of including cell volume measurements to the toolbox of standard physiological analysis of coccolithophores in field and laboratory studies.


Introduction
Anthropogenic carbon dioxide absorbed by the surface ocean rapidly alters the seawater carbonate speciation, resulting in an increase of seawater CO 2 and HCO 3 − concentrations (ocean carbonation) and a decrease of CO 3 2 − and pH (ocean acidification). This process is accompanied by additional environmental changes such as surface warming, stratification and the decreased supply of nutrients to the surface mixed layer (Sarmiento et al., 1998). These multiple changes in the oceanic environment are acting simultaneously and with different regional and temporal intensities (Boyd and Doney, 2002;Boyd et al., 2008). It has become increasingly important to investigate how multiple drivers affect the physiological response of phytoplankton to predict future eco-physiological changes in phytoplankton communities (Boyd and Hutchins, 2012;Boyd et al., 2015a;Brennan and Collins, 2015;Riebesell and Gattuso, 2015). In today's ocean, light supply and the availability of macro-and micronutrients in the surface mixed layer represent the main limiting factors for phytoplankton growth (Davey et al., 2008;Moore et al., 2008;Marinov et al., 2010). While it is projected that the underwater light supply for phytoplankton growth will be enhanced due to shallower mixed layers, the nutrient supply from the underlying water column will be decreased (Boyd et al., 2008). Thus, it seems reasonable to assume that in a future ocean nutrient availability will have a pronounced role in limiting phytoplankton growth.
Coccolithophores are the dominant pelagic calcifiers in the ocean and play an important role in the cycling of carbon over both modern and geological time scales due to their ability to photosynthesize and to produce minute calcium carbonate scales (coccoliths) in the sunlit layer of the ocean. The formation and subsequent export and deposition of coccoliths to the sediments reduce alkalinity of the surface ocean, decreasing the efficiency of the 'biological pump' to sequester anthropogenic carbon dioxide. Estimating the distribution and productivity of coccolithophores in comparison with noncalcifying phytoplankton represents an important component for predictive biogeochemical models to assess possible changes in climatic feedback mechanisms. The response of coccolithophores to changing carbonate chemistry has been intensively investigated in nutrient-replete batch laboratory culture experiments and is overall well understood Meyer and Riebesell, 2015). The combined effects of changing carbonate chemistry and nutrient availability are less well understood and have been mostly documented for the model species Emiliania huxleyi isolated from northern hemisphere waters (Sciandra et al., 2003;Leonardos and Geider, 2005;Borchard et al., 2011;Lefebvre et al., 2012;Müller et al., 2012;Engel et al., 2014). However, coccolithophores from low-latitude oceans and especially the Southern Ocean remain understudied even though the Southern Ocean is one of the world's regions affected most severely by anthropogenic acidification (Orr et al., 2005;Gille, 2008). Emiliania huxleyi is represented in the Southern Ocean by several morphotypes, of which the ecotypes A and B/C are mainly found in the open ocean (Cubillos et al., 2007;Krueger-Hadfield et al., 2014). These ecotypes differ in cell size, coccolith morphology, photosynthetic pigment compositions and physiological response sensitivity to environmental conditions (Cook et al., 2011(Cook et al., , 2013. Southern Ocean E. huxleyi ecotype A and B/C exhibit lower physiological rates (photosynthesis, calcification and growth rate) compared with its northern hemisphere counterpart and, additionally, a higher sensitivity to changing seawater carbonate speciation (Müller et al., 2015). The Southern Ocean represents a geographically isolated ecosystem for phytoplankton due to the Subantarctic and Polar fronts which reduce the possibility of lateral gene transfer and recombination. It is therefore unknown if mechanistic physiological concepts developed for northern hemisphere ecotypes (e.g. Bach et al., 2015) can be transferred to their Southern Ocean counterparts. Here, we investigated the sensitivity of Southern Ocean E. huxleyi ecotype A to future ocean acidification scenarios under nutrient-limited growth using exponentially growing chemostat cultures to which controlled amounts of nitrate and phosphate were supplied. These physicochemical conditions were identical to those applied in our previous study (Müller et al., 2015), facilitating direct comparison of the effects of ocean acidification between nutrientreplete and -limited growing cultures of E. huxleyi.

Material and methods
Culture conditions Emiliania huxleyi originating from the Southern Ocean (strain EHSO 5.14-ecotype A; isolated from 50°S and 149°E by S Cook in February 2007) was grown under axenic conditions as asexual diploids at 14°C in 0.2 μm filtered natural seawater (collected offshore of Bruny-Island, Tasmania) with a salinity of 35 psu and a continuous photon flux density of 100-115 μmol photons m − 2 s − 1 . Emiliania huxleyi was cultured under continuous light over more than 6 months prior to experimental use to ensure full entrainment of the cellular division cycle and other intracellular circadian oscillations. The strain was obtained from the Algae Culture Collection at the Institute of Marine and Antarctic Studies at the University of Tasmania, Australia. Macro-and micronutrients were added according to f/20 and f/80, respectively, corresponding to 88 μmol l − 1 nitrate and 3.6 μmol l − 1 phosphate (Guillard, 1975), which resulted in improved growth behaviour (Müller et al., 2015).

Experimental chemostat setup
All experiments were conducted in chemostat culture vessels consisted of water-jacketed 2 l cylinders (filled to 1.8 l) connected to a circulating water bath maintained at a constant temperature of 14°C (light conditions as described above). Chemostat culture vessels were connected to the seawater media supply tanks (10 l cubitainers) by the gastight silicon tubing system running through one peristaltic pump (Gilson-Minipuls, Gilson S.A.S., Villiers le Bel, France) per chemostat. Before experimental utilization, the culture vessels and all tubing were cleaned and rinsed completely with a 10% HCl solution. The chemostat vessels were filled for 24 h with 10% HCl and subsequently rinsed with MilliQwater and sterile seawater. Culture media supplying the chemostat experiments were prepared in 10 l cubitainers (supply tanks), which were pre-washed with 10% HCl and rinsed with deionized water before utilization. The cubitainers were tested for CO 2 permeability by filling them with acidified seawater (pCO 2 of~2100 μatm) leaving no headspace and, subsequently, the total dissolved inorganic carbon (C T ) concentration was monitored over a 1 month period. The C T concentration in the cubitainer had no significant change over the course of the test period (C T = 2085.5 ± 1.5 μmol kg − 1 , n = 5).
Seawater was adjusted to the target carbonate chemistry speciation by addition of HCl and NaHCO 3 . Afterwards, macro-and micronutrients were added according to f/20 and f/80, respectively, except for the limiting nutrient (phosphate or nitrate). When nitrogen was the limiting element, initial nitrate concentrations were set to 5.3 ± 0.9 μmol l − 1 (1s.d., n = 4), resulting in an N:P ratio of~2.8 and when phosphorus was the limiting element, initial phosphate concentrations were set to 0.32 ± 0.6 μmol l − 1 (1s.d., n = 4), resulting in an N:P ratio of~137. Culture medium was transferred from the chemostat supply tanks (cubitainers) to the precleaned chemostat vessels via acid cleaned tubes passing a 0.2-μm sterile acid cleaned filter. When the chemostat vessels were filled with 1800 ml of culture medium, the supply was stopped, an exponential growing pre-culture of E. huxleyi (acclimated for 4-6 generations to target pCO 2 levels) was inoculated and the vessel was sealed airtight. Emiliania huxleyi (starting density of 90-130 cells ml − 1 ) was allowed to grow exponentially to the maximum population density (growth of 8-9 generations depending on nitrate or phosphate availability) and afterwards the medium inflow (dilution rate) from the supply tanks to the chemostat vessels was restarted. The chemostat vessels were operated at a constant dilution rate (D = 0.20 ± 0.01 day − 1 ), which was periodically checked by weighting the incoming medium. After 7-15 days under limitation (acclimation period), E. huxleyi reached equilibrium state conditions (constant cell number with less than 7% variations) and was allowed to grow for another 6-10 days (3-7 generations) under equilibrium before the dilution was stopped and the chemostat culture was sampled. During equilibrium conditions, cell number, coccosphere, cell and coccolith volume were checked daily with a Coulter Multisizer 4 and the chemostat supply tanks were sampled every second day for C T and total alkalinity (A T ). At the end of the chemostat experiments, samples were taken from the chemostat vessels for C T , A T , cell number, coccosphere, cell and coccolith volume, particulate organic carbon (POC) and total particulate carbon (TPC). Cell densities at the termination of the experiment ranged from 29 × 10 3 to 50 × 10 3 cells ml − 1 to avoid major changes in seawater carbonate chemistry and light limitation due to self-shading. Nutrient concentrations (nitrate +nitrate and phosphate) were measured from the chemostat vessel at the end of the experiment.
Experiments were conducted under axenic conditions but bacterial contamination cannot be excluded. Coulter Multisizer 4 analysis of possible bacteria and detritus particles ranging from 0.12 to 2.5 μm 3 indicated a maximum abundance of 1.8 × 10 6 ml − 1 . This bacterial abundance is comparable to previous E. huxleyi chemostat experiments, contributing less than 2% of the total POC (Lefebvre et al., 2012;Müller et al., 2012;Engel et al., 2014), and we therefore assume bacterial activity to be negligible.

Carbonate chemistry
The carbonate system was monitored via C T and A T measurements. Dissolved inorganic carbon and A T were analysed as the mean of triplicate measurements with the infrared detection method using an Apollo SciTech DIC-Analyzer (Apollo SciTech, Newark, DE, USA) (Model AS-C3) and the potentiometric titration method (Dickson et al., 2003), respectively. Data were corrected to Certified Reference Materials (CRM; Scripps Institution of Oceanography, La Jolla, CA, USA). Consecutive measurements of the reference material resulted in an average precision of 499.8% for both C T and A T . Carbonate system parameters were calculated from temperature, salinity, C T and A T (mean values from the start and end of experiments) using CO2SYS (version 2.1 by E Lewis and D W R Wallace), with the stoichiometric equilibrium constants for carbonic acid given in Roy et al. (1993).
Cell numbers, coccosphere, cell and coccolith volume Samples for cell number and coccosphere/cell volume were processed directly after sampling and each measured three times with a Coulter Multisizer 4. Afterwards, the samples were acidified with HCl (0.1 mM) to dissolve all free and attached coccoliths and subsequently measured again to determine the cell and coccolith volume of E. huxleyi (Müller et al., 2012). Estimates of average sphere, cell and coccolith volumes were associated with a random error of o4%, which was determined by repeated measurements of identical E. huxleyi culture material (n = 10).

Elemental analyses
For each experiment, three sub-samples were filtered onto precombusted quartz filters (450°C for 4 h) and frozen at − 20°C. TPC and POC were measured at the Central Science Laboratory of the University of Tasmania (CSL-UTAS) on separate filters using a Thermo Finnigan EA 1112 Series Flash Elemental Analyser (Thermo Fisher Scientific, Waltham, MA, USA). The filter for POC analysis was treated with fuming HCl (~10 h) to remove all inorganic carbon (Hedges and Stern, 1984). Particulate inorganic carbon (PIC) was calculated from the difference of TPC and POC. Particulate organic nitrogen (PON) was analysed simultaneously with the POC measurements. Cell quota of particulate matter PM (PM = PIC, POC and PON) per cell was calculated as where PM filter is the mass (pg) of particulate matter per filter, V is the volume (ml) filtrated and N is the number of cells per ml. Repeated analysis of identical E. huxleyi culture material (n = 5) resulted in a random error of o5% for TPC, POC and PON while PIC was associated with an error of o8% due to error propagation. Cellular particulate matter was additionally calculated on a per cell volume basis as where cellV is the volume per cell (μm 3 per cell). Production rates of PIC, POC and PON were calculated by multiplying the cell quota (PM/cell or PM/cellV) by the applied chemostat dilution rate, which equals cellular growth rate (μ).

Nutrient analyses
Samples for phosphate and nitrate+nitrite analyses were sterile filtered and stored frozen until analysis.
Fitting procedure, statistical analysis and model application Chemostat data were compared with results from nutrient-replete conditions by fitting the nutrientreplete data to a first-or second-degree polynomial in a least-square sense using the MATLAB function 'polyfit'. Polynomial prediction intervals of 95% and 99% were calculated with the MATLAB function 'polyconf'. Thus, all chemostat data located outside the 95% or 99% prediction intervals of nutrientreplete conditions can be regarded as significantly or highly significantly different, respectively. Production rates of POC and PIC were normalized to the nutrient-specific maximum measured rates, which resulted in relative (rel.) production rates ranging between 0 and 1. Relative PIC production was fitted to a recent developed model . Maximum PIC production rates were encountered at 422, 343 and 366 μatm pCO 2 for EHSO 5.14 under replete, nitrogen-and phosphorus-limited conditions, respectively. The model describes the response of coccolithophore calcification rate as a function of seawater carbonate chemistry, with the sensitivity parameters a, b, c and d: Fitting procedure was done as described in Bach et al. (2015) using the function 'lsqnonlin' in MATLAB, which solves nonlinear least-squares with optional boundary parameters.

Carbonate chemistry and nutrient availability
The manipulation of the seawater carbonate chemistry in the chemostat supply tanks resulted in a significant change in C T concentrations (ranging from 2065 to 2304 μmol kg − 1 , n = 8) whereas the A T remained relatively constant (2299 ± 12 μmol kg − 1 , n = 8). This resulted in a pCO 2 and pH (total scale) range in the chemostat supply tanks from 392 to 1677 μatm and from 8.06 to 7.48, respectively. Dissolved inorganic carbon consumption by coccolithophore growth was in general higher under phosphorus limitation but remained below 5% in all experiments, which resulted in small alterations of carbonate chemistry speciation in the chemostat vessels at the termination of the experiments (Table 1). At nitrogen-induced chemostat equilibrium conditions, nitrate+nitrite concentrations were near or below the detection limit (o0.2 μmol l − 1 ) and phosphate concentrations were 41.5 μmol l − 1 . When phosphorus was the limiting nutrient, no phosphate was detectable but nitrate+nitrite concentrations were 480 μmol l − 1 .
Cell quota and geometry Coccolithophore growth rate was determined by the chemostat dilution rate (0.2 day − 1 ) and below the expected maximum growth rate for E. huxleyi strain EHSO 5.14 for the applied pCO 2 levels (see Müller et al., 2015). Coccosphere, cell and coccolith volumes as well as cellular quota of POC, PIC and PON, respectively were throughout higher under phosphorus limitation compared with cell quota and coccosphere, cell and coccolith volume under nitrogen limitation (Figure 1 and Table 2). Highest coccosphere and cell volumes were observed under phosphorus limitation and elevated pCO 2 of 1100 μatm whereas lowest volumes were detected under nitrogen limitation and elevated pCO 2 (Table 2). PIC and PON per cell decreased with pCO 2 under nitrogen and phosphorus limitation whereas POC per cell was highest at pCO 2 of around 1100 μatm. No coccolith volume could be determined at the highest tested pCO 2 value under nitrogen limitation because of insufficient coccolith production, which is also indicated by the low PIC content per cell of 0.7 pgC cell − 1 . Coccolith volume increased with pCO 2 under nitrogen and phosphorus limitation from 0.43 to 0.47 and from 1.40 to 1.45 μm 3 , respectively (Table 2). Coccolithophore sphere, cell and coccolith volumes under nutrient limitation were highly significantly different from nutrient-replete conditions (Figures 1a and b). Production rates per cell of POC and PIC were higher under phosphorus compared with nitrogen limitation and indicated similar trends in regard to the applied pCO 2 range (Figure 1). Under nitrogen limitation both POC and PIC production per cell were significantly different from nutrient-replete conditions, whereas under phosphorus limitation this was only indicated for POC production per cell (Figures 1c and d). The cellular PIC:POC ratio linearly decreased from ambient to elevated pCO 2  Figure 1f) and was not significantly different from nutrientreplete conditions.

Experimental setup
All chemostat experiments were conducted under continuous light exposure to desynchronize the cellular division cycle (cell cycle) of E. huxleyi, which results in an independence of sampling time to cellular volume and POC quota (Müller et al., 2008(Müller et al., , 2015. Thus, a bias due to different sampling times can be excluded. Alterations of physicochemical culture conditions (light, temperature and nutrient limitation) have the potential to modify the duration of certain cell cycle phases (G1, S, G2 and M) in phytoplankton (Olson et al., 1986;Vaulot et al., 1987;Müller et al., 2008), which can shift the beginning and end of cell cycle phases to different location within the course of a complete cycle. Therefore, it is possible to sample different physiological states related to certain positions in the cell cycle of a synchronized population (light:day cycle)  when sampling at the same time of the day. This potential bias can be avoided when the population's cell cycle is desynchronized and the population is equally distributed in all cell cycle phases. Chemostat experiments are designed to limit the exponential growth of cells to a desired steady-state level lower than the maximum possible growth rate under the applied physicochemical conditions (e.g. Sciandra et al., 2003;Leonardos and Geider, 2005;Borchard et al., 2011;Lefebvre et al., 2012;Müller et al., 2012;Engel et al., 2014). This design is distinctively different from nutrient-starvation batch experiments (e.g. Vaulot et al., 1987;Parpais et al., 1996;Langer et al., 2012Langer et al., , 2013Rouco et al., 2013). In these latter experiments, the dynamic transition from the exponential to the stationary growth phase is investigated, in which conditions become unfavourable for cell division until the growth rate approximates zero and cells loose viability. Thus the physiological state of cells in starvation experiments (stationary growth phase) is distinctly different from the physiological state observed in nutrient-replete batch and chemostat experiments (exponential growth phase). This implies that physiological results (e.g. cell quota and production rates) of nutrient-starvation experiments have to be compared with great caution with results from nutrient-replete batch and chemostat experiments. On the other hand, production rates of particulate material from the mid-exponential growth phase at nutrient-replete conditions (e.g. batch cultures) are comparable to exponential growing cultures under nutrient limitation because these rates include accounting for the specific growth (division) rates under each condition. For example, it seems counterintuitive that phosphorus-limited cells of E. huxleyi strain EHSO 5.14 have a higher POC quota (20.5 pgC cell − 1 ; Table 2) compared with nutrient-replete conditions at similar conditions (12.6 pgC cell − 1 ; Müller et al., 2015). This increased cell quota under phosphorus limitation, however, is a result of the reduced division rate and concomitant accumulation of organic matter inside the cell. When accounting for the respective growth rates, the results become intuitively understandable with a lower POC production rate under phosphorus limitation compared with nutrient-replete conditions (4.1 compared with 5.7 pgC cell − 1 day − 1 , respectively; see also Figure 1).

Physiological response to ocean acidification under nutrient-limited and -replete conditions
Anthropogenic ocean acidification involves multiple changes in the speciation of seawater carbonate chemistry such as the increase of CO 2(aq) , HCO 3 − and H + and, on the other hand, the decrease of CO 3 2 − . Changes in these parameters have direct influence on coccolithophore physiology as they are associated with cellular functions, such as photosynthesis, calcification and enzyme reactions. An increase of CO 2(aq) and HCO 3 − results in enhanced growth, calcification and photosynthesis rates of phytoplankton when keeping pH, respectively, the H + concentration, constant (Bach et al., 2011;McMinn et al., 2014). Under ocean acidification scenarios (increase of CO 2(aq) , HCO 3 − and decrease of pH, CO 3 2 − ), calcification in the coccolithophore E. huxleyi is generally reduced due to the higher energetic costs for intracellular H + homeostasis and the elevation of the calcium carbonate saturations state (Ω calcite ) at the site of calcification (Raven and Crawfurd, 2012). Changing carbonate speciation, however, is one factor that has to be considered when assessing possible changes in coccolithophore physiology in a future ocean. Additional environmental factors are concomitantly changing (e.g. temperature, light and nutrient supply). The importance of multiple driver response has been emphasized (Boyd and Hutchins, 2012) and understanding the single and combined responses will provide a fundamental basis to develop unifying concepts (Riebesell and Gattuso, 2015).
In this study, nutrient-limited chemostat experiments were conducted with the dominant Southern Ocean E. huxleyi ecotype A that was previously investigated in laboratory batch experiment under nutrient-replete conditions (Müller et al., 2015). The same experimental conditions (temperature and light irradiance) and comparable pCO 2 gradients (336-1551 μatm) were applied and cellular growth was limited by the supply rate of either nitrogen or phosphorus.
Nitrogen limitation induced clear reduction in POC prod , PIC prod per cell and cellular volume in E. huxleyi compared with non-limited conditions (Figure 1). Nitrogen is crucial for protein synthesis and the accumulation of cellular biomass. Reduced nitrogen availability presumably results in physiological adjustments towards minimal cellular biomass and reserves requirements for cell division, which is accompanied by reduced average cell volumes (Figure 1a). PIC:POC ratios were generally lower under nitrogen limitation in comparison with nutrient-replete conditions, which is in good agreement with previous chemostat studies indicating a reduction of PIC:POC under nitrogen limitation (Müller et al., 2012). This suggests that under nitrogen limitation physiological preference is given towards maintaining photosynthetic activity compared with calcification. Some studies report increased PIC:POC ratios under nitrogen starvation (Paasche, 2002;Müller et al., 2008). These observations were retrieved from batch experiments in which E. huxleyi entered the stationary growth phase due to nitrogen starvation. This is, however, not comparable to the limited exponential growth achieved in chemostat experiments (see the section above) and the increased PIC:POC ratio presumably results from continuous cellular organic resource consumption under nitrogen starvation.
Phosphorus-limited growth of E. huxleyi induced a reduction in POC prod and PIC prod per cell compared with nutrient-replete growth (Figure 2). However, only the reduction in POC prod per cell was determined to be significantly different compared with nutrient-replete conditions. This is presumably related to the higher associated error of PIC data resulting in broader prediction intervals. Interestingly, the PIC:POC ratio under phosphorus limitation followed a similar trend as observed under nutrientreplete conditions, which indicates that both POC prod and PIC prod per cell are equally affected by reduced phosphorus supply. Highest coccosphere and cell volumes were detected in phosphorus-limited cells of morphotype A (Figure 1a). This confirms the observations described in the literature of E. huxleyi 'overproducing' coccoliths under phosphorus limitation and covering the cell in multiple layers of coccoliths (see the review of Paasche, 2002). Phosphorus is highly required for nucleic acid and phospholipid membrane synthesis (Geider and LaRoche, 2002) and the presence of sufficient phosphorus reserves are crucial to ensure completion of cell division and to prevent corruption of nuclear material (Antia et al., 1990). The 'overproduction' of coccoliths is a result of an induced elongation of the G1 phase of the cellular division cycle by phosphorus limitation (Müller et al., 2008). Calcification and photosynthesis take place in the G1 phase and an increased accumulation of POC and PIC occurs in the cell before sufficient amounts of phosphorus are concentrated to initiate functional cell division. This leads to increased cell and coccosphere volumes while PIC and POC production rates per cell are lower compared with nutrient-replete conditions (Figures 1c and d).
One of the most striking differences between phosphorus and nitrogen limitation is the difference in coccolith volume (Figure 1b). Highest coccolith and cell volumes were observed under phosphorus limitation, in line with previous studies, indicating an elevated calcium content of coccoliths produced under reduced phosphorus availability (Paasche, 1998). An overall correlation was observed between cell and coccolith volume regardless of nutrient conditions: V coccolith = 0.015 V cell +0.076 (r 2 = 0.95, Po0.001, n = 13), confirming previous observations of the relationship between coccolithophore cell diameter and coccolith volume (Müller et al., 2012;Aloisi, 2015).
Figure 2 POC (a) and PIC (b) production rates per cell volume of E. huxleyi at nutrient-limited and -replete conditions and relative POC (c) and PIC (d) production rates per cell volume (normalized to the nutrient-specific maximum measured rate) with additional data from phosphorus limitation experiments (conducted at the same temperature of 14°C; Borchard et al., 2011), indicating the relative change induced by pCO 2 . Solid lines represent best-fit regressions (first-or second-degree polynomials) through nutrient replete, nitrogen and phosphorus limited data (black, blue and red, respectively) with 95% and 99% prediction intervals (dashed and dotted lines, respectively). Nutrient-limited POC and PIC production rates per cell of E. huxleyi followed similar patterns over the applied pCO 2 gradient compared with nutrientreplete conditions (Figure 1). This similarity amplified when accounting for the differences in cellular volume among the nutrient treatments. Calculating the POC and PIC production rates per cell specific volumes resulted in identical rates of nitrogen-and phosphorus-limited treatments while the offset from the nutrient-replete treatments decreased over the applied pCO 2 range (Figures 2a and b), suggesting that the degree of nutrient limitation was reduced with increasing pCO 2 levels. This was indeed the case in our experiments because growth rates under nitrogen and phosphorus limitation were fixed to 0.20 day − 1 while maximum growth rates under nutrient-replete conditions decreased from 0.45 to 0.21 day − 1 with increasing pCO 2 levels (Müller et al., 2015). The pCO 2 induced changes in the relative POC and PIC production per cell volume (normalized to the nutrient-specific maximum measured rates at 422, 343 and 366 uatm pCO 2 for replete, nitrate and phosphorus limited conditions, respectively) indicated no difference and relative rates under nutrient limitation were not significantly different from nutrient-replete conditions (Figures 2c and d). This suggests the absence of combined effects of pCO 2 and macronutrient limitation on the POC and PIC production rates per cell volume of E. huxleyi within the applied pCO 2 range.
It remains to be demonstrated if this independence of macronutrient and pCO 2 impacts on production rates can be transferred to micronutrient limitation (e.g. iron or zinc). Zinc limitation triggers similar physiological changes compared with phosphorus limitation, with an increase in cell volume and elevated numbers of coccoliths per cell (Schulz et al., 2004). It has been speculated that zinc and phosphorus limitation have similar effects on the cellular division cycle (impeding transition from the G1 to the S phase), which possibly could explain the similarity in observed physiological response (Müller et al., 2008). Iron limitation, on the other hand, introduces a reduction in cell volume (similar to nitrogen limitation), which is presumably associated with physiological adjustments to meet cellular iron requirement and/or impairment of the photosynthetic machinery and concomitant reduction of cellular organic material build-up (Muggli and Harrison, 1996;Schulz et al., 2004). Future studies focusing on the effects of micronutrient limitation and the cellular division cycle of E. huxleyi might reveal promising results helping to understand general response mechanisms. The absence of a difference in POC and PIC production rates between nitrogen and phosphorus limitation, after accounting for changes in cell volume, stresses the importance of including this trait to the toolbox of standard physiological measurements of coccolithophores in field and laboratory studies. This will then allow assessments of potential responses to multiple drivers in the future ocean to proceed in an additive way, for example, by first estimating cell volume changes from changing nutrient availability and then adding the effects of increasing pCO 2 using cell volume normalized experimental results.
Recently, a unifying concept has been developed to understand the mechanistic response of northern hemisphere coccolithophores to ocean acidification , as a balancing of positive response to increased bicarbonate availability, negative impacts of decreasing pH and a minimum requirement for molecular CO 2 . Here, we applied this model for Southern Ocean E. huxleyi ecotype A and included the chemostat data of this study together with the data available for strain EHSO 5.14 and 5.30 under nutrient-replete conditions (Müller et al., 2015). The significant correlation of all measured and modelled rates for calcification ( Figure 3a) and the absence of a significant difference between nutrient-replete and -limited conditions ( Figure 3b) indicates that (1) this concept can be applied to both nutrient-replete and macronutrient-limited exponentially growing cells of E. huxleyi and (2) the Southern Ocean E. huxleyi ecotype A exhibits similar physiological regulating mechanisms for calcification in response to changing seawater carbonate chemistry compared with northern hemisphere strains, which is intriguing regarding their geographical distance. However, a high phenotypic variability has been reported for E. huxleyi (Blanco-Ameijeiras et al., 2016) and it remains to be shown whether this concept can be transferred to other ecotypes. The calculated sensitivity parameters for Southern Ocean E. huxleyi ecotype A (Figure 3) do differ from the parameters reported for the northern hemisphere ecotype A . This is not surprising because the model does not account for other environmental parameters such as temperature, light and salinity, which can influence the response sensitivity of E. huxleyi to changing seawater carbonate chemistry (Sett et al., 2014). However, it is intriguing that this relatively simple model can satisfactorily describe the response of cellular calcification to seawater carbonate chemistry, thus motivating further experimentation and model development including additional environmental parameters.
The complex interplay of globally and regionally changing environmental drivers on phytoplankton physiology has received recent attention (Boyd et al., 2015a) and sophisticated experiments have been conducted to give detailed explanations and predictions of regional species-specific future performance (Boyd et al., 2015b). Most physiological rates of phytoplankton species follow optimum (e.g. temperature) or saturation (e.g. nutrient availability) curve behaviour in regard to changing environmental conditions. The response to ocean acidification, on the other hand, is presumably a combination of substrate limitation (CO 2 and HCO 3 − ) and H + -inhibition, which results in an optimum curve like behaviour Müller et al., 2015).
Depending on the species-and strain-specific optima and half saturation constants to specific environmental parameters, the collated results of laboratory experiments comparing ambient to single point predicted future oceanic conditions may lead to a diverse range of species-specific physiological responses (positive, negative or no-effects). The recorded collection of changing environmental parameters, due to anthropogenic climate change, is accumulating with conducted research and environmental monitoring, which is leading to an increased complexity of multiple driver permutation. While it is of the utmost importance to increase our capability to predict future ecosystem functions by experimentally testing the relevant parameters, we emphasize that understanding the principal physiological response to single environmental parameters and their interplay will help to develop unifying physiological concepts.

Conclusions
Southern Ocean E. huxleyi ecotype A exhibits an identical relative physiological response (in terms of photosynthesis and calcification) to ocean acidification under nutrient-replete and nutrient-limited conditions (nitrogen and phosphorus), indicating independent effects of macronutrient limitation and ocean acidification. Our results agree well with a recently developed unifying concept  and provide evidence of our increasing capacity to understand the principal physiological response of coccolithophores to multiple changing environmental conditions. This is useful for the design of multi-driver experiments, the assessment of future Southern Ocean ecological impacts and the interpretation of coccolithophore fossil records.