Biogeochemical implications of comparative growth rates of Emiliania huxleyi and Coccolithus species

Coccolithophores, a diverse group of phytoplankton, make important contributions to pelagic calcite production and export, yet the comparative biogeochemical role of species other than the ubiquitous Emiliania huxleyi is poorly understood. The contribution of different coccolithophore species to total calcite production is controlled by interspecies differences in cellular calcite, growth rate and relative abundance within a mixed community. In this study we examined the relative importance of E. huxleyi and two Coccolithus species in terms of daily calcite production. Culture experiments compared growth rates and cellular calcite content of E. huxleyi (Arctic and temperate strains), Coccolithus pelagicus (novel Arctic strain) and Coccolithus braarudii (temperate strain). Despite assumptions that E. huxleyi is a fast-growing species, growth rates between the three species were broadly comparable (0.16–0.85 d) under identical temperature and light conditions. Emiliania huxleyi grew only 12 % faster on average than C. pelagicus, and 28 % faster than C. braarudii. As the cellular calcite content of C. pelagicus and C. braarudii is typically 30–80 times greater than E. huxleyi, comparable growth rates suggest that Coccolithus species have the potential to be major calcite producers in mixed populations. To further explore these results we devised a simplistic model comparing daily calcite production from Coccolithus and E. huxleyi across a realistic range of relative abundances and a wide range of relative growth rates. Using the relative differences in growth rates from our culture studies, we found that C. pelagicus would be a larger source of calcite if abundances of E. huxleyi to C. pelagicus were below 34 : 1. Relative abundance data collected from North Atlantic field samples (spring and summer 2010) suggest that, with a relative growth rate of 88 %, C. pelagicus dominated calcite production at 69 % of the sites sampled. With a more extreme difference in growth rates, where C. pelagicus grows at 1 / 10th of the rate of E. huxleyi, C. pelagicus still dominated calcite production in 14 % of the field. These results demonstrate the necessity of considering interactions between inter-species differences in growth rates, cellular calcite and relative abundances when evaluating the contribution of different coccolithophores to pelagic calcite production. In the case of C. pelagicus, we find that there is strong potential for this species to make major contributions to calcite production in the North Atlantic, although estimates of relative growth rates from the field are needed to confirm our conclusions.

gest that, with a relative growth rate of 88 %, C. pelagicus dominated calcite production at 69 % of the sites sampled.With a more extreme difference in growth rates, where C. pelagicus grows at 1 / 10th of the rate of E. huxleyi, C. pelagicus still dominated calcite production in 14 % of the field.These results demonstrate the necessity of considering interactions between inter-species differences in growth rates, cellular calcite and relative abundances when evaluating the contribution of different coccolithophores to pelagic calcite production.In the case of C. pelagicus, we find that there is strong potential for this species to make major contributions to calcite production in the North Atlantic, although estimates of relative growth rates from the field are needed to confirm our conclusions.
Understanding whether different species grow at comparable or vastly different rates is key to understanding the relative calcification of these species within natural communities.Emiliania huxleyi has a relatively low cellular calcite content (∼ 0.4-0.5 pmol C cell −1 ; Table 1 and Fig. 1) compared with larger, more heavily calcified species such as Coccolithus pelagicus (∼ 16.6 pmol C cell −1 ; Table 1 and Fig. 1).With a similar growth rate (e.g.0.7 d −1 ), at a cellular level C. pelagicus would have a calcification rate approximately 30-40 times greater (11.6 pmol C cell −1 d −1 ) than E. huxleyi (0.28-0.35 pmol C cell −1 d −1 ).Alternatively, if C. pelagicus grew at only 1 / 10th of the growth rate of E. huxleyi (e.g.0.07 d −1 ), then the difference in calcification between the two would be greatly reduced to around 3-4 times (although C. pelagicus would still represent ∼ 75 % of the total calcite production).
Besides relative growth rates (the growth rate of Coccolithus relative to E. huxleyi), the distribution and relative abundance of the different species are important factors in determining whether Coccolithus will dominate calcite production.While E. huxleyi is ubiquitously distributed throughout the oceans, the biogeography of C. pelagicus only covers the Arctic Ocean and the sub-polar Northern Hemisphere (McIntyre and Bé, 1967;McIntyre et al., 1970), with a particular prevalence in the sub-polar North Atlantic (Milliman, 1980;Tarran et al., 2001).As such, C. pelagicus has the potential to be a major oceanic calcite producer in this region.Coccolithus braarudii, a closely related taxa of C. pelagicus with an even greater cellular calcite content (39.1 pmol C cell −1 ; Table 1 and Fig. 1), has a more limited range, restricted to coastal and upwelling areas (Giraudeau et al., 1993;Cachao and Moita, 2000;Ziveri et al., 2004;Cubillos et al., 2012).However, where present, C. braarudii also has the potential to dominate calcite production.
Although studies concerning coccolithophore growth and calcite production have concentrated mainly on E. huxleyi, the potential for other species to be biogeochemically important has been previously highlighted in studies concerning coccolith export (Broerse et al., 2000;Ziveri et al., 2000Ziveri et al., , 2007;;Baumann et al., 2004).Coccolithus pelagicus is a major contributor to the downwards flux of calcite in the northern North Atlantic (Ziveri et al., 2000), while other larger coccolithophore species such as Calcidiscus leptoporus, Helicosphaera carteri and Gephyrocapsa oceanica are significant contributors in other regions (Ziveri et al., 2007).The relative abundance of C. pelagicus in the downward flux has been shown to increase with depth, which is likely to be due to the greater susceptibility of smaller coccospheres, such as those of E. huxleyi, to disintegration and remineralisation (Ziveri et al., 2000).Therefore, C. pelagicus can dominate coccolith calcite export despite relatively low abundances in surface waters.
We set about to experimentally test the basic hypothesis that under identical growth conditions (light, nutrients, temperature) E. huxleyi would grow at a significantly faster rate than either of the Coccolithus species, C. pelagicus and C. braarudii.Furthermore, we also collected a number of ancillary cellular parameters (e.g.cell size, cell chlorophyll content) and examine these in a comparative sense between the different species.Lastly, the biogeochemical implications of growth rates and relative cell abundances are assessed using model and field data.

Experimental design
Monoclonal cultures of Coccolithus pelagicus (RCC4092) and an Arctic strain of Emiliania huxleyi (RCC3533) were obtained in June 2012 through single cell isolations from surface water samples collected in the Greenland Sea (67.83 • N, 16.42 • W and 66.79 • N, 25.14 • W, respectively) during the 2012 UK Ocean Acidification Arctic cruise (JR271).These cultures have been deposited into the Roscoff Culture Collection (RCC).North Atlantic Ocean strains of Coccolithus braarudii (RCC1198) and E. huxleyi (RCC1228) were obtained from the RCC.
Cultures were grown in sterile-filtered (0.2 µm) modified K/20 medium (modified from Keller et al., 1987;following Gerecht et al., 2014); aged natural seawater was enriched with 28.8 µM nitrate and 1.8 µM phosphate.Experiments on parallel cultures of either the Arctic strains (C.pelagicus and E. huxleyi RCC3533) or the Atlantic strains (C.braarudii and E. huxleyi RCC1228) were carried out over a range of temperature and light conditions, under a 12 h light-12 h dark cycle.
To reflect a realistic in situ environment (Poulton et al., 2010;Ryan-Keogh et al., 2013), different experimental conditions were used for the Arctic and Atlantic cultures.The Arctic strain experiments were carried out at 6, 9 and 12 • C, with a daily photon flux ranging from 1.30 to 8.21 mol photons m −2 d −1 (30-190 µmol photons m −2 s −1 ) between experiments, while the Atlantic strain experiments were carried out at 12, 14, 16 and 19 • C, with a daily photon flux ranging from 1.94 to 10.54 mol photons m −2 d −1 (45-244 µmol photons m −2 s −1 ).Cells were acclimated to experimental conditions for approximately 10 generations and grown in dilute batch cultures in duplicate.Cultures were grown in ventilated flasks and to low cell densities to avoid biological effects on the carbonate system (150 000-470 000 cells mL −1 , 4500-8700 cells mL −1 and 5300-16 000 cells mL −1 for E. huxleyi, C. braarudii and C. pelagicus, respectively) and sampled during the mid-exponential phase to avoid nutrient limitation (Langer et al., 2009;Hoffman et al., 2014).
For determination of cell density, samples were taken daily or every other day and counted immediately in triplicate using either a Sedgwick rafter cell for C. braarudii and C. pelagicus (Langer et al., 2006) or a Coulter Multisizer™3 (Beckman Coulter) for E. huxleyi (Langer et al., 2009).Cell density was plotted against time, and growth rates (µ) were calculated by exponential regression (Langer et al., 2006).
Biometric measurements of coccolithophores were made on samples collected on cellulose nitrate (0.8 µm) and polycarbonate (0.8 µm) filters, and prepared following Poulton et al. (2010) and Daniels et al. (2012), respectively.Light microscopy was used for all biometric measurements of Coccolithus (Gibbs et al., 2013), while a combination of light microscopy and scanning electron microscopy (SEM) was used to study E. huxleyi.Measurements of coccolith size and the number of coccoliths per coccosphere were used to estimate cellular calcite content following the relationship of Young and Ziveri (2000).Cellular particulate organic carbon (POC) was estimated from measured internal cell diameters and cell biovolume following Menden-Deuer and Lessard (2000).Samples for determination of cellular chlorophyll a (Chl a) were collected on Fisherbrand MF300 filters (effective pore size 0.7 µm), extracted in 8 mL of 90 % acetone (HPLC grade, Sigma) for 24 h and analysed on a Turner Designs Trilogy Fluorometer calibrated using a solid standard and a chlorophyll a extract.All experimental data included in the paper are available from the data repository PANGAEA (Publishing Network for Geoscientific & Environmental Data) via Sheward et al. (2014).

Field samples
Samples for coccolithophore abundance were collected from three RRS Discovery cruises spanning the Irminger and Iceland basins of the North Atlantic during the period of April to August 2010.Two cruises (D350, D354) were part of the (UK) Irminger Basin Iron Study (IBIS), while the third cruise (D351) occupied the Extended Ellett Line.In all three cruises, surface water samples (0.2-1 L) were filtered through cellulose nitrate (0.8 µm) and polycarbonate (0.45 or 0.8 µm) filters, oven dried (30-40 • C, 6-12 h) and stored in Millipore PetriSlides.The filters were examined using a Leo 1450VP scanning electron microscope, with coccolithophores identified following Young et al. (2003), and enumerated from 225 fields of view (Daniels et al., 2012).The detection limit was estimated to be 0.2-1.1 cells mL −1 .All field data included in the paper are available from the British Oceanographic Data Centre (BODC) via Daniels et al. (2014).
While our maximum growth rate of E. huxleyi (0.85 d −1 ) was lower than in some recent studies (e.g.0.98-1.64d −1 ; Langer et al., 2009), they are well within the range of reported growth rates (0.4-1.9 d −1 ; Paasche, 2002).Strainspecific variability is likely to partly contribute to this large range in growth rates (e.g.Langer et al., 2009).However, it is also likely that our lower maximum growth rates are due to the effect of the day length used in our study (12 L/12 D), as day lengths shorter than 16 h have been observed to reduce phytoplankton growth rates (Paasche, 1967).Although our E. huxleyi growth rates were lower than those obtained in 16 h day length studies (e.g.Langer et al., 2009;Hoppe et al., 2011), they were similar to another 12 h day length study (0.6-1 d −1 ; Iglesias- Rodriguez et al., 2008).This is also the case for C. braarudii and C. pelagicus; the maximum growth rate of C. braarudii (0.58 d −1 ) was below that observed in 16 h day length studies (0.73-0.82 d −1 ; Langer et al., 2006;Gibbs et al., 2013), but above both 12 h (0.42-0.5 d −1 ; Taylor et al., 2007;Gerecht et al., 2014) Gibbs et al. (2013).Given these differences between experiments, and no literature consensus on recommended day length (Probert and Houdan, 2004), we are therefore confident that our growth rates are representative of these coccolithophore species.
In general, a decrease in absolute growth rates was coupled with a smaller difference in the relative growth rates of E. huxleyi and Coccolithus (Fig. 2).As the variability in growth rate was primarily driven by temperature, this suggests that growth rates of Coccolithus and E. huxleyi may be most comparable in cold waters (< 10 • C), while the growth rate of E. huxleyi will become increasingly greater relative to Coccolithus in temperate waters.As a cold-water species (Winter et al., 1994), with a biogeography spanning the Arctic and sub-polar Northern Hemisphere (McIntyre and Bé, 1967;McIntyre et al., 1970), C. pelagicus could therefore potentially dominate calcite production in this region.As C. braarudii is a more temperate species, seemingly present only in coastal waters of the North Atlantic (Cachao and Moita, 2000;Daniels et al., 2012) and upwelling pockets (Giraudeau et al., 1993;Cubillos et al., 2012), we expect the difference in growth rate between C. braarudii and E. huxleyi to be greater in areas where they are both present.However, as  a heavily calcified species, where the coccosphere calcite of one cell is equivalent to ∼ 78 cells of E. huxleyi (Table 1), C. braarudii still has the potential to dominate calcite production in these regions.

Modelling relative calcite production
The potential for C. pelagicus and C. braarudii to dominate calcite production in their respective environments is dependent on both their relative growth rates and cellular calcite inventories, as well as the relative abundance of these species compared to other coccolithophores.In the context of our study, we consider daily contributions to calcite production, as this is the minimal time length over which we can realistically expect relative abundances to be least variable.Also, much of the work measuring calcite production by natural field communities is based on daily integrals (e.g.Poulton et al., 2010;Poulton et al., 2013).We examine the potential relative daily calcite production by modelling a simplified community comprised of just E. huxleyi and either C. pelagicus or C. braarudii.Assuming steady state in terms of the cellular quota across a day, calcite production for a given species is the product of its growth rate (µ), cellular calcite (C) and abundance (N ) (Leynaert et al., 2001;Poulton et al., 2010).Therefore, we can calculate the percentage of calcite production by a specific species (%CP sp ), such as Coccolithus, within a mixed community, using the following equation: (1) The model was parameterised using a range of relative growth rates that spans the range measured in our culture experiments (Fig. 2, Table 2), but which has also been extended down to 10 % to investigate the effect of Coccolithus having a much lower relative growth rate.The relative abundance of Coccolithus and E. huxleyi in our simple model community is represented as the ratio of E. huxleyi to Coccolithus and was varied from 0 to 80. Cellular calcite values for each species were experimentally determined (Table 1).The percentage calcite production by Coccolithus is inversely related to its relative growth rate, cellular calcite and abundance, and linearly related to the ratio of E. huxleyi to Coccolithus (demonstrated in Fig. 3).As the ratio of E. huxleyi to Coccolithus increases, or the relative growth rate of Coccolithus decreases, a decrease in the percentage calcite production by Coccolithus is observed (Fig. 3).Coccolithus braarudii is the major source (> 50 %) of calcite production in 56 % of the model, and 64 % of the model when considering only the range of relative growth rates of C. braarudii observed in this study (51-88 %, Fig. 3a).At its average relative growth rate (72 %), C. braarudii will dominate (> 50 %) calcite production if the ratio of E. huxleyi to C. braarudii is less than 53 : 1, whilst with the same growth   1) and thus only dominates 29 % of its total model, and 44 % of the model when constrained to observed relative growth rates (74-110 %).When growing at its average observed relative growth rate (88 %), C. pelagicus dominates calcite production when the ratio of E. huxleyi to C. pelagicus is less than 34 : 1 (Fig. 3b).Equivalent growth rates require a ratio less than 39 : 1 for C. pelagicus to dominate cellular calcite production, whilst a growth rate of only 10 % that of E. huxleyi results in calcite production from C. pelagicus being only 3.5 times that of an E. huxleyi cell.Although a greater relative abundance of C. pelagicus is required to dominate calcite production compared to C. braarudii, we still find that it would also be a large source of calcite unless both relative growth rates and abundances are low.
Although we have modelled the effect of growth rate and relative abundance on the role of Coccolithus as a calcite producer, the relative calcite production of the two species in these models is highly dependent on the cellular calcite quotas attributed to both E. huxleyi and Coccolithus (Table 1), as calcite production is the product of growth rate and cellular calcite.Estimates of the cellular calcite content of E. huxleyi vary significantly between studies (Balch et al., 1996;Paasche, 2002;Langer et al., 2009;Poulton et al., 2010), which is likely due to both ecophysiological and methodological differences (Young and Ziveri, 2000;Poulton et al., 2010Poulton et al., , 2013;;Hoffman et al., 2014).Our estimates of E. huxleyi cellular calcite (0.43-0.52 pmol C cell −1 ) are similar to recent estimates based on similar biometric measurements (Hoffman et al., 2014) and are within the range of literature values (0.22-1.1 pmol C cell −1 ; Fritz and Balch, 1996;Paasche, 2002;Hoppe et al., 2011).Our value for C. braarudii cellular calcite is greater than previously measured (28 pmol C cell −1 , Langer et al., 2006;  17 pmol C cell −1 , Gerecht et al., 2014), while the value for C. pelagicus cellular calcite is lower (26 pmol C cell −1 ; Gerecht et al., 2014).
To address the impact of variability in cellular calcite on calcite production, we have varied the parameters of our model by concurrently increasing the calcite content of E. huxleyi and decreasing that of Coccolithus, by 1 standard deviation each (Table 1), or vice versa (Fig. 3c-f).In doing this, we capture most of the reported range of E. huxleyi calcite as it is the equivalent of varying E. huxleyi RCC3533 calcite by 0.23-0.75pmol C cell −1 and RCC1228 by 0.33-0.79pmol C cell −1 , while the value for Coccolithus is held constant.
Reducing the calcite content of C. pelagicus (12.7 pmol C cell −1 ) and C. braarudii (32.5 pmol C cell −1 ) and increasing that of E. huxleyi (0.57-0.66 pmol C cell −1 ) reduces the dominance of Coccolithus in the model (Fig. 3cd).Thus C. braarudii dominates only 37 % of the total model (Fig. 3c), 43 % of the model when constrained to observed relative growth rates, and calcifies at a rate equivalent to 49 cells of E. huxleyi when growth rates are the same.With the same reductions in cellular calcite content, C. pelagicus is the major calcite producer in only 17 % of the total model (Fig. 3d), 26 % of the model when constrained to observed relative growth rates, and with the same growth rate it will dominate calcite production if the ratio of E. huxleyi to C. pelagicus is less than 22 : 1.
An increase in the calcite content of C. pelagicus (20.5 pmol C cell −1 ) and C. braarudii (44.9 pmol C cell −1 ), coupled with a decrease in that of E. huxleyi (0.29-0.38 pmol C cell −1 ), results unsurprisingly in an increased dominance of both C. braarudii (Fig. 3e) and C. pelagicus (Fig. 3f).Coccolithus braarudii dominates 75 % of the total model and 93 % of the observation-constrained model, while C. pelagicus dominates 53 % of the total model and 81 % of the observation-constrained model.
Cellular calcite clearly has a significant influence on our calculation of percentage calcite production and therefore needs to be constrained more tightly, particularly in the case of Coccolithus.However, we still observe notable levels of calcite production deriving from Coccolithus rather than E. huxleyi in the models using even the lowest values of cellular calcite for Coccolithus.

The importance of relative abundance
The model scenarios clearly highlight the importance of relative cellular calcite quotas, relative growth rates and relative abundances when determining the relative role of E. huxleyi and Coccolithus in calcite production.While cellular calcite and growth rates will affect relative calcite production at a cellular level, it is the relative abundance of E. huxleyi and Coccolithus within a population that will determine the proportion of calcite production that derives from Coccolithus.Using data from field communities, we can examine whether populations exist where C. pelagicus has the potential to be a significant calcite producer.
Coccolithophore abundances were determined from samples collected on three cruises in the Irminger and Iceland basins of the North Atlantic, a region in which both E. huxleyi and C. pelagicus are present (McIntyre and Bé, 1967).A physicochemical description of the region is available in Ryan-Keogh et al. (2013), which indicates nutrient replete conditions for the phytoplankton community in spring and nutrient depleted (iron and/or nitrate) conditions in summer.
Although other species of coccolithophore were present, we have extracted only the abundances of E. huxleyi and C. pelagicus, so that the data are comparable to our model scenarios in Sect.3.2.Of the 37 samples analysed, E. huxleyi and C. pelagicus were observed in 29 samples, with E. huxleyi present in a further 6 samples in which C. pelagicus was absent (Fig. 4).When present, concentrations of E. huxleyi ranged from 2 to 980 cells mL −1 , while C. pelagicus ranged from 0.1 to 74 cells mL −1 .The relative abundance of E. huxleyi to C. pelagicus (0.7-85) was generally comparable to our modelled range, with a relatively low median average of 12.7.However, in two samples (Supplement Table S1), the relative abundance was much higher (155-212), such that C. pelagicus was unlikely to be a significant calcite producer in these samples.
Assuming the original model scenario of measured cellular calcite (Table 1, Fig. 3a and b) and the average relative growth rate for C. pelagicus of 88 %, the minimum relative abundance of E. huxleyi to C. pelagicus required for E. huxleyi to dominate calcite production (34 : 1) was exceeded in only 5 out of 29 samples.Taking into account those samples in which C. pelagicus was absent, C. pelagicus is a greater calcite producer than E. huxleyi in 69 % of the samples.If equivalent growth rates are assumed, then C. pelagicus remains the major calcite producer in 69 % of the samples.
Under the more conservative model scenario (Fig. 3d), with a relative growth rate of 88 %, C. pelagicus remains the major calcite producer in 57 % of the samples, which is reduced to 51 % if the lowest measured relative growth rate (74 %) is used.If C. pelagicus has a higher nutrient requirement and lower nutrient affinity than E. huxleyi, then in lownutrient conditions we would expect a lower relative growth rate.As we do not know the relative nutrient affinities, we have used an extreme in our original model where C. pelagicus has a relative growth rate of 10 %.Under this scenario, C. pelagicus is the major calcite producer in 14 % of the samples, although it would still form a significant component of the total calcite production (7-49 %) in other samples when present.
Using experimentally determined relative growth rates and cellular calcite quotas, in conjunction with relative abundances from field populations, we have shown that C. pelagicus is likely to be a major source of calcite in the sub-polar North Atlantic.Data on relative abundances of E. huxleyi and C. braarudii in field communities were not available for an equivalent comparison study.

Implications of cell size differences
While the difference in growth rates between E. huxleyi and Coccolithus is comparatively small, the difference in cell volume of C. pelagicus (∼ 1100 µm 3 ) and C. braarudii (∼ 2100 µm 3 ) compared to E. huxleyi (∼ 50 µm 3 ) is relatively large.These differences are reflected in their cellular Chl a and cellular calcite : POC ratio ( species having similar ratios of carbon : Chl a (25-36 g g −1 ) across the experimental conditions.Larger cells have a lower surface-area-to-volume ratio, which reduces the diffusive nutrient uptake per unit volume of the cell (Lewis, 1976;Finkel et al., 2009), and thus maximal growth rates generally increase with decreasing cell size (Sarthou et al., 2005).Hence, although we expect E. huxleyi maximal (optimal) growth rates to be higher than Coccolithus, the relatively small difference in growth rate (Fig. 2) compared to cell volume (Table 1) implies that Coccolithus must have efficient (competitive) nutrient uptake pathways, or that these experimental conditions are less optimal for E. huxleyi than Coccolithus.
It is also worth considering the implications of relative differences in cell size and surface area to volume for nutrient requirements to support growth.From our estimates of cellular POC (Table 1) and assuming Redfield stoichiometry (Redfield, 1958), we can also estimate that the cellular particulate organic nitrogen (PON) and particulate organic phosphorus (POP) content of E. huxleyi, C. pelagicus and C. braarudii is, respectively, 0.10, 2.0 and 3.6 pmol N cell −1 and 0.006, 0.12 and 0.22 pmol P cell −1 .Our estimates of cellular quotas for E. huxleyi are similar to Langer et al. (2013), who measured cellular quotas of 0.69 pmol C cell −1 , 0.12 pmol N cell −1 , and 0.003 pmol P cell −1 .Cellular quotas of both C. pelagicus and C. braarudii have recently been measured by Gerecht et al. (2014).While the cellular PON (1.9 pmol N cell −1 ) and POP (0.19 pmol P cell −1 ) of C. pelagicus were generally similar to our study, the value for cellular POC was slightly larger (20 pmol C cell −1 ), suggesting a lower nutrient requirement per unit POC.However, Gerecht et al. ( 2014) report C. braarudii cellular quotas of POC (13 pmol C cell −1 ) and PON (1.5 pmol N cell −1 ) that are much lower than their values for C. pelagicus.This is unexpected, as it is generally accepted that C. braarudii is a larger species of coccolithophore than C. pelagicus (Geisen et al., 2004) and we would therefore expect a higher POC content for C. braarudii than C. pelagicus (Table 1) if POC scales with cell size.Clearly further cellular measurements of POC, PON and POP for different coccolithophore species are needed to fully examine cellular nutrient requirements.
For culture media with a given nitrate concentration of 10 µmol N L −1 , the maximum cumulative cell concentration that could be supported using our estimated cellular PON would therefore be ∼ 1 × 10 5 , ∼ 5000 and ∼ 2800 cells mL −1 , respectively, for E. huxleyi, C. pelagicus and C. braarudii.This corresponds to cumulative calcite concentrations, using cellular calcite quotas from Table 1, of ∼ 50, ∼ 80 and ∼ 110 µmol C L −1 .Therefore despite lower cell densities, for a given nutrient concentration, a population of C. pelagicus and C. braarudii would be a greater source of calcite than E. huxleyi.
Emiliania huxleyi regularly forms seasonal blooms in excess of 1000 cells mL −1 , particularly in the high latitudes of the Northern and Southern hemispheres (Tyrrell and Merico, 2004;Poulton et al., 2013).For a bloom with a magnitude of 1000 cells mL −1 , this would require a nitrate concentration of only ∼ 0.1 µmol N L −1 .Comparatively, although rare, C. pelagicus has also been reported in concentrations exceeding 1000 cells mL −1 in the high-latitude North Atlantic (Milliman, 1980), requiring a much larger nitrate concentration of 2 µmol N L −1 .The seasonal drawdown of nitrate in the North Atlantic is estimated be ∼ 10 µmol N L −1 (Sanders et al., 2005;Ryan-Keogh et al., 2013), and thus a C. pelagicus bloom of 1000 cells mL −1 represents the utilisation of a significant amount of the available nutrients.For a bloom of this magnitude to occur, we would expect C. pelagicus to be a significant proportion of the total phytoplankton community with a relatively low mortality rate, as nutrient drawdown will be related to gross production by the total phytoplankton community.Reduced mortality has also been discussed as a possible factor in the formation and persistence of E. huxleyi blooms in the southeast Bering Sea (Olson and Strom, 2002).
The function of coccoliths is not well understood but may have a significant role in reducing mortality by providing a certain level of protection from zooplankton grazing (Young, 1994;Tyrrell and Young, 2009).If this is the case, then we would speculate that C. pelagicus has a relatively lower mortality than E. huxleyi due to both its larger cell size and its much larger and heavier coccosphere.A lower mortality may explain how C. pelagicus is able to form high-density populations, while the large nutrient requirement would restrict C. pelagicus blooms to populations where it heavily dominates the plankton community, and this may explain the scarcity of reported C. pelagicus blooms.

Conclusions
The data we have presented show that, when grown in parallel under identical experimental conditions, the relative difference in growth rates between E. huxleyi and Coccolithus species was generally small (12 and 28 %, respectively, for C. pelagicus and C. braarudii), although E. huxleyi generally grew significantly faster than both C. pelagicus and C. braarudii.Using relative growth rates and estimates of cellular calcite to model relative calcite production, we have also shown that, when in a suitable relative abundance to E. huxleyi, both C. pelagicus and C. braarudii have the potential to dominate relative and absolute calcite production.
The relative abundance of E. huxleyi and C. pelagicus was determined from samples collected from the Irminger and Iceland basins in the North Atlantic.This showed that, using our standard model scenario with C. pelagicus growing at 88 % of the growth rate of E. huxleyi, we would expect C. pelagicus to be the major calcite producer in 69 % of the field samples.Using a more conservative model reduced this to 57 %, while the scenario of an extreme difference in growth rates led to C. pelagicus only dominating 14 % of the samples.Therefore, we would expect C. pelagicus to be a major source of calcite in the sub-polar North Atlantic across a Biogeosciences, 11, 6915-6925, 2014 www.biogeosciences.net/11/6915/2014/spectrum of relative growth rates.With a present-day distribution constrained to the polar and sub-polar Northern Hemisphere, C. pelagicus is unlikely to be a dominant calcite producer on a global scale.However, the fossil record of C. pelagicus shows that it has remained a major contributor to sedimentary calcite for the last 65 million years (Gibbs et al., 2013), and therefore there is the strong potential that it was also a major producer in the surface ocean in the past.
There are a number of other extant coccolithophore species that have high cellular calcite content relative to E. huxleyi (e.g.Calcidiscus leptoporus, Helicosphaera carteri) and are known to have high contributions to deep sea calcite fluxes, and therefore may similarly make significant contributions to pelagic calcite production.Further studies elucidating the relative growth rates of these species compared to E. huxleyi, in culture and in the field, as well as their relative abundances in mixed coccolithophore communities are therefore needed to fully examine their potential to dominate calcite production.Lastly, investigations of community composition and calcification rates are also needed to examine the contribution of different species to total calcite production.Despite a small relative difference in growth rates, there were large differences in cell size.Estimates of the cellular nutrient requirements suggest that for a given nutrient concentration, despite a much smaller maximum cell density, both C. pelagicus and C. braarudii would be a greater source of calcite than E. huxleyi.These results have significant implications for how we view calcite production in natural coccolithophore communities and which coccolithophores are keystone species for oceanic biogeochemical cycles.
The Supplement related to this article is available online at doi:10.5194/bg-11-6915-2014-supplement.

Figure 3 .
Figure 3. Contour plots of how percentage calcite production by Coccolithus varies with the abundance ratio of Emiliania huxleyi to Coccolithus and the growth rate of Coccolithus relative to E. huxleyi, for modelled communities of Coccolithus braarudii and E. huxleyi (a, c, e) and Coccolithus pelagicus and E. huxleyi (b, d, f).Plots (a) and (b) show model with input using calcite quotas from Table 1, (c) and (d) have increased E. huxleyi and decreased Coccolithus calcite content by 1 standard deviation from average values in Table 1, and (e) and (f) have decreased E. huxleyi and increased Coccolithus calcite by 1 standard deviation away from average values given in Table 1.Dotted lines indicate the average relative growth rate as determined from the culture experiments.

Table 1 .
Coccolithophore strain-specific values of cell diameter, cellular calcite, cellular particulate organic carbon (POC), cellular chlorophyll (Chl) and cellular calcite : POC.Values reported are averaged over experiments, with ±1 standard deviation.
Buitenhuis et al., 2008)enhuis et al., 2008)day length experiments.Although there are few studies of C. pelagicus, our maximum growth rate (0.49 d −1 ) was greater than the 12 h day length study (0.36 d −1 ) by Gerecht et al. (2014) but lower than a 16 h day length experiment (0.58 d −1 ) by

Table 2 .
Experiment culture strains, temperature, daily irradiance and growth rates, with ±1 standard deviation for the experiments.Atlantic: RCC1198 and RCC1228; Arctic: RCC4092 and RCC3533.

Table 1 )
, with the