Calcite production by Coccolithophores in the South East Pacific Ocean: from desert to jungle

BIOSOPE cruise achieved an oceanographic transect from the Marquise Islands to the Peru-Chili upwelling (PCU) via the centre of the South Paciﬁc Gyre (SPG). Water samples from 6 depths in the euphotic zone were collected at 20 stations. The concentrations of suspended calcite particles, coccolithophores cells and detached coccoliths 5 were estimated together with size and weight using an automatic polarizing microscope, a digital camera, and a collection of softwares performing morphometry and pattern recognition. Some of these softwares are new and described here for the ﬁrst time. The coccolithophores standing stocks are usually low and reach maxima west of the PCU. The coccoliths of Emiliania huxleyi, Gephyrocapsa spp. and Crenalithus 10 spp. (Order Isochrysidales) represent 50% of all the suspended calcite particles detected in the size range 0.1–46 µ m (21% of PIC in term of the calcite weight). The latter species are found to grow preferentially in the Chlorophyll maximum zone. In the SPG their maximum concentrations was found to occur between 150 and 200 m, which is very deep for these taxa. The weight and size of coccoliths and coccospheres are 15 correlated. Large and heavy coccoliths and coccospheres are found in the regions with relative higher fertility in the Marquises Island and in the PCU. Small and light coccoliths and coccospheres are found west of the PCU. This distribution may correspond to that of the concentration of calcium and carbonate ions.


Introduction
The coccolithophores represent an important group of unicellular algae. They are found in abundance from high latitudes where they form large blooms which are detected by satellites (Balch et al., 2007;Brown and Yoder, 1994), at low latitudes both in oligotrophic (e.g. Okada and McIntyre, 1979) and upwelling (e.g. Giraudeau and Bailley, 1995) zones. They are responsible for about half of the total oceanic carbonate pro-and Kilpatrick, 1996). Several laboratory and mesocosms experiments have shown a decrease in the production of calcium carbonate by the coccolithophores under increasing CO 2 (e.g. Engel et al., 2005;Riebesell et al., 2000). The increase of CO 2 in the atmosphere will results in a decrease of the pH of oceanic waters, which may have dramatic consequences on oceanic calcifiers (Felly et al., 2004;Orr et al., 2005). It 10 is therefore urgent to analyse in greater detail how coccolithophores are calcifying in Today's Ocean.
The South Pacific Gyre (SPG) is the most oligotrophic zone in Today's Ocean, and it is one of sparsely sampled open ocean area (Claustre and Maritorena, 2003), in particular for coccolithophores. The primary objective of BIOSOPE was to study the South 15 Pacific Gyre along a transect through the central part of the SPG to the Peru-Chili Upwelling (PCU). We document here the variations of the coccolithophore standing stock along this transect, as well as the absolute abundance of detached coccoliths and of other small suspended calcite particles. We also study their size and weight, in order to describe how coccolithophore are calcifying in opposite natural trophic environments. Introduction

Setting
The BIOSOPE cruise in the southern Pacific, on board the French Research Vessel l'Atalante (26 October to 11 December 2004) completed a transect of about 8000 km that began in the mesotrophic waters west of the Marquises archipelago and ended in 5 the eutrophic waters off the coastal waters of Chile (Fig. 1). This represents the largest possible trophic gradient that can be investigated in today's world ocean. The South Pacific Gyre (SPG) is the most oligotrophic region of the world's ocean. Two features may explain why this broad geographic area possess the lowest surface chlorophyll concentration estimated through satellite imagery (0.019 mg Chlam −3 ): First, it has the 10 largest pycnocline depth recorded in the world ocean hydrological database (>200 m); second the flux of atmospheric dust (e.g. iron) is extremely low (Claustre et al., 2007 1 ). In contrast, the PCU system and the Marquise area (Equatorial ocean upwelling) are bathed by nutrient richer waters. The sea surface temperature and salinity recorded during the cruise varied from 13 15 to 28 • C and from 34 to 36.5 PSU, respectively, with higher values toward the West and lower values toward the East.
Samples for the study of the coccolithophorids were taken according to the Depth of the 20 Chlorophyll Maximum (DCM) at every station. At most stations, water samples were taken at 6 water depths: at the surface (actually 5 m), between the surface and the DCM, at the DCM and two samples below the DCM. In most cases 4 litres of sea-water EGU were filtered on a nitrate cellulose membrane with a diameter of 47 mm and a pore size of 0.45 µm. At the last 4 stations of the transect (in the PCU) the diameter of the membrane was 23 mm and four litres of water was filtered. In consequence the quantity of particles in these filtrats was extremely high and often the coccoliths could have remained hidden during subsequent analysis. The absolute number given for those 5 stations have therefore large chance to have been underestimated. The membranes were quickly dried and stored at room temperature. Once in the laboratory, a quarter of each membrane was mounted between slide and cover-slip and fixed with Canada Balsam which has the property to render the membrane optically transparent. Additionally a small fragment of the filter was examined using a Hitachi 3000N Scanning 10 Electron Microscope (SEM).

Grabbing frames
A Polarizing Optical Microscope (LEICA DMRBE) with a 50X oil immersion objective was used for automatic scanning of slides in cross-polarized light. Microscope stage motions and focus were computer-controlled. For each sample, forty fields of view 15 were grabbed by a 2 Megapixel Spot Insight camera. Each frame is 240×180 µm 2 with a pixel area of 0.0225 µm 2 . The amount of light going through the sample was precisely controlled.

Analyzing calcite particles
We developed a new software using LabView (National Instruments) which automat- 20 ically detects and measures all birefringent particles from grabbed frames, hereafter called "Particle Analyser VI". It takes advantage of the fact that only birefringent crystals are illuminated in cross-polarized light; the other crystals and the background remains dark. There is a relation between the thickness and the brightness of crystals, and this can been calibrated for a transfer function (Beaufort, 2005 EGU than background, and measures their surface. We placed a lower threshold at 3 pixels (0.07 µm 2 ) to get rid off background noise; and an arbitrarily chosen upper threshold at 74 000 pixels (1683 µm 2 equivalent to circular particles having a 46 µm diameter as for example foraminifera). This upper-threshold is large enough to analyse all particles in the nannoplankton size range including aggregates. Knowing the volume filtered in 5 millilitre (Vf), the surface of the membrane (Sm), the number (Nf) and the surface (Sf) of the frames grabbed, and the total number of particle analysed by sample (Nt), the number of particles per millilitre N is: The Particle Analyser VI automatically measures the "lightness" (L) of all the frames 10 as the sum of all Grey Levels pixel values. A transfer function has been established following the protocol established in (Beaufort, 2005), but applied to samples prepared with cellulosic membranes instead of smear slides. In recalibrating we poured different amounts (precisely weighted) of pure calcite powder into known volumes of water. These suspensions were filtered on membranes of the same type as used during the 15 BIOSOPE transect, and processed as described above. The relation between Grey Levels and weight on the membrane now may serve as a transfer function (Fig. 2).
where w is the weight in pg per pixel (0.0225 µm 2 ); GL is the Grey Level measured per pixel (average of all the frames divided by the number of pixel per frame) 20 The calcite weight per millilitre (W) is calculated as following: Where Np is the number of pixel per frame (=2×10 6 ). The values are given in pg ml −1 . Particulate Inorganic Carbon (PIC) is often given in mmol CaCO 3 m −3 . PIC values for the fraction smaller than 46 µm (PIC <46 µ ) in this unit are obtained by dividing W by 10 5 . ANN the coccosphere recognition used here is not species specific. But coccospheres from other orders (Syracosphaerales, Zygodiscales and Coccolithales) are generally not recognised by this ANN.
All the frames have been computed with SYRACO; when an object belonging to one of the 2 classes is detected, its image is saved in a class specific output frame. 15 These output frames are used to perform morphometry and to check the reliability of the recognition. We verified the reproducibility of our technique by counting manually the number of coccospheres in all the frames in 20 samples. The results obtained by the automated and the manual approaches are extremely similar and often identical. In only two samples the number of coccospheres was higher as determined by 20 manual counts. This was due the presence of aggregates of coccospheres in densely populated membranes.
For the coccolith however the number specimen recognized by SYRACO was lower than those determined by human counts. This is not the case when sample are prepare on smear slides (Beaufort and Dollfus, 2004). In the present case, the samples 25 were prepared with membranes that cannot be mounted absolutely flat on the slides, and thus significant portions of the fields of view are out of focus (e.g. Fig. 3a), also coccoliths are often tilted on the mesh of the membrane, often coccoliths are in contact, EGU or forming small aggregates which are not recognized by SYRACO. Because of the large generalisation capability of the ANN, a significant amount of objects that more or less resemble the targeted pattern are included in the specific output frames. In the case of coccospheres, these "invading" objects are "manually" erased from the frame. For the coccoliths, they are automatically withdrawn from the 5 analysis by another new software developed in LabView.
This software, hereafter called "Coccolith Analyser VI", automatically measures coccoliths and coccospheres. It reads the specific output frames and analyses all objects. In the case of coccoliths it first looks for four landmarks characteristic of the coccoliths. If these are not found, the analysis of this object ends. This pre-processing elimi-10 nates all non-coccoliths objects that were wrongly recognized by SYRACO. However we found that this process withdraws from the analysis up to 25% of the coccoliths. In the case of coccospheres, all the objects are analysed (incorrectly identified coccospheres were erase manually, see above). The Coccolith Analyser VI measures the grey level of the objects, their diameter and their surface, and tabulates the results. 15 There is a bias in the measurement of the diameter of the small and dim objects, such as coccolith of −0.6 µm. This due to the fact that we apply a Grey Level threshold below which is defined background. This threshold erodes 2 pixels in the periphery of the dim objects. The pixel size being 0.15 µm and 4 pixels being eroded in total when the length is measured, we added 0.6 µm to the coccolith length results. By comparing 20 optical measurement with SEM measurement, it appears that for small placolith like E. huxleyi the entire distal shield is not detected in cross-polarized light. The measures have to be multiplied by a 1.25 factor. When these corrections are applied the correspondence between SEM and optical measurements on small placoliths are in good agreement. No correction was applied to coccospheres for which SEM and optical 25 measurements are matching.
It should be noted that in a theoretical case of a pure E. huxleyi sample, the size distribution estimated by SYRACO and the Coccolith Analyser VI will narrower that that estimated with the Calcite Analyser VI because SYRACO detects only well preserved, EGU well oriented and isolated coccoliths whereas the Calcite Analyser VI will measure all particles, including aggregated, broken, out of focus and tilted coccoliths.

Importance and composition of the Isochrysidales
Emiliania huxleyi and several species belonging to the genus Gephyrocapsa and Crenalithus represent all the calcifying taxa of the marine Isochrysidales Order (de Vargas 5 et al. in press). We will call this complex "EGC" (for Emiliania, Gephyrocapsa and Crenalithus ranked in order of abundance). SYRACO has been trained to recognize the EGC complex and is therefore the focus of this paper. The specific composition of EGC varied significantly in the BIOSPE sample. We therefore analyse with a Scanning Electron Microscope the samples. This analysis reveals that East of Easter Island 10 (about 110 • W) the EGC dominates the coccolithophore community with relative abundance ranging from 60 to 100%. West of Easter Island the coccolithophore concentration diminishes and EGC represents 40% on average of the coccolithophore community.

Spatial distribution of calcite particles
The concentrations of suspended calcite particles, detached coccoliths and coccospheres show very similar patterns of distribution in the BIOSOPE transect ( Fig. 4): Maximum concentrations are found between 80 • and 100 • W, associated with the sub-5 tropical front (Claustre et al., 2007 1 ).
The concentration in coccospheres is generally very low (average of 9/ml with a maximum of 150/ml). That of detached coccoliths ranges from 11 to 1200 coccoliths per millilitre with an average of 150. The amount of suspended calcite particles and the total weight of calcite per millilitre were in average 733 particles/ml and 11 200 pg/ml 10 (or PIC <46 µ =0.11 mmol CaCO 3 m −3 ) respectively. The corrected total weight of the EGC detached coccoliths and coccospheres is 2431 pg/ml (or 0.024 mmol CaCO3/m3), which represent 21% of the PIC <46 µ . Large aggregates that may be rich in coccoliths composed a large part of remaining 79%.
The spatial distributions of detached coccoliths and suspended calcite particles 15 present two larger scatters of higher concentrations around 95 • W (between 50 and 100 m depth) and around 85 • W at about 30 m depth. Coccospheres are found in great abundance only in the second scatter. SEM examination of samples in the former scatter confirms the presence of numerous coccoliths of E. huxleyi, with very rare coccospheres. This "cloud" of detached coccoliths may correspond to a recent bloom of 20 E. huxleyi.
The observed pattern of density distribution of calcite particles is confirmed by the study of in situ optical properties described in Twardowski et al. (2007a) 3 (i.e. the ratio of backscattering to scattering) is dependant on size distribution of particle assemblage analysis and confirms the relative "patchy" distribution of these biogenic particles.

Grain size distribution of suspended calcite particles, detached coccoliths and coccospheres
Ninety five percent of the 416 000 suspended calcite particles analysed in the BIOSOPE samples have a surface inferior to 20 µm 2 or a diameter inferior to 5 µm (in 10 the 0.1-46 µm range). The distribution is unimodal and slightly skewed toward larger particles, with a mode at 3.2 µm 2 (Fig. 5a). The distribution of detached coccoliths and coccospheres are unimodals with modes at 3.2 µm 2 and 40 µm 2 , respectively (Fig. 5a).
Interestingly, the mode of the suspended calcite particles is the same than that of the detached EGC coccoliths. The number of detached coccoliths (mostly E. huxleyi plus 15 some Gephyrocapsa) represents 1/5 of all suspended calcitic particles. Sample ST18 at 30 m is almost monospecific (E. huxleyi represents more 95% of the coccolith assemblage). In the size range (1-10 µm 2 ) of E. huxleyi, we observed very few particles in the view fields that were not of this species (e.g. Figs. 3a, b). The number of coccoliths detected by SYRACO and the Coccolith Analyser VI represents 20 only 40% of the suspended calcite particles in the same size range (Fig. 5b). That means our system missed 60% of coccolith because they were out of focus, tilted, broken or aggregated. Applying a correcting factor of 2.5 to the entire suite of samples, we can now estimates that the EGC coccoliths represent 50% of all the suspended calcite particles detected in the range 0.1-46 µm.

Size and weight distribution
The diameter and weight of the coccoliths and coccospheres show the same spa-5 tial distribution (Fig. 4). These parameters have in general higher values in eutrophic (PCU) or mesotrophic (Marquesas) zones and lowest values between 80 and 100 • W (Fig. 7). In oligotrophic area, these values are larger in the deep photic zone. There are significant correlations (Fig. 8) between the station average diameter of coccoliths and coccospheres (r=0.87). The same is true for their weights (r=0.88). Also there are significant correlation between station average of the weight and the diameter of the coccoliths (r=0.97) and of the coccopshere (r=0.94).

Depth profiles
Morphometric and abundance data show depth profiles which are similar to that of the chlorophyll concentration ( Fig. 9), implying that maxima of the coccolithophores EGU distribution of that number.

Abundance distribution
The EGC coccospheres stocks estimated in the South East Pacific are low with a median value of 4000 cell per litre. The lowest values are found at Station GYR at the 5 centre of the South Pacific Gyre. However in the centre of the gyre at all stations coccolithophores were continuously present down to 300 m. The average stock at Station GYR2 was 1250 cell per litres. This is equivalent to 375×10 6 cells m-2 in a 300 m thick water column and this represents only the stock of marine Isochrysidales (EGC) which represent only a small fraction (1/3) of the coccolithophores in that area. The stocks of EGC estimated in this study are in the same range as previously reported for the tropical Pacific, 1-240 cell/ml (Hagino and Okada, 2006), 0-60 cell/ml (Balch and Kilpatrick, 1996), 1-100 cell/ml (Ohkouchi et al., 1999;Okada and Honjo, 1973). 0-60 cell/ml (Giraudeau and Beaufort, 2007 (Hagino and Okada, 2006). This is equivalent to what is found in BIOSOPE, where up to 150 cell/ml were observed west of the CPU. The E. huxleyi abundance drops from this 150 cell/ml outside the CPU to 9 cell/ml at maximum in the CPU. This is very different from what has been observed in other upwelling systems. For example higher numbers of coccospheres of E. huxleyi were observed at the centre rather 20 than outside the Benguela uwpelling (∼250 cell/ml) (Giraudeau and Bailley, 1995). In the case of BIOSOPE experiment, the abundance of coccospheres decreases sharply from the edge to the centre of PCU (a caution note should be given here because smaller filters has been used in the PCU; see the material and method section). huxleyi and the number of suspended calcite particles (and therefore, the PIC). Emiliania has been seen as one of the most important calcite producers (e.g. Westbroek et al., 1993) or at the opposite, it has been considered to represent only an insignificant share of the oceanic calcite production (Paasche, 2002;Ziveri et al., 2007), because this species secrete one of the lightest coccoliths (Beaufort and Heussner, 1999;Young 10 and Ziveri, 2000). We show here that most of the fine calcite particles in the BIOSOPE transect have to be attributed to EGC coccoliths (essentially of E. huxleyi) production. Calcification in the Tropical Pacific is very high, (equal the rate of photosynthesis) and the turnover times of calcite in the euphotic zone ranges from 3 to 10 days (Balch and Kilpatrick, 1996). These high turnover rates of calcite induce a high ballasting of organic matter by carbonate particles and a high depletion of Ca ++ ion in the euphotic zone (Balch et al., 2007). Because of the high abundance of detached coccoliths, and coccospheres, the ballasting due to E. huxleyi coccolith must have been particularly efficient around 90 • W-30 • S.

Weight and size relation between coccolith and coccosphere 20
An interesting aspect of this study, is the fact that there is a close (r=relationship between the diameter of the coccoliths and of the coccospheres in the EGC complex. A factor of ∼1.9 can be used to estimate the diameter of a coccosphere from the length of a coccolith. Also the number of coccoliths per coccosphere is 15 in average without changes through the BIOSOPE transect. These values could be used in pa-EGU 4.4 Calcification, cell diameter and carbonate chemistry: The most calcified EGC are found in the Marquises area and Peru-Chili Upwelling (PCU). This could results from the high fertility of these areas, if we rely on the recent culturing experiments showing that E. huxleyi is more calcified in waters rich in P and N in batch cultures  or after addition of nutriments in mesocosms 5 (Engel et al., 2005). The problem is that in these studies the number of cell was also elevated. In BIOSOPE, the highest number of coccospheres was found between 80 and 100 • W and it is also in the same samples that the least calcified Isochrysidales were found. The number of coccospheres in the PCU may have been underestimated, but not in the Marquise area. There is no relation between the number of coccospheres 10 and their weight of CaCO 3 . In a comparison of numerical simulation and observed data from seasonal blooms in the Bering Sea, it has been shown that the E. huxleyi production benefits greatly from an increase in the concentration of carbonate ion in the surface water resulting from the increase in phytoplankton production (Merico et al., 2006). These authors hypothesised that in a zone of seasonal blooms, E. hux- 15 leyi would calcify more after a spring bloom in response to the increase in carbonate ion concentration. This hypothesis could explain why the heaviest coccospheres are observed in the eutrophic and mesotrophic areas of the BIOSOPE experiment. At the reverse, the least calcified Isochrysidales are found at the subtropical front in the highest coccosphere abundance zone of the BIOSOPE experiment. Because it is not a 20 highly productive area, the production of coccoliths may have decreased the carbonate ion concentration, making calcification more difficult for E. huxleyi. Also (Balch et al., 2007) recently suggested that high PIC turnover such as those recorded in the tropical Pacific, would induce a depletion of calcium ion in the photic zone as a response of losses of PIC ballasted particles.

25
Finally we observed a strong negative correlation between surface oxygen concentration recorded during the cruise (Goyet et al., 2007) and carbonate weight of the coccospheres (r=0.93). (Shiraiwa, 2003)  EGU centration on calcification and photosynthesis of coccolithophores. But because of the high number of coccospheres in that zone, a direct relation with oxygen (Warburg effect) is not considered here. The low oxygen content is seen as an oceanographic signature of the distinct ocean chemistry of this area which has a strong impact on the coccolithophore calcification. We did not find strong correlation between salinity (and 5 temperature) and any morphological parameter we measured on the coccolithophores. Recently some relation between the length of E. huxleyi coccoliths and salinity has been suggested (Bollmann and Herrle, 2007). We do not find this relation here (r<0.5): Although the smallest coccoliths are found in relatively low salinity waters, the longest coccoliths were found in the CPU also with low salinity. Our data suggest that the 10 shape (size and weight) of coccoliths and coccospheres is dependant on the carbonate chemistry and productivity of the water in which they are secreted.

Deep production of marine Isochrysidales
In the South Pacific Gyre, coccolithophores are growing at great depths. For example at Station STB11, Florisphaera profunda is found between 200 and 300 m (maximum 15 abundance at 250 m) (Fig. 10a) and at Station GYR2 it is found at 170 m and possibly bellow whereas the maximum abundances of Isochrysidales was found at 150-170 m (Fig. 10b). Station STB11 is one of the rare case in which maximum abundance of EGC was found above the Deep Chlorophyll Maximum (DCM) (Fig. 10a). Except at Station STB11, maximum abundance occurs at about 120 m, i.e., deeper than usually found 20 for coccoliths in oligotrophic area (e.g. Okada and Honjo, 1973;Okada and McIntyre, 1979). A possibility is that these coccospheres were not of living cells but the sinking remains of coccolithophores that grew at shallower depths. Several lines of evidence argue against this: 1) the maximum abundances of coccospheres are in the Deep Chlorophyll Maxima (DCM). 2) the production in the upper photic zone is too low to fuel 25 the coccosphere maxima where coccosphere abundance is 3 times larger than above. This is particularly true for Florisphaera profunda which is found only below 200 m. 3) the community vertical structure is typical of oligotrophic area, 4) It is interesting to note 3282 stress" would be weaker at greater depth.
In conclusion, the system investigated can be considered as an endmember of oligotrophic systems with the deepest chlorophyll maximum and the clearest waters ever reported (Morel et al., 2007). The cococlithophore assemblage is typically adapted to these conditions with maximum cell density being in general closely associated with 10 the deep Chlorophyll maximum. Furthermore from pigment signature is it very clear that below the chlorophyll maximum and up to depth of 250 and above, the dominating (sometime the only one) carotenoids is 19 ′ -hexnoyloxyfucoxanthin, the marker of prymnesiophyceae (Ras et al., 2007 4 ). This observation had to be put in line with the layer of high backscattering ratio (the calcite marker) that is recorded at ∼240 m (Twardowski 15 et al., 2007b) at the GYR station.
4.6 Implication of deep production for alkenone paleothermometry and satellite calcite detection When the temperature difference between the surface and the level of maximum abundance of the EGC, the represent of the marine Isochrysidales, is calculated, it appears 20 that for 1/3 of the stations, the difference is above 2 • C (Fig. 11). The Isochrysidales are the producers of alkenones used in paleoceanography as sea surface temperature (SST) proxy. Ohkouchi et al. (1999) described some discrepancies between SST estimates from North Pacific surface sediments and the observed SST at the same location, that could be attributed to the fact that alkenones were produced in the DCM.

EGU
Also Conte et al. (2006) found some differences between the alkenone calibration curve based on surface sediment (Muller et al., 1998) and their calibration based on mixedlayer water measurements. But those differences were essentially recorded in high latitudes in absence of DCM. Our results would indicate that it is may be excessive to infer SST from an alkenone record core taken below the South Pacific Gyre because 5 alkenone would have been produced far below the surface (there is no suitable sediments to establish such a record in the Central Southern Pacific, Rea et al., 2006). But it has been shown that alkenones are produced exclusively in the mix layer depth, and above the DCM in ALOHA Station in the oligotrophic North Pacific Gyre (Prahl et al., 2005). Either Station ALOHA was similar to Station STB11 where E. huxleyi was 10 abundant above DCM (Fig. 10), or the secretion of alkenones by E. huxleyi is light dependent. In that case the deep production of Isochrysidales observed in SPG would not temper the SST reconstruction based on alkenones.
From the 115 samples analysed in BIOSOPE, 62% of the coccoliths were found at depth below 30 m, and therefore undetectable by satellite. This indicates that a 15 large part of the calcite production from huge oceanic areas cannot be inferred by remote detection. It is interesting to note that coccolith blooms detected by satellite are always in regions of shallow organic production (high latitudes, continental shelves, and upwelling zones) (Balch et al., 2007;Brown and Yoder, 1994).

20
In the South Pacific Gyre coccolithophores grow in low abundance and calcify. The production is spread on a 300 m water column. When integrated to that entire depth, the stock of marine Isochrysidales, which represent a 1/3 of the coccolith community in that area, is 375 million cells per m 2 .
As found in other coccolithophores study of the Tropical Pacific, the stocks observed 25 during BIOSOPE are low. However, EGC coccoliths compose a significant fraction of Particulate Inorganic Carbone (PIC) (around 50% in term of number of particles and 3284 EGU 21% in term of weight). Broken coccoliths and aggregates of these same taxa may represent a large part of the remaining PIC. Therefore a large amount of the fine calcite particles in the BIOSOPE transect have to be attributed to EGC coccoliths and essentially to E. huxleyi) production. Calcification in the Tropical Pacific is very high, (equal the rate of photosynthesis) which induce 5 a high ballasting of organic matter by carbonate particles and a high depletion of Ca ++ ion in the euphotic zone (Balch et al., 2007). Because of the high abundance of detached coccoliths, and coccospheres, the ballasting due to E. huxleyi coccolith must have been particularly efficient especially around 90 • W-30 • S where E. huxleyi is found in great abundance. 10 There is a close relationship between the diameter of the coccoliths and of the coccospheres in the EGC complex. The most calcified EGC are found in the Marquises area and Peru-Chili Upwelling (PCU). This could results from the high fertility of these areas: high phytoplankton production can induce an increase in the concentration of carbonate ion in the surface water which will benefit for the coccosphere calcification. 15 At the reverse, the least calcified EGC are found west of the PCU in the highest coccosphere abundance zone of the BIOSOPE experiment. Because it is not a highly productive area, the production of coccoliths may have decreased the carbonate and calcium ion concentrations, making calcification more difficult for E. huxleyi.
In the South Pacific Gyre, coccolithophores are growing at great depths: the maxi-20 mum abundances of EGC were found between 150 and 170 m. The Deep Chlorophyll maximum is not only the place of maximum abundance of EGC, but also an area in which they secrete heavier coccoliths and have larger cells.