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Beyond the pCO2 records provided by ice core measurements, the quantification of atmospheric CO2 concentrations and changes thereof relies on proxy data, the development of which represents a foremost challenge in paleoceanography. In the paleoceanographic toolbox, the coccolithophores occupy a notable place, as the magnitude of the carbon isotopic fractionation between ambient CO2 and a type of organic compounds that these photosynthetic microalgae synthesize (the alkenones) represents a relatively robust proxy to reconstruct past atmospheric CO2 concentrations during the Cenozoic. The isotopic composition of coeval calcite biominerals found in the sediments and also produced by the coccolithophores (the coccoliths) have been found to record an ambient CO2 signal through culture and sediment analyses. These studies have, however, not yet formalized a transfer function that quantitatively ties the isotopic composition of coccolith calcite to the concentrations of aqueous CO2 and, ultimately, to atmospheric CO2 levels. Here, we make use of a microseparation protocol to compare the isotopic response of two size-restricted coccolith assemblages from the North Atlantic to changes in surface ocean CO2 during Termination II (ca. 130–140 ka). Performing paired measurements of the isotopic composition (δ13C and δ18O) of relatively large and small coccoliths provides an isotopic offset that can be designated as a “differential vital effect”. We find that the evolution of this offset follows that of aqueous CO2 concentrations computed from the ice core CO2 curve and an independent temperature signal. We interpret this biogeochemical feature to be the result of converging carbon fixation strategies between large and small cells as the degree of carbon limitation for cellular growth decreases across the deglaciation. We are therefore able to outline a first-order trend between the coccolith differential vital effects and aqueous CO2 in the range of Quaternary CO2 concentrations. Although this study would benefit from further constraints on the other controls at play on coccolith geochemistry (growth rate, air–sea gas exchange, etc.), this test of the drivers of coccolith Δδ13C and Δδ18O in natural conditions is a new step in the development of a coccolith paleo-CO2 probe.

The genus Palusphaera Lecal-Schlauder emend. R.E. Norris is a distinctive modern coccolithophore that accommodates monomorphic, monothecate coccospheres with one type of spine-bearing heterococcoliths. Having examined a set of scanning electron microscopy images from our collections, we were able to demonstrate that four distinct species of Palusphaera exist in the modern oceans, including the type species Palusphaera vandelii, an informally proposed form, Palusphaera sp. 1 (type robusta) that is herein taxonomically ratified as Palusphaera crosiae sp. nov., and a distinctive morphotype, which is formally described as Palusphaera bownii sp. nov. Biometric analyses and observations on the morphologies of numerous P. vandelii specimens revealed the existence of a further form, and its commonly distinguished coccolith morphotypes are therefore taxonomically separated from Palusphaera vandelii and established as a discrete species, Palusphaera probertii sp. nov.

Coccoliths are plates of biogenic calcium carbonate secreted by calcifying marine phytoplankton; annually these phytoplankton are responsible for exporting >1 billion tonnes (1015 g) of calcite to the deep ocean. Rapid and reliable methods for assessing the degree of calcification are technically challenging because the coccoliths are micron sized and contain picograms (pg) of calcite. Here we pioneer an opto‐eletrochemical acid titration of individual coccoliths which allows 3D reconstruction of each individual coccolith via in situ optical imaging enabling direct inference of the coccolith mass. Coccolith mass ranging from 2 to 400 pg are reported herein, evidencing both inter‐ and intra‐species variation over four different species. We foresee this scientific breakthrough, which is independent of knowledge regarding the species and calibration‐free, will allow continuous monitoring and reporting of the degree of coccolith calcification in the changing marine environment., Acid dissolution enables 3D image reconstruction of coccoliths, providing mass estimation in the range of 2 to 400 picograms.

Oceanic red beds (ORBs) are present in Upper Cretaceous and Danian deep-marine deposits in the Basque–Cantabrian Basin of northern Spain. The presence and regularity of the succession of marl–limestone couplets is exceptional based on the macroscopic, microscopic and geochemical evidence collected. Five types of marl–limestone couplets are identified based on the colour, and a high maximum sedimentation rate (3.6 cm ka–1 ) is noted. The oxidizing activity of deep, cold-water masses is indicated by the oxygen isotope signal in the lower–upper Maastrichtian and Danian sections and the presence of the boreal inoceramidSpyridoceramus tegulatus.In theory, the variation in colour from grey to greenish-yellow, purple and pink up to red tones correlates with the Fe2+/(Fe2++Fe3+) ratio. It is interpreted as the possible palaeoenvironmental transit of particles that sediment out slowly in oxic environments when they circulate through cooler, oxidizing water masses. The colour is considered to be a depositional feature, and hematite, detected by X-ray diffraction, is the main staining agent, without discarding the possible redistribution of previous oxyhydroxides passing to hematite as a final product. The cell filling of the foraminifer shells does not incorporate appreciable amounts of Fe and Mg during diagenesis. Bacterial activity is detected using scanning electron microscopy images, both in the coccolith debris and in the detrital micas, although there is uncertainty as to its importance in the staining process.

The localization of organic material within biominerals is central to developing biomineral formation mechanisms. Coccoliths, morphologically sophisticated calcite platelets of intracellularly calcifying coccolithophores, are not only eco-physiologically important, but also influence biogeochemical cycles through mass production. Despite their importance and over a century of research, the formation mechanism of coccoliths is still poorly understood. Crucial unsolved questions include the localization of organic material within coccoliths. In extracellular calcifiers the discovery of an organics-containing nano-structure within seemingly single crystals has led to the formulation of a two-step crystallization mechanism. Coccoliths are traditionally thought of as being formed by a different mechanism, but it is unclear whether coccolith crystals possess a nano-structure. Here we review the evidence for and against such a nano-structure. Current SXPD analyses suggest a nano-structure of some kind, while imaging methods (SEM, TEM, AFM) provide evidence against it. We suggest directions for future research which should help solve this puzzle. STATEMENT OF SIGNIFICANCE: Coccolithophores, unicellular calcifying algae, are important primary producers and contribute significantly to pelagic calcium carbonate export. Their calcite platelets, the coccoliths, are amongst the most sophisticated biomineral structures. Understanding the crystallization mechanism of coccolith crystals is not only central to coccolithophore cell biology but also lies at the heart of biomineralization research more generally. The crystallization mechanism of coccoliths has remained largely elusive, not least because it is still an open question whether the micron sized coccolith crystals are pure calcite, or contain organic material. Here we review the state of the art and suggest a way to solve this central problem.

A quantitative analysis of coccoliths is presented in 18 core-top samples ranging between 26° N and 21° N and covering two major deep brine pools in the northern part of the Red Sea. Non-brine sites are characterized by rich coccoliths that may reach up to 3.31 × 109 coccoliths/g made by 22 species, whereas brine sites of Shaban and Kebrit Deeps with additional two non-brine sites are characterized by a decline in coccoliths/g (3.25 × 108 coccoliths/g), Shannon diversity, CaCO3 (%), and high TOC (%). Carbonate dissolution, inferred by qualitative observation and quantitative indices, was only observed at one brine site GeoB7828 in Kebrit Deep. This suggests that the decline in coccolith assemblages may not entirely be attributed to carbonate dissolution. The major decline, however, is probably related to the suspension of fecal pellets and marine aggregates containing delicate coccolith shields within a nepheloid layer and subsequently grazed by zooplankters in which reduced the numbers of coccolith that reached the bottom of the brine sites, or alternatively a deep-sea flow current that carried and remobilized some suspended particles outside the brine pool. Latitudinal fluctuations of eutrophic/oligotrophic coccoliths suggest profound trophic changes in the photic zone in the northern part of the Red Sea. C. braarudii, a valuable nutrient-indicator species is here reported for the first time, along with G. oceanica, H. carteri as well as biogenic opal dominating the assemblage between 26° N and 24° N, suggesting elevated nutrient conditions and supporting recent high chl-a records, whereas areas between 21° N and 23° N lie under oligotrophic conditions due to the presence of U. sibogae, U. tenuis, R. clavigera, F. profunda, and S. pulchra.

Coccolithophores are a diverse group of calcifying microalgae that have left a prominent fossil record on Earth. Various coccolithophore relics, both organic and inorganic, serve as proxies for reconstruction of past oceanic conditions.Emiliania huxleyi is the most widely distributed representative of the coccolithophores in modern oceans, and is known to engage in dynamic interactions with bacteria. Algal-bacterial interactions influence various aspects of algal physiology and alter algal alkenone unsaturation (UK’37), a frequently used organic coccolithophore-derived paleotemperature proxy. Whether algal-bacterial interactions influence inorganic coccolithophore-derived paleo-proxies, is yet unknown.A commonly used inorganic proxy for past productivity and sea surface temperature is the Sr/Ca ratio of the coccolith calcite. Interestingly, during interactions between bacteria and a population of calcifying algae, bacteria were shown to physically attach only to non-calcified algal cells, suggesting an influence on algal calcification.In this study we explore the effects of algal-bacterial interactions on calcification and coccolith Sr/Ca ratios. We find that while bacteria attach only to non-calcified algal cells, coccolith cell coverage and overall calcite production in algal populations with and without bacteria, is similar. Furthermore, we find that Sr/Ca values are impacted only by water temperature and algal growth rate, regardless of bacterial influences on algal physiology. Our observations reinforce the robustness of coccolith Sr/Ca ratios as a paleo-proxy independent of microbial interactions, and highlight a fundamental difference between organic and inorganic paleo-proxies.Summary StatementThe current research investigates the effect of microbial interactions on coccolith Sr/Ca ratio and overall calcification in the coccolithophore Emiliania huxleyi. We co-cultured E. huxleyi with the marine bacterium Phaeobacter inhibens and compared coccolith Sr/Ca between different growth stages in a range of temperatures. Our results indicate that coccolith Sr/Ca depends on temperature and algal growth rate, and remains robust despite significant bacterial influences on algal physiology.

Based on materials obtained during Cruise 37 of the R/V Professor Logatchev in the Russian exploration area, the species composition, distribution, and preservation of nanno- and microfossil carbonate shells in biogenic bottom sediments within the Pobeda hydrothermal cluster (17°07.45′–17°08.7′ N MAR) were studied. Physicochemical parameters and total carbonate content were measured and their changes along column sections were revealed. Based on the coccolith assemblages, the studied sediments are assigned to the upper part (acme) of the biostratigraphic zone Emiliania huxleyi. It has been established that the hydrothermal activity zone (Pobeda hydrothermal cluster) is marked usually by a decrease of both total population and species diversity of microrganisms in the lower part of columns. The column is also marked by downsection decrease of Eh, pH, and carbonate content. These changes are attributed to the influence of diffuse hydrothermal fluids.

Organic-rich layers (sapropels), preserved in eastern Mediterranean marine sediment records, represent pronounced perturbations to thermohaline circulation and environmental conditions in the basin, in response to enhanced African monsoon activity and subsequent massive freshwater discharge. During the most recent event, Sapropel S1 formed between 10.8 and 6.1 ka, when freshwater-driven stratification caused seafloor anoxia below ~1,800-m depth, as a result of both failure of deep water formation and enhanced productivity. Here we analyze coccolith assemblages from the open eastern Mediterranean that form a west-east transect across the basin and provide insights on past environmental changes. We focus on holococcoliths, which are specifically produced by coccolithophores as part of their life cycle during the haploid phase. Since holococcolith calcification is characterized by nanocrystals highly susceptible to dissolution, we are testing their potential preservation under different bottom environmental conditions, including the effect of postdepositional oxidation. A comparison with benthic foraminifera assemblages in a core recovered close to Lybia reveals that holococcolith preservation is enhanced during seafloor reventilation and benthic foraminiferal repopulation in the middle to upper part of the record, before the actual sapropel termination. There are two such events of improved deep-water oxygenation in the Aegean and Adriatic Seas at 8.2 and 7.4 ka. The latter episode marks the onset of the transition to restored circulation in the eastern Mediterranean Sea, due to resumption of deep-water formation in the southern Aegean Sea and the conclusion of enhanced biogenic productivity.

The globally averaged calcite compensation depth has deepened by several hundred metres in the past 15 Myr. This deepening has previously been interpreted to reflect increased alkalinity supply to the ocean driven by enhanced continental weathering due to the Himalayan orogeny during the late Neogene period. Here we examine mass accumulation rates of the main marine calcifying groups and show that global accumulation of pelagic carbonates has decreased from the late Miocene epoch to the late Pleistocene epoch even though CaCO3 preservation has improved, suggesting a decrease in weathering alkalinity input to the ocean, thus opposing expectations from the Himalayan uplift hypothesis. Instead, changes in relative contributions of coccoliths and planktonic foraminifera to the pelagic carbonates in relative shallow sites, where dissolution has not taken its toll, suggest that coccolith production in the euphotic zone decreased concomitantly with the reduction in weathering alkalinity inputs as registered by the decline in pelagic carbonate accumulation. Our work highlights a mechanism whereby, in addition to deep-sea dissolution, changes in marine calcification acted to modulate carbonate compensation in response to reduced weathering linked to the late Neogene cooling and decline in atmospheric partial pressure of carbon dioxide. A redistribution of marine calcifiers along with a reduction in weathering led to increased seafloor carbonate deposition during the late Neogene, according to a global compilation of carbonate mass accumulation rate records from sediment cores.

Coccolithophores produce the bulk of ocean biogenic calcium carbonate but this process is predicted to be negatively affected by future ocean acidification scenarios. Since coccolithophores calcify intracellularly, the mechanisms through which changes in seawater carbonate chemistry affect calcification remain unclear. Here we show that voltage-gated H+ channels in the plasma membrane of Coccolithus braarudii serve to regulate pH and maintain calcification under normal conditions, but have greatly reduced activity in cells acclimated to low pH. This disrupts intracellular pH homeostasis and impairs the ability of C. braarudii to remove H+ generated by the calcification process, leading to specific coccolith malformations. These coccolith malformations can be reproduced by pharmacological inhibition of H+ channels. Heavily-calcified coccolithophore species such as C. braarudii, which make the major contribution to carbonate export to the deep ocean, have a large intracellular H+ load and are likely to be most vulnerable to future decreases in ocean pH.

International audience; Heterococcoliths are calcite platelets produced inside diploid coccolithophore cells and extruded to form a covering on the cell surface called a coccosphere. The size of coccoliths is an important parameter sometimes used to identify species, and it is observed to be influenced in extant species by abiotic parameters (e.g., CO2, light). However, the variable distribution of coccolith sizes occurring within a single coccosphere questions the mechanisms controlling coccolith size. A relationship between cell/coccosphere size and mean coccolith size was previously identified, called the “coccolithophore size rules”. In this study, we query the mechanisms controlling the size of a coccolith during coccolithogenesis. A culture experiment on Gephyrocapsa huxleyi strain RCC1216 shows that coccolithogenesis occurs during the cell growth G1 interphase and newly produced coccoliths get bigger as the cell grows. These observations provide parameters for the development of two numerical models used to simulate the coccolith size distribution within a coccolithophore population. Neither model can accurately reproduce an empirical monoclonal coccolith size distribution, indicating that additional factors influence coccolith size. According to our results, coccolith size is only clearly related to cell size at the time of its formation. We confirm that application of the coccolithophore size rules model should be limited to inferring average cell dimensions from (fossil) coccolith biometry, and that comparisons are valid only in multipopulational studies. The coccolith size rule model – the constraining effect of coccolith production during G1 interphase cell growth on coccolith size – proposed here is applicable only for some placolith-forming species.

International audience; While numerous studies have highlighted the central role of Southern Ocean (SO) dynamics in modulating rapid increases in atmospheric CO2 concentrations during deglaciations, fewer studies have yet focused on the impact of the Biological Carbon Pump - and more specifically the Carbonate Counter Pump (CCP) - in contributing to increase the CO2 concentration in oceanic surface waters and thus, in the atmosphere. Here, we present micropaleontological (coccolith, planktonic foraminifera) and geochemical (CaCO3, CaXRF, δ13CN. pachyderma) constraints from sediment core MD04-2718 retrieved in the Polar Front Zone of the Indian Ocean covering the time interval spanning Marine Isotope Stage (MIS) 12 to MIS 10 (440,000-360,000 years). We compare our results with published records from the SO to reconstruct past changes in CCP and upwelling dynamics and understand their leverage on the ocean-atmosphere portioning of CO2. We demonstrate that the sharp increase in atmospheric pCO2 during Termination V was likely associated with enhanced deep-water ventilation in the SO, that promoted the release of previously sequestered CO2 to the ocean surface as the westerly wind belt and the frontal system migrated southwards. Enhanced CCP is observed later, during MIS 11, and is likely the consequence of higher sea surface temperature and higher nutrient availability due to the reinvigoration of SO upwelling leading to increased coccolith (and to a lesser degree, planktonic foraminifera) production and export. The low eccentricity signal recorded during MIS 11 might have additionally strengthened the CCP, exerting a specific control on Gephyrocapsa morphotypes. In addition to the strong global biological productivity and higher carbon storage on land, these synergistic mechanisms may have permitted to shape the distinctive 30 ka-long pCO2 plateau characteristic of MIS 11.

The global ocean acts as a climate regulator through the uptake of Earth’s excess heat and the absorption of about 30% of anthropogenic CO2 emissions since 1750. Southern Ocean waters are warming faster than the global ocean average and their low temperatures and moderate alkalinity make this region especially vulnerable to ocean acidification. Coccolithophores are a major group of calcifying phytoplankton and an important component of the Southern Ocean carbon cycle. Controlled laboratory experiments on Emiliania huxleyi (the most abundant coccolithophore) over a broad range of carbonate chemistry scenarios suggest that this taxon may be susceptible to ongoing environmental change. However, it remains uncertain whether Southern Ocean coccolithophore populations have been modified by environmental change during the industrial era. The main reason for this knowledge gap is the lack of observational data since the onset of the Industrial Revolution. In particular, continuous monitoring of key Southern Ocean ecosystems only started a few decades ago, a period insufficiently long to permit assessments of whether anthropogenic impacts on the environment have affected coccolithophore populations beyond their natural state. In order to overcome this limitation, here we take advantage of the preservation capacity of coccolithophores in the sedimentary record to provide a benchmark of their pre-industrial state. We compare the morphotype assemblage composition and morphometric parameters in coccoliths of E. huxleyi from a suite of Holocene-aged sediments south of Tasmania with annual sediment trap records retrieved at the Southern Ocean Time Series observatory in the Australian sector of the Subantarctic Zone. Our results suggest that carbonate dissolution in the sediments reduced the coccolith mass and length of the coccoliths but, coccolith thickness appeared to be decoupled from dissolution. The biogeographical distribution of coccolith thickness in subtropical and subantarctic sediments mirrored the distribution of E. huxleyi morphotypes, highlighting the important role of E. huxleyi assemblage composition on the control of coccolith morphometrics. Moreover, comparison of coccolith assemblages from the sedimentary record with those collected from subantarctic sediment traps indicates that modern E. huxleyi coccoliths are about 2% thinner than those from the pre-industrial Holocene. The subtle change in coccolith thickness is in stark contrast with previous work that documented a dramatic reduction in shell calcification in the planktonic foraminifera Globigerina bulloides that resulted in a shell-weight decrease of 30-35%, most likely induced by ocean acidification. Overall, our results underscore the variable sensitivity of different marine calcifying plankton groups to ongoing environmental change in the Southern Ocean.

Some microorganisms, such as coccolithophores, produce an intricate exoskeleton made of inorganic solids. Coccoliths, the calcium carbonate scales of coccolithophores, are examples of the precise bioproduction of such complex 3D structures. However, the understanding of the cellular mechanisms that control mineral formation inside the cell, specifically the ability of these microalgae to transport high fluxes of inorganic building blocks, is still limited. Recently, using cryo-electron and X-ray microscopy, several intracellular compartments are shown to store high concentrations of calcium and phosphorous and are suggested to have a dominant role in the intracellular mineralization pathway. Here, live-cell confocal microscopy and fluorescent markers are used to examine the dynamics of ion stores in coccolithophores. Using calcein and 4',6-diamidino-2-phenylindole (DAPI) as fluorescent proxies for calcium and polyphosphates, the experiments reveal an unexpected plethora of organelles with distinct fluorescent signatures over a wide range of strains and conditions. Surprisingly, the fluorescent labeling does not show changes along the calcification process and is similar between calcifying and noncalcifying cells, suggesting that these ion pools may not be a dynamic avenue for calcium transport. In such a case, the enigma behind the ability of coccolithophores to sustain intracellular calcification still awaits comprehensive elucidation.

In this paper, we provide the first evidence of a bloom formation of coccolithophore Syracosphaera halldalii in a salt‐wedge estuary of Krka River (Mediterranean Sea), during stratified conditions. The increase of coccolithophore abundance was firstly noticed in August 2017 and occurred again in October 2017. The increased abundance appeared over wide salinity (16.9–35.3) and temperature ranges (16.4–23.3ºC). On these two occasions, the highest abundance of S. halldalii confirmed by SEM counts reached 2.51 × 106 cells L−1 at halocline in October. The contribution of S. halldalii to total coccolithophore assemblage was more than 97%. Vertical distribution of total coccolithophores abundance varied over time and followed the temperature curve, with the highest abundances co‐occurring with maximum temperature in the water column. A positive association of coccolithophore abundances with nitrates was found, but pulses of both organic nitrogen and nitrites seem to be important for sustaining the bloom after the initial proliferation. During the entire study period, S. halldalii was found only in the heterococcolith‐bearing phase, and the absence of the holococcolith‐bearing phase could be related to increased turbulence in the estuary. SEM analysis of cell morphology was in concordance with earlier descriptions, with apical coccoliths and body coccoliths of approximately equal size and both having distinct protruding teeth along the inner rim. A single exothecal coccolith was found in association with two S. halldalii cells indicating possible dithecatism of this species.

Coccolithophores represent a major component of the marine phytoplankton and contribute to the bulk of biogenic calcite formation on Earth. These unicellular protists produce minute calcite scales (coccoliths) within the cell, which are secreted to the cell surface. Individual coccoliths and their arrangements on the cell surface display a wide range of morphological variations. This review explores some of the recent evidence that points to similarities and differences in the mechanisms of calcification, focussing on the transport mechanisms that bring substrates to, and remove products from the site of calcification, together with new findings on factors that regulate coccolith morphology. We argue that better knowledge of these mechanisms and their variations is needed to inform more generally how different species of coccolithophore are likely to respond to changes in ocean chemistry. STATEMENT OF SIGNIFICANCE: Coccolithophores, minute single celled phytoplankton are the major producers of biogenic carbonate on Earth. They also represent an important component of the ocean's biota and contribute significantly to global carbon fluxes. Coccolithophores produce intricate calcite scales (coccoliths) internally that they secrete onto their external surface. This review presents some recent key findings on the mechanisms underlying the production of coccoliths. It also considers the factors that regulate the rate of production as well as the variety of shapes of individual coccoliths and their arrangements at the cell surface. Understanding these processes is needed to allow better predictions of how coccolithophores may respond to changing ocean chemistry associated with climate change.

The Early Eocene Thermal Maximum 2 (ETM2) at ~53.7 Ma is one of multiple hyperthermal events that followed the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma). The negative carbon excursion and deep ocean carbonate dissolution which occurred during the event imply that a substantial amount (103 Gt) of carbon (C) was added to the ocean-atmosphere system, consequently increasing atmospheric CO2(pCO2). This makes the event relevant to the current scenario of anthropogenic CO2 additions and global change. Resulting changes in ocean stratification and pH, as well as changes in exogenic cycles which supply nutrients to the ocean, may have affected the productivity of marine phytoplankton, especially calcifying phytoplankton. Changes in productivity, in turn, may affect the rate of sequestration of excess CO2 in the deep ocean and sediments. In order to reconstruct the productivity response by calcareous nannoplankton to ETM2 in the South Atlantic (Site 1265) and North Pacific (Site 1209), we employ the coccolith Sr/Ca productivity proxy with analysis of well-preserved picked monogeneric populations by ion probe supplemented by analysis of various size fractions of nannofossil sediments by ICP-AES. The former technique of measuring Sr/Ca in selected nannofossil populations using the ion probe circumvents possible contamination with secondary calcite. Avoiding such contamination is important for an accurate interpretation of the nannoplankton productivity record, since diagenetic processes can bias the productivity signal, as we demonstrate for Sr/Ca measurements in the fine (

Unicellular marine microalgae are responsible for one of the largest carbon sinks on Earth. This is in part due to intracellular formation of calcium carbonate scales termed coccoliths. Traditionally, the influence of changing environmental conditions on this process has been estimated using poorly constrained analogies to crystallization mechanisms in bulk solution, yielding ambiguous predictions. Here, we elucidated the intracellular nanoscale environment of coccolith formation in the model species Pleurochrysis carterae using cryoelectron tomography. By visualizing cells at various stages of the crystallization process, we reconstructed a timeline of coccolith development. The three-dimensional data portray the native-state structural details of coccolith formation, uncovering the crystallization mechanism, and how it is spatially and temporally controlled. Most strikingly, the developing crystals are only tens of nanometers away from delimiting membranes, resulting in a highly confined volume for crystal growth. We calculate that the number of soluble ions that can be found in such a minute volume at any given time point is less than the number needed to allow the growth of a single atomic layer of the crystal and that the uptake of single protons can markedly affect nominal pH values. In such extreme confinement, the crystallization process is expected to depend primarily on the regulation of ion fluxes by the living cell, and nominal ion concentrations, such as pH, become the result, rather than a driver, of the crystallization process. These findings call for a new perspective on coccolith formation that does not rely exclusively on solution chemistry.

Culture experiments with coccolithophore algae—the dominant group of marine calcifying phytoplankton—imply a strong sensitivity in growth rate, degree of cellular calcification, and cell size to changes in the carbon chemistry of their growth environment. These results underpin recent studies that have explored how these physiological parameters have varied on geological time scales, in response to changing surface ocean habitats and the concentrations of carbon in the ocean-atmosphere system. Here, we add to this work with a study of reticulofenestrid coccolith size—the dominant coccolithophore family of the Cenozoic—over the Oligocene to Early Miocene time interval. We examine sediments from contrasting latitudes and regional environmental settings, comparing sites using coherent, updated age models to distinguish globally synchronous trends in cell size from regional trends. Our results confirm several changes in coccolith size—which is strongly correlated to cell size—that are globally reproducible within the ~1 Myr age uncertainty, including a reduction in mean size by >2 μm from 30.2 to 27 and 24.5 to 23 Ma, and then increase in mean size after 20 Ma. The main difference among regions is the presence/absence of coccoliths larger than 8 μm. We evaluate which scenarios of change in carbon dioxide, temperature, and nutrient availability could have exerted selective pressure on cell size for different size classes to produce the observed size trends at each studied site. These million-year scale adaptations of ancient coccolithophores contribute to the understanding of phytoplankton physiology in the transition to the modern “icehouse” world. ©2020. American Geophysical Union. All Rights Reserved. ISSN:2572-4525 ISSN:2572-4517

A recommendation of: Suchéras-Marx B, Giraud F, Simionovici A, Tucoulou R, and Daniel I (2020). Evidence of high Sr/Ca in a Middle Jurassic murolith coccolith species. PaleorXiv, dcfuq, version 7, peer-reviewed by PCI Paleo. DOI: 10.31233/osf.io/dcfuq This upload contains: (1) the recommendation by the editor, and (2) the complete editorial process. Both can also be found on the PCI Paleo website (DOI: 10.24072/pci.paleo.100006).

Biomineralization by calcifying microalgae is a precisely controlled intracellular calcification process that produces delicate calcite scales (or coccoliths) in the coccolithophore Emiliania huxleyi (Haptophycea). Despite its importance in biogeochemical cycles and the marine environment globally, the underlying molecular mechanism of intracellular coccolith formation, which requires calcium, bicarbonate, and coccolith-polysaccharides, remains unclear. In E. huxleyi CCMP 371, we demonstrated that reducing the calcium concentration from 10 (ambient seawater) to 0.1 mM strongly restricted coccolith production, which was then recovered by adding 10 mM calcium, irrespective of inorganic phosphate conditions, indicating that coccolith production could be finely controlled by the calcium supply. Using this strain, we investigated the expression of differentially expressed genes (DEGs) to observe the cellular events induced by changes in calcium concentrations. Intriguingly, DEG analysis revealed that the phosphatidylinositol-specific phospholipase C (PI-PLC) gene was upregulated and coccolith production by cells was blocked by the PI-PLC inhibitor U73122 under conditions closely associated with calcium-induced calcification. These findings imply that PI-PLC plays an important role in the biomineralization process of the coccolithophore E. huxleyi.

Coccolithophores are calcifying microalgae that carry characteristic calcite platelets (coccoliths) on their surfaces. Most coccolithophore species exhibit diploid and haploid life cycle stages, each adjusted to different environmental conditions. The diploid life cycle stage of the coccolithophore C. braarudii is heavily calcifying with calcification rates that exceed the rates of photosynthesis. Haploid life-cycle stages are often weakly calcifying, generating significantly less H+ from the intracellular calcification reaction. We show how these different cellular “H+ burdens” require substantially different physiological molecular strategies to regulate intracellular pH under changing environmental conditions. Voltage-gated H+ channels (Hv) have been shown to play a role in the release of H+ in the diploid life cycle previously (Taylor et al. 2011). Combining scanning electron microscopy, electrophysiology, gene expression approaches and physiological measurements, we here show a direct link between the function of proton channels and coccolith formation of the diploid but not the haploid life-cycle stage. Our data also indicate how the different mechanisms for acid-base regulation of the diploid and haploid life-cycle stages may result in different sensitivities towards ocean acidification.

Unidad de excelencia María de Maeztu MdM-2015-0552 Published alkenone εp records spanning known glacial pCO2 cycles show considerably less variability than predicted by the diffusive model for cellular carbon acquisition and isotope fractionation. We suggest this pattern is consistent with a systematic cellular enhancement of the carbon supply to photosynthesis via carbon concentrating mechanisms under the case of carbon limitation during low pCO2 glacial time periods, an effect also manifest under carbon limitation in experimental cultures of coccolithophores as well as diatoms. While the low-amplitude εp signal over glacial pCO2 cycles has led some to question the reliability of εp for reconstructing long-term pCO2, the [CO2]aq in the tropical oceans during glacial pCO2 minima represents the most extreme low CO2 conditions likely experienced by phytoplankton in the Cenozoic, and the strongest upregulation of carbon concentrating mechanisms. Using a statistical multilinear regression model, we quantitatively parse out the factors (namely light, growth rate, and [CO2]aq), that contribute to variation in εp in alkenone-producing algae, which confirms a much smaller dependence of εp on [CO2]aq in the low [CO2]aq range, than inferred from the hyperbolic form of the diffusive model. Application of the new statistical model to two published tropical εp records spanning the late Neogene produces much more dynamic pCO2 estimates than the conventional diffusive model and reveals a significant pCO2 decline over the last 15 Ma, which is broadly consistent with recent results from boron isotopes of foraminifera. The stable isotopic fractionation between coccolith calcite and seawater dissolved inorganic carbon (here Δcoccolith-DIC) also shows systematic variations over glacial-interglacial cycles which may, following future experimental constraints, help estimate the degree of upregulation of parts of the algal carbon concentrating mechanism over glacial cycles.

Marine phytoplankton such as bloom-forming, calcite-producing coccolithophores, are naturally exposed to solar ultraviolet radiation (UVR, 280–400 nm) in the ocean's upper mixed layers. Nevertheless, the effects of increasing carbon dioxide (CO2)-induced ocean acidification and warming have rarely been investigated in the presence of UVR. We examined calcification and photosynthetic carbon fixation performance in the most cosmopolitan coccolithophorid, Emiliania huxleyi, grown under high (1000 µatm, HC; pHT: 7.70) and low (400 µatm, LC; pHT: 8.02) CO2 levels, at 15 ∘C, 20 ∘C and 24 ∘C with or without UVR. The HC treatment did not affect photosynthetic carbon fixation at 15 ∘C, but significantly enhanced it with increasing temperature. Exposure to UVR inhibited photosynthesis, with higher inhibition by UVA (320–395 nm) than UVB (295–320 nm), except in the HC and 24 ∘C-grown cells, in which UVB caused more inhibition than UVA. A reduced thickness of the coccolith layer in the HC-grown cells appeared to be responsible for the UV-induced inhibition, and an increased repair rate of UVA-derived damage in the HC–high-temperature grown cells could be responsible for lowered UVA-induced inhibition. While calcification was reduced with elevated CO2 concentration, exposure to UVB or UVA affected the process differentially, with the former inhibiting it and the latter enhancing it. UVA-induced stimulation of calcification was higher in the HC-grown cells at 15 and 20 ∘C, whereas at 24 ∘C observed enhancement was not significant. The calcification to photosynthesis ratio (Cal ∕ Pho ratio) was lower in the HC treatment, and increasing temperature also lowered the value. However, at 20 and 24 ∘C, exposure to UVR significantly increased the Cal ∕ Pho ratio, especially in HC-grown cells, by up to 100 %. This implies that UVR can counteract the negative effects of the “greenhouse” treatment on the Cal ∕ Pho ratio; hence, UVR may be a key stressor when considering the impacts of future greenhouse conditions on E. huxleyi.

A 19-year (1998–2016) continuous dataset is presented of coccolithophore Emiliania huxleyi distributions and activity, i.e. the release of CaCO3 in water and the decrease of uptake of dissolved CO2 by Emiliania huxleyi cells (e.g. Kondrik et al., 2018a), in Arctic and sub-Arctic seas. The dataset is based on optical remote-sensing data (mostly OC CCI data) with assimilation of different relevant in situ observations, preprocessed with authorial algorithms. Alongside bloom locations, we provide both detailed information on E. huxleyi impacts on carbon balance and the sub-datasets of quantified coccolith concentrations, particulate inorganic carbon content and CO2 partial pressure in water driven by coccolithophores. All data are presented on a regular 4×4 km grid at a temporal resolution of 8 days. The paper describes the theoretical and methodological basis for all processing and modelling steps. The data are available on Zenodo: https://doi.org/10.5281/zenodo.1402033.

Coccolithophores play important roles in marine biochemistry due to the processes of calcification and photosynthesis. Coccoliths are produced intracellularly, and cells produce coccoliths with stable isotopes distinct from theoretically precipitated inorganic calcite due to the influences of coccolithophore physiology, which are the so-called vital effects. The coccolith isotopic vital effects show large variations between species and hamper the use of coccolith isotopes in paleoceanography. In addition, learning the coccolith isotopic vital effects can help to better understand the carbon fractionation in coccolithophore cell, so as to provide a new insight in reconstructing sea water carbonate system (e.g., from alkenone δ 13 C or directly from coccolith δ 13 C) in geological past. In the present study, we investigated the morphological parameters, growth rate, and coccolith stable isotope compositions of Gephyrocapsa, the most universal alkenone-producing coccolithophore across the Pleistocene. These data allow us to estimate the influences of Gephyrocapsa morphology as well as the growth rate on coccolith vital effects. The results showed that Gephyrocapsa morphology, as well as coccolith isotopes, varied with species and morphotype changes, which were evolutionarily forced over the past 400 ka. The small Gephyrocapsa morphotypes (G. caribbeanica and Gephyrocapsa

We present the dissolution kinetics of mixed planktic foraminifera, the benthic foraminifera Amphistegina, the coccolithophore Emiliania huxleyi, and the soft coral Rhythismia fulvum in seawater . Dissolution rates were measured across a large range of saturation states (Ω = 0.99–0.2) by dissolving 13 C-labeled calcites in natural seawater undersaturated with respect to calcite. 13C-label was incorporated into biogenic calcite by culturing marine calcifiers in 13C-labeled natural seawater. Net dissolution rates were calculated as the slope of seawater δ13 C versus time in a closed seawater-calcite system. All calcites show distinct, nonlinear, dependencies on seawater saturation state when normalized by mass or by specific surface area. For example, coccolith calcite dissolves at a similar rate to inorganic calcite near equilibrium when normalized by surface area, but dissolves much more slowly far from equilibrium. Mass loss from foraminiferal tests is correlated with a decrease in Mg/Ca of the solid, indicating that Mg-rich phases are preferentially leached out at even mild undersaturations. Dissolution also appears to strongly affect test B/Ca. Finally, we provide an interpretation of surface area-normalized biogenic calcite dissolution rates as a function of their Mg and organic carbon content. Near-equilibrium dissolution rates of all calcites measured here show a strong, nonlinear dependence on Mg content. Far-from-equilibrium dissolution rates decrease strongly as a function of organic carbon content. These results help to build a framework for understanding the underlying mechanisms of rate differences between biogenic calcites, and bear important implications for the dissolution of high-Mg calcites in view of ocean acidification .

There has been some doubt cast upon the results of dinoflagellate cyst studies previously undertaken at Stevns Klint, Denmark, one of the classic outcrops of the Cretaceous–Palaeogene (K–Pg) boundary. A re-examination of both the uppermost Maastrichtian chalks and an expanded section of the Fish Clay (Fiskeler Member) has identified some differences between our findings and the earlier work. The white coccolith chalk of the uppermost Maastrichtian (Sigerslev Member) is placed in the Palynodinium grallator Zone. The last occurrence of P. grallator lies within the Hojerup Member, which represents a shallower-water marine succession that is characterized by a series of dune-like structures. This confirms a latest Maastrichtian age for the P. grallator Zone. Within the Fiskeler Member, a number of new taxa appear, including Carpatella cornuta, Xenicodinium reticulatum, and Fibrocysta spp. The highest samples in the Fiskeler Member indicate a progressive transition towards more proximal environments, with no dinoflagellate cysts recorded, having been replaced in the samples by pollen grains.

Coccolithophore responses to changes in carbonate chemistry speciation such as CO2 and H+ are highly modulated by light intensity and temperature. Here, we fit an analytical equation, accounting for simultaneous changes in carbonate chemistry speciation, light and temperature, to published and original data for Emiliania huxleyi, and compare the projections with those for Gephyrocapsa oceanica. Based on our analysis, the two most common bloom-forming species in present-day coccolithophore communities appear to be adapted for a similar fundamental light niche but slightly different ones for temperature and CO2, with E. huxleyi having a tolerance to lower temperatures and higher CO2 levels than G. oceanica. Based on growth rates, a dominance of E. huxleyi over G. oceanica is projected below temperatures of 22 ∘C at current atmospheric CO2 levels. This is similar to a global surface sediment compilation of E. huxleyi and G. oceanica coccolith abundances suggesting temperature-dependent dominance shifts. For a future Representative Concentration Pathway (RCP) 8.5 climate change scenario (1000 µatm fCO2), we project a CO2 driven niche contraction for G. oceanica to regions of even higher temperatures. However, the greater sensitivity of G. oceanica to increasing CO2 is partially mitigated by increasing temperatures. Finally, we compare satellite-derived particulate inorganic carbon estimates in the surface ocean with a recently proposed metric for potential coccolithophore success on the community level, i.e. the temperature-, light- and carbonate-chemistry-dependent CaCO3 production potential (CCPP). Based on E. huxleyi alone, as there was interestingly a better correlation than when in combination with G. oceanica, and excluding the Antarctic province from the analysis, we found a good correlation between CCPP and satellite-derived particulate inorganic carbon (PIC) with an R2 of 0.73, p

Ocean surface acidification due to increasing atmospheric CO2 concentration is currently attracting much attention. Coccolithophores distribute widely across the world's oceans and represent a carbon sink containing about 100 million tonnes of carbon. For this reason, there is concern about dissolution of their shells, which are made of calcium carbonate, due to decreasing pH. In this study, intracellular calcification, photosynthesis, and mass transport through biomembranes of Emiliania huxleyi were modelled numerically for understanding biological response in calcifying organisms. Unknown parameters were optimised by a generic algorithm to match existing experimental results. The model showed that the production of calcium carbonate rather than its dissolution is promoted under an acidified environment. Calcite remains at saturation levels in a coccolith even when it is below saturation levels in the external seawater. Furthermore, a coccolith can dissolve even in water where calcite saturation exceeds 1, because the saturation may be below the threshold level locally around the cell membrane. The present model also showed that the different calcification rates of E. huxleyi with respect to rising CO2 concentrations reported in the literature are due to differences in experimental conditions; in particular, how the CO2 concentration is matched. Lastly, the model was able to reproduce differences in calcification rates among coccolithophore species. The above biochemical-kinetic model was then incorporated into an ecosystem model, and the behaviour of coccolithophores in the ecosystem and the influence of increases in CO2 concentration on water quality were simulated and validated by comparison with existing experimental results. The model also suggests that increased CO2 concentration could lead to an increase in the biomass ratio of coccolithophores to diatoms at high CO2 concentrations, particularly in oligotrophic environments, and to a consequent decrease in pH due to calcium dissolution.

Coccolith assemblages in two gravity cores (KX21-2 and KX12-1) from the central Western Pacific Warm Pool (WPWP) have been analyzed with SYRACO. The variations of nutricline and primary productivity (PP) have been reconstructed based on these assemblages. The results show that the coccolith assemblages were dominated by Florisphaera profunda, Gephyrocapsa and Emiliania huxleyi over the last 380 kyr. Variations of nutricline and primary productivity can be divided into three intervals. Interval I (about 380–300 kyr): PP was high and nutricline was shallow; Interval II (about 300–160 kyr): PP decreased dramatically for a short time after the acme of G. caribbeanica in Mid-Brunhes while nutricline became deeper; Interval III (about 160 kyr–present): PP fluctuated at low levels and nutricline was deep. Variations of each coccolith taxon and PP were highly correlated in the two cores, which means that the geological environment is similar in the two cores. Spectrum analysis is performed for all coccolith taxons and PP, and the 19-kyr cycle is the most prominent. It means that the production of coccolithophores in the WPWP is mainly controlled by precession.

The calcite plates, or coccoliths, of haptophyte algae including Emiliania huxleyi are formed in intracellular vesicles in association with water–soluble acidic polysaccharides. These coccolith–associated polysaccharides (CAPs) are involved in regulating coccolith formation and have been recovered from sediment samples dating back to ∼180 Ma. Paired measurements of the carbon isotopic compositions of CAPs and coccolith calcite have been proposed as a novel paleo–pCO2 barometer, but additional proxy validation and development are still required. Here we present culture results quantifying carbon isotopic offsets between CAPs and other cellular components: bulk organic biomass, alkenones, and calcite. E. huxleyi was grown in nitrate–limited chemostat experiments at growth rates (µ) of 0.20–0.62/d and carbon dioxide concentrations of 10.7–17.6 µmol/kg. We find that CAPs are isotopically enriched by 4.5–10.1‰ relative to bulk organic carbon, exhibiting smaller isotopic offsets at faster growth rates and lower CO2 concentrations. This variability suggests that CAPs record a complementary signature of past growth conditions with different sensitivity than alkenones or coccolith calcite. By measuring the isotopic compositions of all three molecules and minerals of self-consistent origin, the ratio µ/[CO2(aq)] may be reconstructed with fewer assumptions than current approaches.

Dinoflagellates and coccolithophores are two of the most important groups of phytoplankton in the modern oceans. These groups originated in the Triassic and radiated through the early Mesozoic, rising to ecological prominence. Within this long-term radiation, important short-term intervals of evolutionary and ecological change can be recognised. The Bajocian (Middle Jurassic,similar to 170-168 Ma) was characterised by an important ecological transition within the coccolithophores, and the radiation of one of the principal families of cyst forming dinoflagellates, the Gonyaulacaceae. During the Early Bajocian, the coccolith genus WahZnatteria diversified and expanded ecologically to dominate coccolith floras, a situation which continued for the remainder of the Mesozoic. This pattern was paralleled within dinoflagellate cyst floras by the ecological dominance of the genus Dissiliodiniun in the mid-palaeolatitudes. These phenomena appear to be linked to a positive carbon isotope shift, and an interval of enhanced productivity driven by a shift to a more humid climate, enhanced continental weathering and nutrient flux, or by changes in ocean circulation and upwelling. The latest Early Bajocian to earliest Bathonian was then characterised by the rapid increase in diversity of dinoflagellate cysts within the family Gonyaulacaceae. Through this interval, the Gonyaulacaceae transitioned from being a relatively minor component of dinoflagellate cyst floras, to becoming one of the prominent groups of cyst-forming dinoflagellates, which has persisted to the Holocene. In Europe, the pattern of this radiation was strongly influenced by sea level, with the increase in gonyaulacacean diversity reflecting a major second-order transgression. On a finer scale, the main pulses of first appearances correlate with third-order transgressive episodes. A rise in sea level, coupled with changes in the tectonic configuration of ocean gateways, appears to have controlled the pattern of plankton diversification in Europe. These palaeoceanographic changes may have enhanced water-mass transfer between Europe, the northwest Tethys Ocean and the Hispanic Corridor, which promoted the floral interchange of dinoflagellates. Whilst sea-level rise and associated large-scale palaeoenvironmental shifts appear to have controlled the pattern of dinoflagellate cyst appearances in several regions outside Europe, there is no direct correlation between dinoflagellate cyst diversity and sea-level rise on a global scale. Although the Bajocian was transgressive in several regions, widespread flooded continental area was also present throughout the preceding Aalenian, an interval of low gonyaulacacean diversity. Moreover, although the Middle Jurassic was an interval of major climatic cooling, there was a similar to 5 myr gap between the onset of cooling and the radiation of gonyaulacaceans during the Bajocian. The Bajocian was, however, marked by a major evolutionary radiation in the pelagic realm, including ammonites, giant suspension feeding fishes and planktonic foraminifera. These phenomena may indicate an underlying ecological driver to the radiation of dinoflagellates during the Bajocian evolutionary explosion which could represent an extension of the Mesozoic Marine Revolution.

An increase in the atmospheric pCO2 and temperature is expected to reduce ocean CO32− concentration, make oceans warmer and alter ocean circulation patterns. This will also affect the production and biogeographic distribution of marine calcifying organisms including coccolithophores. The lowering of oceanic CO32− is expected to interfere with the coccolithophore calcification process and cause malformation of coccoliths, whereas changes in the oceanic temperature and circulation patterns may shift their biogeographic boundaries. In this study, we have investigated Emiliania huxleyi coccolith and coccosphere size response to the wide-ranging physico-chemical conditions of the Indian sector of the Southern Ocean between latitudes 38oS and 58oS during the austral summer of 2010 (January–February). This study helps to understand the response of E. huxleyi coccolith/coccosphere morphometry and mass changes to the fluctuating temperature, salinity, CO32−, pCO2, and nutrient values. Our results show that in the Indian sector of the Southern Ocean, E. huxleyi coccoliths are larger and coccospheres are smaller in the Subtropical Zone (STZ). In contrast, coccoliths size is smaller and coccospheres size is larger in the Subantarctic Zone (SAZ), which is due to the decrease in Sea Surface Temperature, Sea Surface Salinity and increase in nutrient concentrations. In the Indian sector of the Southern Ocean, E. huxleyi shows a north-to-south morphotype shift from the heavily calcified ‘Group A’ (E. huxleyi morphotype A) to the weakly calcified ‘Group B’ (E. huxleyi morphotypes B/C, C) forms. We demonstrate that although weakly calcified E. huxleyi morphotypes (morphotypes B/C and C) comprise less mass than that of the E. huxleyi morphotype A, due to the large-sized coccospheres and numerous coccoliths per coccosphere, ‘Group B’ coccospheres precipitate large amount of CaCO3 in the SAZ compared to ‘Group A’ coccospheres located in the STZ. We have documented the presence of large E. huxleyi overcalcified coccospheres with large-sized coccoliths in the southernmost cold, high pCO2, and nutrient-rich waters which show extracellular calcite precipitation. The energy dispersive spectrometry analysis indicates the presence of a large amount of Mg in the overcalcified E. huxleyi specimens. We suspect that E. huxleyi in the colder nutrient-rich waters, with future projected changes in the carbonate chemistry, may adapt to low pH, high pCO2 conditions through extracellular Ca and Mg mineralization.

Marine multicellular organisms inhabiting waters with natural high fluctuations in pH appear more tolerant to acidification than conspecifics occurring in nearby stable waters, suggesting that environments of fluctuating pH hold genetic reservoirs for adaptation of key groups to ocean acidification (OA). The abundant and cosmopolitan calcifying phytoplankton Emiliania huxleyi exhibits a range of morphotypes with varying degrees of coccolith mineralization. We show that E. huxleyi populations in the naturally acidified upwelling waters of the eastern South Pacific, where pH drops below 7.8 as is predicted for the global surface ocean by the year 2100, are dominated by exceptionally over-calcified morphotypes whose distal coccolith shield can be almost solid calcite. Shifts in morphotype composition of E. huxleyi populations correlate with changes in carbonate system parameters. We tested if these correlations indicate that the hyper-calcified morphotype is adapted to OA. In experimental exposures to present-day vs. future pCO2 (400 vs. 1200 µatm), the over-calcified morphotypes showed the same growth inhibition (−29.1±6.3 %) as moderately calcified morphotypes isolated from non-acidified water (−30.7±8.8 %). Under the high-CO2–low-pH condition, production rates of particulate organic carbon (POC) increased, while production rates of particulate inorganic carbon (PIC) were maintained or decreased slightly (but not significantly), leading to lowered PIC ∕ POC ratios in all strains. There were no consistent correlations of response intensity with strain origin. The high-CO2–low-pH condition affected coccolith morphology equally or more strongly in over-calcified strains compared to moderately calcified strains. High-CO2–low-pH conditions appear not to directly select for exceptionally over-calcified morphotypes over other morphotypes, but perhaps indirectly by ecologically correlated factors. More generally, these results suggest that oceanic planktonic microorganisms, despite their rapid turnover and large population sizes, do not necessarily exhibit adaptations to naturally high-CO2 upwellings, and this ubiquitous coccolithophore may be near the limit of its capacity to adapt to ongoing ocean acidification.

Calcifying haptophytes (coccolithophores) sequester carbon in the form of organic and inorganic cellular components (coccoliths). We examined the effect of phosphorus (P) limitation and heat stress on particulate organic and inorganic carbon (calcite) production in the coccolithophore Emiliania huxleyi. Both environmental stressors are related to rising CO2 levels and affect carbon production in marine microalgae, which in turn impacts biogeochemical cycling. Using semi-continuous cultures, we show that P limitation and heat stress decrease the calcification rate in E. huxleyi. However, using batch cultures, we show that different culturing approaches (batch versus semi-continuous) induce different physiologies. This affects the ratio of particulate inorganic (PIC) to organic carbon (POC) and complicates general predictions on the effect of P limitation on the PIC ∕ POC ratio. We found heat stress to increase P requirements in E. huxleyi, possibly leading to lower standing stocks in a warmer ocean, especially if this is linked to lower nutrient input. In summary, the predicted rise in global temperature and resulting decrease in nutrient availability may decrease CO2 sequestration by E. huxleyi through lower overall carbon production. Additionally, the export of carbon may be diminished by a decrease in calcification and a weaker coccolith ballasting effect.

Coccolithophores are major contributors to phytoplankton communities and ocean biogeochemistry and are strong modulators of the optical field in the sea. New discoveries are changing paradigms about these calcifiers. A new role for silicon in coccolithophore calcification is coupling carbonate and silicon cycles. Phosphorus and iron play key roles in regulating coccolithophore growth. Comparing molecular phylogenies with coccolith morphometrics is forcing the reconciliation of biological and geological observations. Mixotrophy may be a possible life strategy for deep-dwelling species, which has ramifications for biological pump and alkalinity pump paradigms. Climate, ocean temperatures, and pH appear to be affecting coccolithophores in unexpected ways. Global calcification is approximately 1–3% of primary productivity and affects CO2 budgets. New measurements of the backscattering cross section of coccolithophores have improved satellite-based algorithms and their application in case I and case II optical waters. Remote sensing has allowed the detection of basin-scale coccolithophore features in the Southern Ocean.

Coccolithophores are a critical component of global biogeochemistry, export fluxes, and seawater optical properties. We derive globally significant relationships to estimate integrated coccolithophore and coccolith concentrations as well as integrated concentrations of particulate inorganic carbon (PIC) from their respective surface concentration. We also examine surface versus integral relationships for other biogeochemical variables contributed by all phytoplankton (e.g., chlorophyll a and particulate organic carbon) or diatoms (biogenic silica). Integrals are calculated using both 100 m integrals and euphotic zone integrals (depth of 1% surface photosynthetically available radiation). Surface concentrations are parameterized in either volumetric units (e.g., m-3) or values integrated over the top optical depth. Various relationships between surface concentrations and integrated values demonstrate that when surface concentrations are above a specific threshold, the vertical distribution of the property is biased to the surface layer, and when surface concentrations are below a specific threshold, the vertical distributions of the properties are biased to subsurface maxima. Results also show a highly predictable decrease in explained-variance as vertical distributions become more vertically heterogeneous. These relationships have fundamental utility for extrapolating surface ocean color remote sensing measurements to 100 m depth or to the base of the euphotic zone, well beyond the depths of detection for passive ocean color remote sensors. Greatest integrated concentrations of PIC, coccoliths, and coccolithophores are found when there is moderate stratification at the base of the euphotic zone.

Coccoliths comprise a major fraction of the global carbonate sink. Therefore, changes in coccolithophores' Ca isotopic fractionation could affect seawater Ca isotopic composition, affecting interpretations of the global Ca cycle and related changes in seawater chemistry and climate. Despite this, a quantitative interpretation of coccolith Ca isotopic fractionation and a clear understanding of the mechanisms driving it are not yet available. Here, we address this gap in knowledge by developing a simple model (CaSri–Co) to track coccolith Ca isotopic fractionation during cellular Ca uptake and allocation to calcification. We then apply it to published and new δ44/40Ca and Sr/Ca data of cultured coccolithophores of the species Emiliania huxleyi and Gephyrocapsa oceanica. We identify changes in calcification rates, Ca retention efficiency and solvation–desolvation rates as major drivers of the Ca isotopic fractionation and Sr/Ca variations observed in cultures. Higher calcification rates, higher Ca retention efficiencies and lower solvation–desolvation rates increase both coccolith Ca isotopic fractionation and Sr/Ca. Coccolith Ca isotopic fractionation is most sensitive to changes in solvation–desolvation rates. Changes in Ca retention efficiency may be a major driver of coccolith Sr/Ca variations in cultures. We suggest that substantial changes in the water structure strength caused by past changes in temperature could have induced significant changes in coccolithophores' Ca isotopic fractionation, potentially having some influence on seawater Ca isotopic composition. We also suggest a potential effect on Ca isotopic fractionation via modification of the solvation environment through cellular exudates, a hypothesis that remains to be tested.

We determined the abundances and concentrations of coccoliths and alkenones in 66 surface sediment samples from the northwest North Atlantic to evaluate the role that surface ocean temperature, salinity, sea-ice cover, and productivity have on the regional distribution of these two biological remains produced by haptophytes in the photic zone. In areas with sea-ice cover of more than 1 month per year, coccolith and alkenone concentrations in sediments are extremely low to nil. Elsewhere, the distribution of coccolith taxa generally displays strong relationships to water temperature, salinity, and productivity. Coccolithus pelagicus is associated with low summer sea-surface temperatures ( 33.5), whereas Helicosphaera carteri seems to follow the path of the North Atlantic Drift. The distribution of Emiliania huxleyi, the dominant alkenone producer, is not strongly correlated with that of alkenones. Calcite dissolution in shelf sediments could explain the occurrence of alkenones in the absence of coccoliths but alkenone production by non-calcifying haptophytes seems to also exert some control on alkenone concentrations in surface sediments, thus blurring alkenone abundance links to coccolithophorid production and their relative preservation.

Extratropical sea surface temperature records from alkenones record a dramatic cooling of up to 17 °C over the last ∼14 Ma, but the relationship between this cooling and greenhouse gas forcing has been elusive due to sparse and contrasting reconstructions of atmospheric CO2 for the time period. Alkenone carbon isotopic fractionation during photosynthesis has previously been used to estimate changes in pCO2 over this interval, but is complicated by significant changes in cell size of the alkenone-producing coccolithophorids over this time period. In this study, we reconstruct carbon isotopic fractionation during photosynthesis ( e p ) using organic compounds trapped within the frustules of pennate diatoms in sediments from the Eastern Equatorial Pacific Ocean at Ocean Drilling Program Site 846 over the last ∼13 Ma. Physical separation of pennate diatoms prior to measuring carbon isotopic fractionation enables us to obtain a record with constant cell geometry, eliminating this factor of uncertainty in our pCO2 reconstruction. In the past ∼11 Ma, e p declines from 15.5 to 10.3‰. Using the classic diffusive model and taking into account variations in opal content, alkenone concentration and coccolith Sr/Ca as indicators of past productivity and growth rate, and sea surface temperature records from the site, we estimate a decline in pCO2 from 454 ( + / − 41 ) to 250 ( + / − 15 ) ppmv between ∼11 and 6 Ma. Models accounting for changing the significance of active carbon uptake for photosynthesis, which likely produce more accurate CO2 estimates, suggest a significant larger pCO2 decline of up to twice that shown by the classic diffusive model (in average from 794 ( + / − 233 ) ppmv at ∼11 Ma to 288 (+/−25) ppmv at ∼6 Ma, considering growth rates varying between 0.5 and 1.7 day−1). Large uncertainties in the pCO2 estimated between ∼8 and 11 Ma using the active uptake model are related to the growth rate used for calculations. Together, these results suggest CO2 forcing for this period of steep decline in temperatures.