Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2

Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short‐term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long‐term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO₂ conditions (1,000 μatm). High CO₂‐selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO₂ (400 μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO₂‐selected cell lines. All of these trait changes persisted when high CO₂‐grown populations were moved back to ambient CO₂ conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short‐term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences.     

A voltage-gated H+ channel underlying pH homeostasis in calcifying coccolithophores

Marine coccolithophorid phytoplankton are major producers of biogenic calcite, playing a significant role in the global carbon cycle. Predicting the impacts of ocean acidification on coccolithophore calcification has received much recent attention and requires improved knowledge of cellular calcification mechanisms. Uniquely amongst calcifying organisms, coccolithophores produce calcified scales (coccoliths) in an intracellular compartment and secrete them to the cell surface, requiring large transcellular ionic fluxes to support calcification. In particular, intracellular calcite precipitation using HCO₃ ⁻ as the substrate generates equimolar quantities of H⁺ that must be rapidly removed to prevent cytoplasmic acidification. We have used electrophysiological approaches to identify a plasma membrane voltage-gated H⁺ conductance in Coccolithus pelagicus ssp braarudii with remarkably similar biophysical and functional properties to those found in metazoans. We show that both C. pelagicus and Emiliania huxleyi possess homologues of metazoan H_v1 H⁺ channels, which function as voltage-gated H⁺ channels when expressed in heterologous systems. Homologues of the coccolithophore H⁺ channels were also identified in a diversity of eukaryotes, suggesting a wide range of cellular roles for the H_v1 class of proteins. Using single cell imaging, we demonstrate that the coccolithophore H⁺ conductance mediates rapid H⁺ efflux and plays an important role in pH homeostasis in calcifying cells. The results demonstrate a novel cellular role for voltage gated H⁺ channels and provide mechanistic insight into biomineralisation by establishing a direct link between pH homeostasis and calcification. As the coccolithophore H⁺ conductance is dependent on the trans-membrane H⁺ electrochemical gradient, this mechanism will be directly impacted by, and may underlie adaptation to, ocean acidification. The presence of this H⁺ efflux pathway suggests that there is no obligate use of H⁺ derived from calcification for intracellular CO₂ generation. Furthermore, the presence of H_v1 class ion channels in a wide range of extant eukaryote groups indicates they evolved in an early common ancestor.     

Stable isotope fractionation of strontium in coccolithophore calcite: Influence of temperature and carbonate chemistry

Marine calcifying eukaryotic phytoplankton (coccolithophores) is a major contributor to the pelagic production of CaCO₃ and plays an important role in the biogeochemical cycles of C, Ca and other divalent cations present in the crystal structure of calcite. The geochemical signature of coccolithophore calcite is used as palaeoproxy to reconstruct past environmental conditions and to understand the underlying physiological mechanisms (vital effects) and precipitation kinetics. Here, we present the stable Sr isotope fractionation between seawater and calcite (Δ^88/86Sr) of laboratory cultured coccolithophores in individual dependence of temperature and seawater carbonate chemistry. Coccolithophores were cultured within a temperature and a pCO₂ range from 10 to 25°C and from 175 to 1,240 μatm, respectively. Both environmental drivers induced a significant linear increase in coccolith stable Sr isotope fractionation. The temperature correlation at constant pCO₂ for Emiliania huxleyi and Coccolithus braarudii is expressed as Δ^88/86Sr = ?7.611 × 10^?3 T + 0.0061. The relation of Δ^88/86Sr to pCO₂ was tested in Emiliania huxleyi at 10 and 20°C and resulted in Δ^88/86Sr = ?5.394 × 10^?5 pCO₂ – 0.0920 and Δ^88/86Sr = ?5.742 × 10^?5 pCO₂ – 0.1351, respectively. No consistent relationship was found between coccolith Δ^88/86Sr and cellular physiology impeding a direct application of fossil coccolith Δ^88/86Sr as coccolithophore productivity proxy. An overall significant correlation was detected between the elemental distribution coefficient (D_Sr) and Δ^88/86Sr similar to inorganic calcite with a physiologically induced offset. Our observations indicate (i) that temperature and pCO₂ induce specific effects on coccolith Δ^88/86Sr values and (ii) that strontium elemental ratios and stable isotope fractionation are mainly controlled by precipitation kinetics when embedded into the crystal lattice and subject to vital effects during the transmembrane transport from seawater to the site of calcification. These results provide an important step to develop a coccolith Δ^88/86Sr palaeoproxy complementing the existing toolbox of palaeoceanography.     

The Effect of cytoskeleton inhibitors on coccolith morphology in Coccolithus braarudii and Scyphosphaera apsteinii

The calcite platelets of coccolithophores (Haptophyta), the coccoliths, are among the most elaborate biomineral structures. How these unicellular algae accomplish the complex morphogenesis of coccoliths is still largely unknown. It has long been proposed that the cytoskeleton plays a central role in shaping the growing coccoliths. Previous studies have indicated that disruption of the microtubule network led to defects in coccolith morphogenesis in Emiliania huxleyi and Coccolithus braarudii. Disruption of the actin network also led to defects in coccolith morphology in E. huxleyi, but its impact on coccolith morphology in C. braarudii was unclear, as coccolith secretion was largely inhibited under the conditions used. A more detailed examination of the role of actin and microtubule networks is therefore required to address the wider role of the cytoskeleton in coccolith morphogenesis. In this study, we have examined coccolith morphology in C. braarudii and Scyphosphaera apsteinii following treatment with the microtubule inhibitors vinblastine and colchicine (S. apsteinii only) and the actin inhibitor cytochalasin B. We found that all cytoskeleton inhibitors induced coccolith malformations, strongly suggesting that both microtubules and actin filaments are instrumental in morphogenesis. By demonstrating the requirement for the microtubule and actin networks in coccolith morphogenesis in diverse species, our results suggest that both of these cytoskeletal elements are likely to play conserved roles in defining coccolith morphology.     

Toxicity of coastal coccolithophores (Prymnesiophyceae, Haptophyta)

Over the last decade, certain coccolithophores have been thesubject of extensive multidisciplinary research. Several speciesof coccolithophore, belonging mainly to the families Pleurochrysidaceaeand Hymenomonadaceae, inhabit inshore coastal waters where theymay occasionally bloom and hence impact aquaculture resources.The toxicity to Artemia salina larvae of 11 species of coccolithophore(nine coastal and two oceanic members of the order Coccolithales)was tested. For the nine coastal species, tests were conductedwith rapidly growing and stationary phase cultures at a rangeof cellular concentrations and for two different exposure times(24 and 48 h). Five of the coastal species (four in the genusPleurochrysis as well as Jomonlithus littoralis) were foundto be toxic to A. salina nauplii. Allelopathic effects of acell-free filtrate of a culture of a toxic coccolithophore werealso tested on three flagellate microalgal species: Scrippsiellatrochoidea, Tetraselmis sp. and Isochrysis galbana. Negativeeffects of the filtrate on growth rates and motility of S. trochoideaand Tetraselmis sp. were recorded, suggesting that the toxinof the coccolithophore tested could be an exotoxin similar tothat produced by other non-calcifying members of the Prymnesiophyceae.The fact that certain coccolithophores were found to be toxicto invertebrates and were shown to exhibit allelopathic activitycould imply negative effects at different trophic levels incoastal areas.     

Emiliania huxleyi increases calcification but not expression of calcification-related genes in long-term exposure to elevated temperature and pCO2

Increased atmospheric pCO₂ is expected to render future oceans warmer and more acidic than they are at present. Calcifying organisms such as coccolithophores that fix and export carbon into the deep sea provide feedbacks to increasing atmospheric pCO₂. Acclimation experiments suggest negative effects of warming and acidification on coccolithophore calcification, but the ability of these organisms to adapt to future environmental conditions is not well understood. Here, we tested the combined effect of pCO₂ and temperature on the coccolithophore Emiliania huxleyi over more than 700 generations. Cells increased inorganic carbon content and calcification rate under warm and acidified conditions compared with ambient conditions, whereas organic carbon content and primary production did not show any change. In contrast to findings from short-term experiments, our results suggest that long-term acclimation or adaptation could change, or even reverse, negative calcification responses in E. huxleyi and its feedback to the global carbon cycle. Genome-wide profiles of gene expression using RNA-seq revealed that genes thought to be essential for calcification are not those that are most strongly differentially expressed under long-term exposure to future ocean conditions. Rather, differentially expressed genes observed here represent new targets to study responses to ocean acidification and warming.     

CACO3 OPTICAL DETECTION WITH FLUORESCENT IN SITU HYBRIDIZATION: A NEW METHOD TO IDENTIFY AND QUANTIFY CALCIFYING MICROORGANISMS FROM THE OCEANS1

Open oceanic calcification is mainly driven by unicellular organisms and in particular by eukaryotes such as coccolithophores and foraminifers. Open ocean microcalcifiers, like most planktonic protists, are characterized by extremely fast generation times and occasional sexual reproduction. Populations can alternate between diploid and haploid stages, which often build different kinds of cell covers. In the most important pelagic calcifiers, the coccolithophores, the diploid and haploid stages, which can self‐replicate and grow independently, display radically different morphologies with different modes of calcification or even with the absence of calcification in at least one life cycle stage. Although life cycle strategies seem likely to fundamentally influence the where and when of open ocean calcification, this issue has yet to be seriously addressed in the natural environment. Here, we introduce a new morphogenetic method, “combined CaCO₃ optical detection with fluorescent in situ hybridization,” or COD‐FISH, which is based on a combination of TSA‐FISH and polarized optical microscopy. This technique allows simultaneous assessment of the taxonomic and life cycle status of single coccolithophore cells collected from the ocean. We demonstrate the application of COD‐FISH using both laboratory culture and field samples and discuss its potential value for assessing the ecology, biodiversity, population structure, and life cycles of coccolithophores and other open ocean unicellular calcifiers.     

Coccolithophore (–CaCO3) flux in the Sea of Okhotsk: seasonality, settling and alteration processes

Coccolithophore fluxes were determined in the Sea of Okhotsk using samples from a 1 year experiment (12 August 1990 to 12 August 1991) with sediment traps at 258 and 1061 m depth. A special study was made on Coccolithus pelagicus, using fragmentation and the degree of etching, as indicators of transport mechanisms. A Corrosion Index for C. pelagicus is developed. The coccolithophore flux pattern at 258 m depth was characterised by a strong seasonality, with flux peaks during autumn 1990 (late November to early December) and spring 1991 (March). The assemblage consisted almost entirely of the two species C. pelagicus and Emiliania huxleyi. During autumn, coccolithophore transportation to 258 m depth mainly occurred within cylindrical fecal pellets and marine snow aggregates of silicoflagellates, and through agglutination on tintinnids. Grazing caused severe fragmentation of coccoliths and disintegration of coccospheres. Marine snow aggregates contained many intact coccospheres of C. pelagicus. During spring, coccolithophores were probably removed from the euphotic zone by the ballast effect of sinking diatoms. The coccolithophore flux peak in spring occurred immediately after the ice had retreated from the trap station, and the trapped assemblage included coccoliths of subtropical species. These features indicate drifting from an ice-free location to the south or east.The coccolith and coccosphere flux at 1061 m was respectively 7 and 12 times lower than at 258 m depth, and maximum fluxes were recorded 2 months later. Increasing carbonate dissolution from 258 to 1061 m depth is expressed in the coccolithophore–CaCO₃ flux reduction of 82%, and in the increasing percentage of etched coccoliths of Coccolithus pelagicus from 32 to >90%.     

Holococcolithophore‐heterococcolithophore (Haptophyta) life cycles: Flow cytometric analysis of relative ploidy levels

Several coccolithophore species are known to exhibit heteromorphic life cycles. In certain species, notably Emiliania huxleyi, the heterococcolith‐bearing phase alternates with a non‐calcifying stage, whereas in others the heterococcolith‐bearing phase alternates with a holococcolith‐bearing phase. Heterococcolithophore‐holococcolithophore life cycles have previously been observed for only one species in culture, but have also been inferred from an increasing number of observations of combination coccospheres. 18S rDNA sequences from pure cultures of both the heterococcolith‐bearing and holococcolith‐bearing phases of Coccolithus pelagicus were identical, providing an additional indication of their identity as different life cycle stages of the same species. Flow cytometric analyses have been undertaken on SybrGreen‐stained nuclei isolated from pure cultures of the two phases of four coccolithophore species (Coccolithus pelagicus, Calcidiscus leptoporus, Coronosphaera mediterranea and Emiliania huxleyi) in order to determine relative DNA content. Results confirm the hypothesis that holococcolithophore‐heterococcolithophore life cycles are haplo‐diploid in nature. Light microscope observations of the processes of sexual fusion and meiosis are reported for two of the experimental species. The results are discussed in the context of the evolution of bio‐mineralization in the coccolithophores and the possible ubiquity of haplo‐diploidy in the haptophytes.     

Spatial dynamics of coccolithophore assemblages in the Equatorial Western-Central Pacific Ocean

The spatial distribution of living coccolithophores was studied in the Western-Central Equatorial Pacific Ocean during November–December, 1990 and September–October, 1992. The highest local concentration of coccolithophores occurred at the thermocline in well-stratified waters, but at sea-surface level in dynamic waters. In total, 111 coccolithophore taxa were recognized, some of which exhibited hydrographically controlled variation in their absolute abundance. Gephyrocapsa oceanica and Oolithotus antillarum were abundant in the upwelling front. Most of the lower photic dwellers were abundant in the tropical to subtropical stations regardless of the water stratification. The coccolithophore flora of well-stratified waters could be distinguished from the upwelling front flora by the higher abundance of Umbellosphaera irregularis and lower abundance of G. oceanica. The temperature mixed-water flora was characterized by a high abundance of Emiliania huxleyi. The vertical distribution of all coccolithophore taxa, except three placolith-bearing species, Gephyrocapsa ericsonii, G. oceanica and E. huxleyi, was controlled by upper photic-zone temperature and water stratification. The upper or lower vertical distribution limits of many coccolithophore taxa coincided with the top of the thermocline. The most common 27 taxa were grouped into four ecological groups, Upper Photic-zone Group (UPG), Middle Photic-zone Group (MPG), Lower Photic-zone Group (LPG) and Omnipresent Group (OPG), on the basis of their vertical distribution. By analyzing the hydrographic control on the vertical distribution of these four ecological groups, four ecological assemblages were recognized: High Temperature; Warm Oligotrophic; Warm Eutrophic; and Temperate Mixed-water Assemblages. In equatorial waters, the total coccolithophore assemblage across the photic-zone was controlled by the population in the upper photic-zone. The UPG monopolized the upper photic-zone flora in the High Temperature Assemblage. In the Warn Oligotrophic-water Assemblage, common OPG accompanied abundant UPG in the upper photic-zone. The upper photic-zone of the Warm Eutrophic Assemblage consisted of UPG, MPG and OPG.Emiliania huxleyi and Gephyrocapsa oceanica, the major component of OPG, displayed intra-specific morphological variations. G. oceanica Type 1 was restricted to the upper photic-zone of well-stratified oligotrophic waters. Conversely, in these waters E. huxleyi Type C and G. oceanica Type 2 only occurred below the thermocline. These two taxa also coexist with G. oceanica Type 3 and E. huxleyi Type A in the upper photic zone of dynamic waters.     

Coccolithophores in the equatorial Atlantic Ocean: response to seasonal and Late Quaternary surface water variability

The present study was initiated to ascertain the significance of coccolithophores as a proxy for paleoceanographic and paleoproductivity studies in the equatorial Atlantic. Data from a range of different samples, from the plankton, surface sediments as well as sediment cores are shown and compared with each other.In general, the living coccolithophores in the surface and subsurface waters show considerable variation in cell numbers and distribution patterns. Cell densities reached a maximum of up to 300×10³ coccospheres/l in the upwelling area of the equatorial Atlantic. Here, Emiliania huxleyi is the dominant species with relatively high cell numbers, whereas Umbellosphaera irregularis and Umbellosphaera tenuis are characteristic for oligotrophic surface waters. Although they are observed in high relative abundances, these species only occur in low absolute numbers. The lower photic zone is dominated by high abundances and considerable cell numbers of Florisphaera profunda.The geographical distribution pattern of coccoliths in surface sediments reflects the conditions of the overlying surface water masses. However, abundances of the oligotrophic species Umbellosphaera irregularis and Umbellosphaera tenuis are strongly diminished, causing an increase in relative abundance of the lower photic zone taxa Florisphaera profunda and Gladiolithus flabellatus.During the past 140,000 years the surface water circulation of the equatorial Atlantic has changed drastically, as can be seen from changes in the coccolithophore species composition, absolute coccolith numbers, as well as coccolith accumulation rates. Significant increases in coccolith numbers and accumulation rates is observed in the southern equatorial Atlantic during the last glacial interval (oxygen isotope stages 2–4), which we attribute to enhanced upwelling intensities and advection of cool nutrient rich waters at this site. In the western equatorial Atlantic we observe an opposite trend with decreasing numbers of coccoliths during glacial periods, which probably is caused by a deepening of the thermocline.     

Effects of multiple drivers of ocean global change on the physiology and functional gene expression of the coccolithophore Emiliania huxleyi

Ongoing ocean global change due to anthropogenic activities is causing multiple chemical and physical seawater properties to change simultaneously, which may affect the physiology of marine phytoplankton. The coccolithophore Emiliania huxleyi is a model species often employed in the study of the marine carbon cycle. The effect of ocean acidification (OA) on coccolithophore calcification has been extensively studied; however, physiological responses to multiple environmental drivers are still largely unknown. Here we examined two‐way and multiple driver effects of OA and other key environmental drivers—nitrate, phosphate, irradiance, and temperature—on the growth, photosynthetic, and calcification rates, and the elemental composition of E. huxleyi. In addition, changes in functional gene expression were examined to understand the molecular mechanisms underpinning the physiological responses. The single driver manipulation experiments suggest decreased nitrate supply being the most important driver regulating E. huxleyi physiology, by significantly reducing the growth, photosynthetic, and calcification rates. In addition, the interaction of OA and decreased nitrate supply (projected for year 2100) had more negative synergistic effects on E. huxleyi physiology than all other two‐way factorial manipulations, suggesting a linkage between the single dominant driver (nitrate) effects and interactive effects with other drivers. Simultaneous manipulation of all five environmental drivers to the conditions of the projected year 2100 had the largest negative effects on most of the physiological metrics. Furthermore, functional genes associated with inorganic carbon acquisition (RubisCO, AEL1, and δCA) and calcification (CAX3, AEL1, PATP, and NhaA2) were most downregulated by the multiple driver manipulation, revealing linkages between responses of functional gene expression and associated physiological metrics. These findings together indicate that for more holistic projections of coccolithophore responses to future ocean global change, it is necessary to understand the relative importance of environmental drivers both individually (i.e., mechanistic understanding) and interactively (i.e., cumulative effect) on coccolithophore physiology.     

Gene expression changes in the coccolithophore Emiliania huxleyi after 500 generations of selection to ocean acidification

Coccolithophores are unicellular marine algae that produce biogenic calcite scales and substantially contribute to marine primary production and carbon export to the deep ocean. Ongoing ocean acidification particularly impairs calcifying organisms, mostly resulting in decreased growth and calcification. Recent studies revealed that the immediate physiological response in the coccolithophore Emiliania huxleyi to ocean acidification may be partially compensated by evolutionary adaptation, yet the underlying molecular mechanisms are currently unknown. Here, we report on the expression levels of 10 candidate genes putatively relevant to pH regulation, carbon transport, calcification and photosynthesis in E. huxleyi populations short-term exposed to ocean acidification conditions after acclimation (physiological response) and after 500 generations of high CO₂ adaptation (adaptive response). The physiological response revealed downregulation of candidate genes, well reflecting the concomitant decrease of growth and calcification. In the adaptive response, putative pH regulation and carbon transport genes were up-regulated, matching partial restoration of growth and calcification in high CO₂-adapted populations. Adaptation to ocean acidification in E. huxleyi likely involved improved cellular pH regulation, presumably indirectly affecting calcification. Adaptive evolution may thus have the potential to partially restore cellular pH regulatory capacity and thereby mitigate adverse effects of ocean acidification.     

Cloning and characterization of two genes encoding sulfate transporters from rice (Oryza sativa L.)*

Two genes were isolated from a rice genomic library and the coding region of their corresponding cDNAs generated by RT-PCR. These single copy genes, designated ORYsa;Sultr1;1 and ORYsa;Sultr4;1, encode putative sulfate transporters. Both genes encode proteins with predicted topologies and signature sequences of the H⁺/SO₄^2- symporter family of transporters and exhibit a high degree of homology to other plant sulfate transporters. ORYsa;Sultr1;1 is expressed in roots with levels of expression being strongly enhanced by sulfate starvation. In situ hybridization experiments revealed that ORYsa;Sultr1;1 expression is localized to the main absorptive region of roots. This gene probably encodes a transporter that is responsible for uptake of sulfate from the soil solution. In contrast, ORYsa;Sultr4;1 was expressed in both roots and shoots and was unresponsive to the sulfur status of the plant. The sequence of ORYsa;Sultr4;1 contains a possible plastid-targeting transit peptide which may indicate a role in transport of sulfate to sites of sulfate reduction in plastids. The role of the transporter encoded by ORYsa;Sultr4;1 is likely to be significantly different fromORYsa;Sultr1;1. These are the first reports of isolation of genes encoding sulfate transporters from rice and provide a basis for further studies involving sulfate transport.     

Long‐term evolutionary and ecological responses of calcifying phytoplankton to changes in atmospheric CO2

Calcifying phytoplankton play an important role in marine ecosystems and global biogeochemical cycles, affecting the transfer of both organic and inorganic carbon from the surface to the deep ocean. Coccolithophores are the most prominent members of this group, being well adapted to low‐nutrients environments (e.g., subtropical gyres). Despite urgent concerns, their response to rising atmospheric carbon dioxide levels (pCO₂) and ocean acidification is still poorly understood, and short‐term experiments may not extrapolate into longer‐term climatic adaptation. Current atmospheric pCO₂ (~390 ppmv) is unprecedented since at least 3 million years ago (Ma), and levels projected for the next century were last seen more than 34 Ma. Hence, a deep‐time perspective is needed to understand the long‐term effects of high pCO₂ on the biosphere. Here we combine a comprehensive fossil data set on coccolithophore cell size with a novel measure of ecological prominence: Summed Common Species Occurrence Rate (SCOR). The SCOR is decoupled from species richness, and captures changes in the extent to which coccolithophores were common and widespread, based on global occurrences in deep‐sea sediments. The size and SCOR records are compared to state‐of‐the‐art data on climatic and environmental changes from 50 to 5 Ma. We advance beyond simple correlations and trends to quantify the relative strength and directionality of information transfer among these records. Coccolithophores were globally more common and widespread, larger, and more heavily calcified in the pre‐34 Ma greenhouse world, and declined along with pCO₂ during the Oligocene (34–23 Ma). Our results suggest that atmospheric pCO₂ has exerted an important long‐term control on coccolithophores, directly through its availability for photosynthesis or indirectly via weathering supply of resources for growth and calcification.     

Distribution of living coccolithophores in the California Current system,southern California borderland

The distribution of living coccolithophores in the California Current system of southern California at 10 m water depth was investigated on two dates in March and June, 1982. Six closely spaced stations were sampled in March, of which three were resampled in June. Thirty-six euphotic species were identified of which four,Emiliania huxleyi, Umbilicosphaera sibogae, Gephyrocapsa oceanica, andRhabdosphaera longistylis, respectively, were the most abundant. Both the “cold” and “warm” morphotypes ofE. huxleyi were present, in varying proportions. Large ranges in community structure, diversity (0.35–2.64 natural bels), and standing crop (1.0 × 10⁴–6.2 × 10⁵cells/l) were recorded. This range of end-member values is approximately that found in the open ocean from 0° to about 65° latitude.The distributions of four coccolithophore assemblages recognized in March samples from the Borderland area appear to reflect the following distinct water masses: (1) California Current; (2) Southern California Counter Current; (3) Transitional Zone; (4) Near Shore. The coccolithophore assemblages from the June stations were more uniform, indicating that the Borderland was experiencing more stable conditions than in March.     

Effects of elevated CO2 on growth,calcification, and spectral dependence of photoinhibition in the coccolithophore Emiliania huxleyi (Prymnesiophyceae)1

We studied the effects of elevated CO₂ concentrations on cell growth, calcification, and spectral variation in the sensitivity of photosynthesis to inhibition by solar radiation in the globally important coccolithophore Emiliania huxleyi. Growth rates and chlorophyll a content per cell showed no significant differences between elevated (800 ppmv) and ambient (400 ppmv) CO₂ conditions. However, the production of organic carbon and the cell quotas for both carbon and nitrogen, increased under elevated CO₂ conditions, whilst particulate inorganic carbon production rates decreased under the same conditions. Biometric analyses of cells showed that coccoliths only presented significant differences due to treatments in the central area width. Most importantly, the size of the coccosphere decreased under elevated CO₂ conditions. The susceptibility of photosynthesis to inhibition by ultraviolet radiation (UVR) was estimated using biological weighting functions (BWFs) and a model that predicts photosynthesis under photosynthetically active radiation and UVR exposures. BWF results demonstrated that the sensitivity of photosynthesis to UVR was not significantly different between E. huxleyi cells grown under elevated and present CO₂ concentrations. We propose that the acclimation to elevated CO₂ conditions involves a physiological mechanism of regulation and allocation of energy and metabolites in the cell, which is also responsible for altering the sensitivity to UVR. In coccolithophores, this mechanism might be affected by the decrease in the calcification rates.     

Interaction of the coccolithophore Gephyrocapsa oceanica with its carbon environment: response to a recreated high-CO2 geological past

Coccolithophores have played a key role in the carbon cycle since becoming dominant in the Cretaceous ocean, and their influence depends fundamentally on how they interact with their external carbon environment. Because the photosynthetic carbon-fixing enzyme Rubisco requires high levels of CO(2) for effective catalysis, coccolithophores are known to induce carbon concentrating mechanisms (CCMs) to raise the level of dissolved inorganic carbon (DIC) in an 'internal pool'. The ocean carbon system has varied greatly over the geological past, suggesting that coccolithophore interactions with that external carbon environment will have changed in parallel. The widespread present-day coccolithophore Gephyrocapsa oceanica was acclimated here to a geological scale change in the seawater carbon system (five times higher DIC and alkalinity). Significant acclimation in response to the external carbon environment was demonstrated by a fourfold increase in the K(m) substrate concentration requirement for half-maximum photosynthetic carbon fixation rates (suggesting that CCMs were down-regulated when ambient carbon was more available). There was, however, no difference in growth rate, morphology or calcification, suggesting that calcification is not coupled to photosynthesis as one of the CCMs induced here and that productivity (growth rate and calcification) is not carbon-limited under representative present-day conditions. Beyond the kinetic parameters of photosynthesis, the only other indication of changed cell physiology seen was the increased fractionation of carbon isotopes into organic matter. These findings demonstrate that G. oceanica changes its carbon-use physiology to maintain consistent photosynthetic carbon fixation in concert with different levels of ambient DIC without changing its morphology or calcification.     

INTRASPECIFIC GENETIC DIVERSITY IN THE MARINE COCCOLITHOPHORE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE): THE USE OF MICROSATELLITE ANALYSIS IN MARINE PHYTOPLANKTON POPULATION STUDIES1

Using primer pairs for seven previously described microsatellite loci and three newly characterized microsatellite loci from the coccolithophore Emiliania huxleyi (Lohm.) Hay and Mohler, we assessed genetic variation within this species. Analysis of microsatellite length variants (alleles) was conducted for 85 E. huxleyi isolates representative of different ocean basins. These results revealed high intraspecific genetic variability within the E. huxleyi species concept. Pairwise comparison of a 1992 Coastal Fjord group (FJ92) (n=41) and a North East Atlantic (NEA) group (n=21), using F_ST as an indicator of genetic differentiation, revealed moderate genetic differentiation (F_ST=0.09894; P=0; significance level=0.05). Gene flow between the FJ92 and NEA groups was estimated to be low, which is in agreement with the moderate levels of genetic differentiation revealed by the microsatellite data. A genetic assignment method that uses genotype likelihoods to draw inference about the groups to which individuals belong was tested. Using FJ92 and NEA as reference groups, we observed that all the E. huxleyi groups tested against the two reference groups were unrelated to them. On a global biogeographical scale, E. huxleyi populations appear to be highly genetically diverse. Our findings raise the question of whether such a high degree of intraspecific genetic diversity in coccolithophores translates into variability in ecological function.     

Morphological variation in the deep ocean-dwelling coccolithophore Florisphaera profunda (Haptophyta)

Detailed analysis of the morphology of Florisphaera profunda from plankton samples collected at three sites in the Atlantic and Pacific Oceans reveals wide variation in this deep ocean-dwelling coccolithophore. In addition to the two varieties described previously, we found a third distinctive form, Florisphaera profunda var. rhinocera var. nov. All three varieties occur at each of the sampling sites. The analysis of monthly samples from different levels in the lower photic zone (LPZ) (100–200?m) at the Hawaii Ocean Time series station suggests that the varieties have similar distributions, which are correlated to primary productivity and the availability of light. The analysis of coccolith and coccosphere size in F. profunda reveals the existence of several size modes in Florisphaera profunda var. profunda and F. profunda var. elongata. The biological significance of these modes, or morphotypes is not known. However, their co-occurrence in single samples from different oceanic areas suggests that they are not ecophenotypes. In the light of recent molecular genetic analyses of intraspecific groups within commonly occurring coccolithophores, the varieties and size morphotypes of F. profunda are of significant interest for the study of marine phytoplankton biodiversity. Coccolithophores inhabiting the LPZ may be adapted to the low light, high nutrient conditions of this layer and hold great potential as a means to reconstruct past oceanographic conditions such as the position of the nutricline. However, coccolithophore biodiversity in the LPZ is poorly documented and the number of species may be much higher than previously thought.