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The analysis of water samples collected during the MD218 CROTALE expedition (February-March, 2019), and the 4th Indian Southern Ocean Expedition (SOE) (January-February, 2010) (re-visited samples) resulted in the occurrence of a new coccolithophore species belonging to the genus Alisphaera, and is named Alisphaera crostae sp. nov. This new species appears monomorphic and shows distinctive morphological differences in terms of its coccolith size, flange structure, and nodules, compared to other known Alisphaera species. In this study, morphological differences between A. crostae and other Alisphaera species are illustrated and compared. We suggest that the genus Alisphaera may have a higher diversity and may possess a wider distribution than previously evidenced, which requires more careful observation [ABSTRACT FROM AUTHOR]

Introduction: The stable carbon isotope ratio of long-chain alkenones produced by marine haptophyte phytoplankton has often been used to estimate past variations in atmospheric CO2 throughout the Cenozoic. However, previous experimental studies and surveys of alkenones from surface sediment and suspended particulate matter document additional environmental and physiological influences on carbon isotopic fractionation in alkenones. Methods: To clarify the non-CO2 effects on the alkenone carbon isotope fractionations, an important alkenone producer, Gephyrocapsa oceanica, was cultured in laboratory. To separate effects of different environment parameters, G. oceanica was grown in continuous cultures under a matrix of environmental conditions in order to explore the influence of temperature independently of CO2(aq). Through careful manipulation of the media carbon system, we can control the variation of the media CO2(aq) independently of temperature solubility. Carbon isotope fractionations from alkenones, coccolith, and particulate organic carbon were measured from this steady state system. Results and Discussion: We find εp in alkenones and particulate organic carbon inversely correlates with temperature, and temperature affects εp more strongly than CO2(aq). The magnitude of the temperature effect can be explained by higher growth rates at warmer temperatures with a similar growth rate dependence as observed in previous cultures in which growth rate was regulated by other factors. Where the past temperature influence on growth rate could be constrained using the UK37 alkenone index in the same samples, our finding offers an approach to deconvolve an important physiological factor affecting ancient alkenones ep, and may therefore improve past pCO2 estimates. [ABSTRACT FROM AUTHOR]

Hydrogen isotope ratios (δ²H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of ²H/¹H fractionation in algal lipids by systematically manipulating temperature, light, and CO2(aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica. We analyze the hydrogen isotope fractionation in alkenones (αalkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the αalkenone with increasing CO2 (aq) and confirm αalkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ²H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with ²H-richer intracellular water, increasing αalkenone. Higher chloroplast CO2(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of αalkenone to CO2(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO2 at the Rubisco site, but rather that chloroplast CO2 varies with external CO2(aq). The pervasive inverse correlation of αalkenone with CO2(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, αalkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis. [ABSTRACT FROM AUTHOR]

Simple Summary: The nutrient strategy is a significant part of Gephyrocapsa huxleyi's physiology. Unlike other studies mainly focusing on photosynthesis, we conducted a short-term incubation to test the difference in phagocytosis capacity between diploid calcified G. huxleyi RCC1266 and haploid non-calcified G. huxleyi PML B92/11 in different light regimes. The results suggested that a higher percentage has been found in the non-calcified strain as compared to the calcified strain, no matter whether it was under light or in the dark. Additionally, the non-calcified strain showed a greater percentage in the dark than under light. Coccolithophores play a significant role in marine calcium carbonate production and carbon cycles, attributing to their unique feature of producing calcareous plates, coccoliths. Coccolithophores also possess a haplo-diplontic life cycle, presenting distinct morphology types and calcification states. However, differences in nutrient acquisition strategies and mixotrophic behaviors of the two life phases remain unclear. In this study, we conducted a series of phagocytosis experiments of calcified diploid and non-calcified haploid strains of coccolithophore Gephyrocapsa huxleyi under light and dark conditions. The phagocytosis capability of each strain was examined based on characteristic fluorescent signals from ingested beads using flow cytometry and fluorescence microscopy. The results show a significantly higher phagocytosis percentage on fluorescent beads in the bacterial prey surrogates of the non-calcified haploid Gephyrocapsa huxleyi strain, than the calcified diploid strain with or without light. In addition, the non-calcified diploid cells seemingly to presented a much higher phagocytosis percentage in darkness than under light. The differential phagocytosis capacities between the calcified diploid and non-calcified haploid Gephyrocapsa huxleyi strains indicate potential distinct nutritional strategies at different coccolithophore life and calcifying stages, which may further shed light on the potential strategies that coccolithophore possesses in unfavorable environments such as twilight zones and the expanding coccolithophore niches in the natural marine environment under the climate change scenario. [ABSTRACT FROM AUTHOR]

The southern waters of the Sumatra‐Java Islands, within the Indo‐Pacific Warm Pool, are highly susceptible to global climate change impacts. Understanding particulate inorganic carbon (PIC) dynamics and environmental responses in this region is critical for assessing climate change effects on marine ecosystems. Utilizing two decades of remote sensing PIC data (2003–2022), we identified five sub‐regions via K‐means clustering: the Sunda Strait, southern coastal/offshore waters of Sumatra Island, and southern coastal/offshore waters of Java Island. Self‐Organizing Map explored interannual PIC variations, while Generalized Additive Models delineated driving factors. In the Sunda Strait, PIC dynamics are influenced by water exchanges between the Indonesian Seas (ISs) and eastern Indian Ocean (EIO). Seasonally, PIC concentrations peak during the east monsoon (boreal summer); decreasing during positive Indian Ocean Dipole (+IOD) and El Niño events. Along the southern Sumatra, PIC variations positively correlate with rainfall and photosynthetically active radiation (PAR). Along the southern Sumatra, upwelling drives PIC variations, with higher concentrations during mature summer monsoon induced upwelling periods. PIC increases during +IOD and decreases during El Niño. Our study highlights the nexus between nutrients, physical factors (e.g., rainfall), and climatic events (e.g., El Niño) on PIC concentrations. Climate‐induced changes in oceanic physical processes modulate nutrient concentrations, thereby governing PIC variations. We propose a mechanistic model to elucidate PIC variations in upwelling areas. Findings suggest that prospective climate variability, encompassing alterations in climate events, rainfall, and temperature, may escalate PIC concentrations within the study area, advocating PIC as a forward‐looking climate change indicator. Plain Language Summary: Understanding the response of particulate inorganic carbon (PIC)—a primary form of oceanic carbon ‐ to environmental changes in the vulnerable southern waters of the Sumatra‐Java Islands is crucial for assessing the marine ecosystem. We used satellite image data to examine the variability of PIC, from 2003 to 2022. Our research showed that PIC concentrations varied across the area. In the Sunda Strait, a narrow body of water between land masses, water movement between the Indonesian Seas and eastern Indian Ocean influences PIC concentrations. In the southern waters of Sumatra Island, we discovered that increased rainfall and sunlight contribute to higher PIC concentrations. In the southern waters of Java Island, upwelling, an ocean physical process that brings nutrient‐rich water to the surface, plays a significant role. Our study delineates the interplay between nutrients, physical factors (e.g., rainfall), and climatic events (e.g., El Niño) on coccolithophore biomass. Climate changes drive alterations in oceanic physical processes, which in turn affect nutrient availability, thereby governing the growth of coccolithophores. Findings suggest prospective climate variability, inclusive of alterations in climate events, rainfall, and temperature, may escalate PIC concentrations within the study area, advocating PIC concentrations alterations as a forward‐looking climate change indicator. Key Points: Climate alters oceanic physical processes, thereby modulating nutrient availability, which in turn directly governs Particulate Inorganic Carbon (PIC) variationsA 20‐year analysis identified three core mechanisms driving PIC spatiotemporal variations in the study areaRising PIC under future climates predicts higher carbon sedimentation rates, marking PIC as a key climate indicator [ABSTRACT FROM AUTHOR]

Introduction: The stable carbon isotope ratio of long-chain alkenones produced by marine haptophyte phytoplankton has often been used to estimate past variations in atmospheric CO2 throughout the Cenozoic. However, previous experimental studies and surveys of alkenones from surface sediment and suspended particulate matter document additional environmental and physiological influences on carbon isotopic fractionation in alkenones.Methods: To clarify the non-CO2 effects on the alkenone carbon isotope fractionations, an important alkenone producer, Gephyrocapsa oceanica, was cultured in laboratory. To separate effects of different environment parameters, G. oceanica was grown in continuous cultures under a matrix of environmental conditions in order to explore the influence of temperature independently of CO2(aq). Through careful manipulation of the media carbon system, we can control the variation of the media CO2(aq) independently of temperature solubility. Carbon isotope fractionations from alkenones, coccolith, and particulate organic carbon were measured from this steady state system.Results and Discussion: We find εp in alkenones and particulate organic carbon inversely correlates with temperature, and temperature affects εp more strongly than CO2(aq). The magnitude of the temperature effect can be explained by higher growth rates at warmer temperatures with a similar growth rate dependence as observed in previous cultures in which growth rate was regulated by other factors. Where the past temperature influence on growth rate could be constrained using the UK’37 alkenone index in the same samples, our finding offers an approach to deconvolve an important physiological factor affecting ancient alkenones εp, and may therefore improve past pCO2 estimates.

We examined the physiological performance in the most cosmopolitan coccolithophorid, Emiliania huxleyi and Gephyrocapsa oceanica, which were treated with 8.3 (ambient O2 (AO)), 4.6 (medium O2 (MO)), and 2.5 (low O2 (LO)) mg L–1 O2 under 400 ppm (ambient CO2 (AC)) and 1000 (high CO2 (HC)) ppm CO2 conditions. Elevated CO2 decreased the specific growth rate of cells cultured under AO and LO conditions in both species, but it increased the rate in the MO-grown E. huxleyi. Regardless of the CO2 levels, diminished O2 concentration inhibited the growth rate in E. huxleyi while accelerating the rate in G. oceanica. LO reduced the particulate organic carbon (POC) production rate compared to the AO treatment in both species. Additionally, the decrease was higher in the HC cultures than in the AC ones. LO also inhibited the production rate of particulate inorganic carbon (PIC) compared to the AO/AC treatment. Due to a higher reduction in the production rate of PIC than POC, the PIC/POC ratio was decreased in the LO treatment compared to the AO/AC treatment. The current study reveals that low O2 can, individually or in combination with high CO2, considerably affect the physiology of marine photoautotrophic organisms. [ABSTRACT FROM AUTHOR]

Coccolithophores play a significant role in marine calcium carbonate production and carbon cycles, attributing to their unique feature of producing calcareous plates, coccoliths. Coccolithophores also possess a haplo-diplontic life cycle, presenting distinct morphology types and calcification states. However, differences in nutrient acquisition strategies and mixotrophic behaviors of the two life phases remain unclear. In this study, we conducted a series of phagocytosis experiments of calcified diploid and non-calcified haploid strains of coccolithophore Gephyrocapsa huxleyi under light and dark conditions. The phagocytosis capability of each strain was examined based on characteristic fluorescent signals from ingested beads using flow cytometry and fluorescence microscopy. The results show a significantly higher phagocytosis percentage on fluorescent beads in the bacterial prey surrogates of the non-calcified haploid Gephyrocapsa huxleyi strain, than the calcified diploid strain with or without light. In addition, the non-calcified diploid cells seemingly to presented a much higher phagocytosis percentage in darkness than under light. The differential phagocytosis capacities between the calcified diploid and non-calcified haploid Gephyrocapsa huxleyi strains indicate potential distinct nutritional strategies at different coccolithophore life and calcifying stages, which may further shed light on the potential strategies that coccolithophore possesses in unfavorable environments such as twilight zones and the expanding coccolithophore niches in the natural marine environment under the climate change scenario.

The elemental composition of marine microorganisms (their C:N:P ratio, or stoichiometry) is central to understanding the biotic and biogeochemical processes underlying key marine ecosystem functions. Phytoplankton C:N:P is species specific and flexible to changing environmental conditions. However, bulk or fixed phytoplankton stoichiometry is usually assumed in biogeochemical and ecological models because more realistic, environmentally responsive C:N:P ratios have yet to be defined for key functional groups. Here, a comprehensive meta‐analysis of experimental laboratory data reveals the variable C:N:P stoichiometry of Emiliania huxleyi, a globally significant calcifying phytoplankton species. Mean C:N:P of E. huxleyi is 124C:16N:1P under control conditions (i.e. growth not limited by one or more environmental stressors) and shows a range of responses to changes in nutrient and light availability, temperature and pCO2. Macronutrient limitation caused strong shifts in stoichiometry, increasing N:P and C:P under P deficiency (by 305% and 493% respectively) and doubling C:N under N deficiency. Responses to light, temperature and pCO2 were mixed but typically shifted cellular elemental content and C:N:P stoichiometry by ca. 30% or less. Besides these independent effects, the interactive effects of multiple environmental changes on E. huxleyi stoichiometry under future ocean conditions could be additive, synergistic or antagonistic. To synthesise our meta‐analysis results, we explored how the cellular elemental content and C:N:P stoichiometry of E. huxleyi may respond to two hypothetical future ocean scenarios (increased temperature, irradiance and pCO2 combined with either N deficiency or P deficiency) if an additive effect is assumed. Both future scenarios indicate decreased calcification (which is predominantly sensitive to elevated pCO2), increased C:N, and up to fourfold shifts in C:P and N:P. Our results strongly suggest that climate change will significantly alter the role of E. huxleyi (and potentially other calcifying phytoplankton species) in marine biogeochemical processes. [ABSTRACT FROM AUTHOR]

The global climate of the early Oligocene was characterized by initiated Antarctic glaciation and meridional overturning circulation, which then led to profound eutrophication in the upper ocean. Generating a high‐resolution coccolith record helps to understand the responses of marine phytoplankton to the newly established environment. Using highly resolved (∼6 kyr time‐resolution) marine sediment samples from Deep Sea Drilling Project Site 522 in the South Atlantic Ocean, we conducted a comprehensive morphological study on coccoliths from the genera Reticulofenestra, Dictyococcites, and Coccolithus, which dominated the study interval between ∼33.1 and 32.8 Ma. Our results showed that the size variations of the three measured genera were significantly correlated (p < 0.01) with each other, indicating homogeneous responses to the environmental changes. Moreover, spectrum analysis on integrated morphologic data of all measured coccoliths showed distinct obliquity (∼40‐kyr) and precession (∼23‐kyr and ∼18‐kyr) cycles. We suggest that these variations were mainly driven by temperate, short‐term ecological fluctuations, which periodically altered the nutrient conditions in the common living habitats of the studied coccolithophores. We proposed two tentative explanations focusing on the obliquity signal. First, the cyclic variation could result from obliquity‐modulated changes in ice volume and variations in ocean circulation intensity, which influenced nutrient export from deep waters to the upper ocean. Alternatively, the changes in coccolith size may indicate the strength of seasonality that influenced upper ocean mixing on the west coast of South Africa. Plain Language Summary: Phytoplankton sizes yield important information to understand changes in marine ecosystems. Coccoliths, the fossil remains of unicellular algae coccolithophores, are preserved in marine sediments and thus are ideal for recording phytoplankton size changes in the past. We present a high‐resolution coccolith record spanning a period of ∼300,000 years in the early Oligocene (∼33 million years ago). Our data showed changes in coccolithophore sizes, corresponding to Earth's orbital cycles. We propose that the increased size of coccolithophore corresponded to nutrient‐rich conditions. Strengthened ocean circulation or enhanced wind‐driven mixing that is related to Earth's orbital cycles are two possible driving mechanisms for the nutrient increase in the upper ocean. Our study provides direct evidence of Earth's orbital forcings on the marine ecosystem in the early Oligocene when Antarctica started to be covered by a continental ice sheet. Key Points: This study presents a high‐resolution morphological record of early Oligocene coccolithophores in the South AtlanticThree measured coccolithophore genera showed homogenous responses to short‐term ecological fluctuationsOcean circulation and seasonality modulated by orbital forcing might have controlled the nutrient availability in the upper ocean [ABSTRACT FROM AUTHOR]

Abstract The global climate of the early Oligocene was characterized by initiated Antarctic glaciation and meridional overturning circulation, which then led to profound eutrophication in the upper ocean. Generating a high‐resolution coccolith record helps to understand the responses of marine phytoplankton to the newly established environment. Using highly resolved (∼6 kyr time‐resolution) marine sediment samples from Deep Sea Drilling Project Site 522 in the South Atlantic Ocean, we conducted a comprehensive morphological study on coccoliths from the genera Reticulofenestra, Dictyococcites, and Coccolithus, which dominated the study interval between ∼33.1 and 32.8 Ma. Our results showed that the size variations of the three measured genera were significantly correlated (p

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. [ABSTRACT FROM AUTHOR]

Coccolithophores feature a haplo-diplontic life cycle comprised of diploid cells producing heterococcoliths and haploid cells producing morphologically different holococcoliths. These life cycle phases of each species appear to have distinct spatial and temporal distributions in the oceans, with the heavily calcified heterococcolithophores (HET) often more prevalent in winter and at greater depths, whilst the lightly calcified holococcolithophores (HOL) are more abundant in summer and in shallower waters. The haplo-diplontic life cycle may therefore allow coccolithophores to expand their ecological niche, switching between life cycle phases to exploit conditions that are more favourable. However, coccolithophore life cycles remain poorly understood and fundamental information on the physiological differences between life cycle phases is required if we are to better understand the ecophysiology of coccolithophores. In this study, we have examined the physiology of HET and HOL phases of the coccolithophore Coccolithus braarudii in response to changes in light and nutrient availability. We found that the HOL phase was more tolerant to high light than the HET phase, which exhibited defects in calcification at high irradiances. The HET phase exhibited defects in coccolith formation under both nitrate (N) and phosphate (P) limitation, whilst no defects in calcification were detected in the HOL phase. The HOL phase grew to a higher cell density under P-limitation than N-limitation, whereas no difference was observed in the maximum cell density reached by the HET phase at these nutrient concentrations. HET cells grown under a light:dark cycle divided primarily in the dark and early part of the light phase, whereas HOL cells continued to divide throughout the 24 h period. The physiological differences may contribute to the distinct biogeographic distributions observed between life cycle phases, with the HOL phase potentially better adapted to high light, low nutrient regimes, such as those found in seasonally stratified surface waters. Coccolithus braarudii life cycle phases exhibit different physiological responses. The heavily calcified heterococcolithophores (HET) life cycle phase is more sensitive to high light. The lightly calcified holococcolithophores (HOL) life cycle phase may be better suited to growth under low phosphate availability. [ABSTRACT FROM AUTHOR]

The possession of a carbon concentrating mechanism (CCM) by a given phytoplankton species will in part determine its response to changes in the dissolved inorganic carbon concentrations in the global ocean. Earlier work on CCMs in the coccolithophore Emiliania huxleyi suggested that this species did not possess a CCM, though more recent studies indicate that a CCM is indeed present. Here we report work showing that another coccolithophore species, Gephyrocapsa oceanica, also possesses a functioning CCM, implying that increasing CO2 concentrations are unlikely to lead to enhanced growth rates. We discuss the protocol required for working with this species, noting that due to its liths being 10 or more times as massive as of those of Emiliania huxleyi, much lower centrifugation speeds (<400 × g) must be used to avoid damage to the cells. [ABSTRACT FROM AUTHOR]

Marine microbes form host-associated biofilm communities that are shaped by complex interactions between bacteria and their host. The roseobacter Phaeobacter inhibens exploits both symbiotic and pathogenic niches while interacting with its microalgal host Emiliania huxleyi. During co-cultivation over extended periods with E. huxleyi, we show that P. inhibens selectively kills two host cell types, the diploid calcifying strain and the haploid flagellated strain. Meanwhile, various non-calcifying diploid strains are resistant to this pathogen or the pathogen is avirulent to this cell type. This differential pathogenesis has the potential of dramatically altering the composition of E. huxleyi blooms, which are typically dominated by the roseobacter-susceptible calcifying strain. This cell type makes calcite plates, which are an important sink in the marine carbon cycle and forms part of the marine paleobotanic record. P. inhibens kills the haploid cells, which have been proposed as critical to the survival of the algae, as they readily escape both eukaryotic predation and viral infection. Consequently, bacteria such as P. inhibens could influence E. huxleyi's life history by selective pathogenesis, thereby altering the composition of cell types within E. huxleyi populations and its bloom-bust lifestyle.

The relationship between temperature (T) and diversity is one of the most important issues in ecology. It provides a key direction not only for exploring the determinants of diversity's patterns, but also for understanding diversity's responses to climate change. Previous studies suggested that T–diversity relationships could be positive, negative, or unimodal. Although these studies accumulated many informative achievements, they might be unsatisfied due to (1) investigating inadequate range of T, (2) selecting incomplete diversity metrics, and (3) making insufficiently detailed analysis of correlation. In this study, species diversity is estimated by four most commonly used diversity metrics and three parameters of species abundance distribution (SAD), and two global datasets of marine phytoplankton (covering a wider range of T) are used to evaluate the T–diversity relationships according to a piecewise model. Results show that all aspects of diversity (except evenness) have the similar relationship with T in the range of lower T, noting that diversity significantly increases as T increases. However, in the range of higher T, diversity may significantly decrease or nearly constant, which indicates that their relationships may be the unimodal or asymptotic. The asymptotic relationship found by this study is assumed that increasing diversity with T will gradually approach the Zipf's law (1:1/2:1/3...). If such assumption can be verified by future investigations, the intrinsic mechanism of the asymptotic relationship is likely to be crucial in understanding the T–diversity relationships. [ABSTRACT FROM AUTHOR]

The effect of light, copper ions, copper oxide nanoparticles on the change in the structural, functional, cytometric, fluorescent parameters of coccolithophore Pleurochrysis sp. was investigated. The culture Pleurochrysis sp. was represented by two cell forms: (1) covered with coccoliths; and (2) not covered, the ratio of which depends from growth conditions. An increase in light from 20 to 650 μE m−2 s−1 led to a decrease in the concentration of cells covered with coccoliths from 90 to 35%. With an increase in light, the decrease in the values of variable chlorophyll a fluorescence was observed, a decrease in the chlorophyll concentration was noted, and an increase in cell volumes and their granularity due to coccoliths 'overproduction' was recorded. A tolerance of Pleurochrysis sp. to the effect of copper was registered, both in the ionic form and in the form of a nanopowder. This is probably due to the morphological (presence of coccoliths) and physiological (ligand production) peculiarities of species. Copper did not affect the ratio of cells covered with coccoliths; its value was about 85%. Growth inhibition, a 2-fold decrease in the intracellular chlorophyll content, a decrease in F v/ F m, and a pronounced cell coagulation were recorded at the maximum Cu2+ concentration (625 μg L−1). The mechanical effect was registered of CuO nanoparticles on the surface of Pleurochrysis sp. coccosphere, which results in the emergence of destroyed and deformed coccoliths. A hypothesis is proposed considering the protective function of coccoliths acting as a barrier when the cells are exposed to nanoparticles and copper ions. This study revealed new aspects of Pleurochrysis sp. physiology, a coccolithophore that causes massive 'blooms', affecting the circulation of biogenic and mineral substances in the marine environment. Two main conclusions are: (1) measure of coccolithophore cells abundance in the sea in summer based on satellite information gives underestimated data relative to real values as only cells covered with coccoliths are registered; and (2) the morphophysiological peculiarities of coccolithophores allow them to vegetate in waters under high anthropogenic load. [ABSTRACT FROM AUTHOR]

Phytoplankton stoichiometry and cell size could result from both phenology and environmental change. Zooplankton graze on primary producers, and this drives both the balance of the ecosystem and the biogeochemical cycles. In this study, we performed incubations with copepods and coccolithophores including different prey sizes and particulate carbon contents by considering phytoplankton biovolume concentration instead of chlorophyll a level (Chl a) as is usually performed in such studies. The egestion of fecal pellet and ingestion rates were estimated based on a gut fluorescence method. The latter was calibrated through the relationship between prey Chl a level and the biovolume of the cell. Chl a/biovolume ratio in phytopkanton has to be considered in the copepod gut fluorescent content method. Both coccolithophore biovolume and particulate inorganic/organic carbon ratios affect the food foraging by copepods. Finally, we observed a non-linear relationship between ingestion rates and fecal pellet egestion, due to the presence of calcite inside the copepod's gut. These results illustrate that both prey size and stoichiometry need to be considered in copepod feeding dynamics, specifically regarding the process leading to the formation of fecal pellets. [ABSTRACT FROM AUTHOR]

Although numerous studies have examined the responses of coccolithophores to ocean acidification, less is known on the fate of those calcifying organisms when they sink to the ocean's aphotic regions. In this study, the coccolithophore Emiliania huxleyi was first grown under a regular 12/12 light/dark cycle at 20°C, exposed to both high (1000 μatm) and ambient CO2 (410 μatm) levels. The cultures were then transferred to continuous darkness for 96 h at 20°C or 16°C. We found that elevated CO2 decreased the specific growth rate while increasing the cellular particulate organic carbon (POC) and nitrogen (PON) contents and the POC/PON ratio of E. huxleyi in the light/dark period. After 96 h of dark acclimation, the cell abundance decreased more obviously at 20°C than at 16°C but showed no significant difference between the two CO2 treatments. The decrease in volumetric POC concentration was most prominent in the high CO2/20°C treatment and least in the ambient CO2/16°C treatment. At 16°C, the PON concentration increased in the high CO2 cultures and exhibited no change in the ambient CO2 cultures. While at 20°C, the PON concentration decreased significantly both under high and ambient CO2 conditions. The final POC/PON ratio showed no significant differences among the different temperature and CO2 treatments. Overall, a higher percentage of POC relative to that of PON was lost in darkness with increasing CO2 concentration, with potential implications for the ocean's nutrient cycle. [ABSTRACT FROM AUTHOR]

Global climate change leads to simultaneous changes in multiple environmental drivers in the marine realm. Although physiological characterization of coccolithophores have been studied under climate change, there is limited knowledge on the biochemical responses of this biogeochemically important phytoplankton group to changing multiple environmental drivers. Here we investigate the interactive effects of reduced phosphorus availability (4 to 0.4 μmol L-1), elevated pCO2 concentrations (426 to 946 μatm) and increasing light intensity (40 to 300 μmol photons m-2 s-1) on elemental content and macromolecules of the cosmopolitan coccolithophore Emiliania huxleyi. Reduced phosphorus availability reduces particulate organic nitrogen and protein contents under low light intensity, but not under high light intensity. Reduced phosphorus availability and ocean acidification act synergistically to increase particulate organic carbon (POC) and carbohydrate contents under high light intensity but not under low light intensity. Reduced phosphorus availability, ocean acidification and increasing light intensity act synergistically to increase the allocation of POC to carbohydrates. Under future ocean acidification and increasing light intensity, enhanced carbon fixation could increase carbon storage in the phosphorus-limited regions of the oceans where E. huxleyi dominates the phytoplankton assemblages. In each light intensity, elemental carbon to phosphorus (C : P) and nitrogen to phosphorus (N : P) ratios decrease with increasing growth rate. These results suggest that coccolithophores could reallocate chemical elements and energy to synthesize macromolecules efficiently, which allows them to regulate its elemental content and growth rate to acclimate to changing environmental conditions. [ABSTRACT FROM AUTHOR]

The marine microalga Emiliania huxleyi is widely distributed in the surface oceans and is prone to infection by coccolithoviruses that can terminate its blooms. However, little is known about how global change factors like solar UV radiation (UVR) and ocean warming affect the host-virus interaction. We grew the microalga at 2 temperature levels with or without the virus in the presence or absence of UVR and investigated the physiological and transcriptional responses. We showed that viral infection noticeably reduced photosynthesis and growth of the alga but was less harmful to its physiology under conditions where UVR influenced viral DNA expression. In the virus-infected cells, the combination of UVR and warming (+4°C) led to a 13-fold increase in photosynthetic carbon fixation rate, with warming alone contributing a change of about 5-7-fold. This was attributed to upregulated expression of genes related to carboxylation and light-harvesting proteins under the influence of UVR, and to warming-reduced infectivity. In the absence of UVR, viral infection downregulated the metabolic pathways of photosynthesis and fatty acid degradation. Our results suggest that solar UV exposure in a warming ocean can reduce the severity of viral attack on this ecologically important microalga, potentially prolonging its blooms., (© 2024 John Wiley & Sons Ltd.)

Summary: Incongruent phylogenies have been widely observed between nuclear and plastid or mitochondrial genomes in terrestrial plants and animals. However, few studies have examined these patterns in microalgae or the discordance between the two organelles.Here we investigated the nuclear–mitochondrial–plastid phylogenomic incongruence in Emiliania–Gephyrocapsa, a group of cosmopolitan calcifying phytoplankton with enormous populations and recent speciations. We assembled mitochondrial and plastid genomes of 27 strains from across global oceans and temperature regimes, and analyzed the phylogenomic histories of the three compartments using concatenation and coalescence methods.Six major clades with varying morphology and distribution are well recognized in the nuclear phylogeny, but such relationships are absent in the mitochondrial and plastid phylogenies, which also differ substantially from each other. The rampant phylogenomic discordance is due to a combination of organellar capture (introgression), organellar genome recombination, and incomplete lineage sorting of ancient polymorphic organellar genomes. Hybridization can lead to replacements of whole organellar genomes without introgression of nuclear genes and the two organelles are not inherited as a single cytoplasmic unit.This study illustrates the convoluted evolution and inheritance of organellar genomes in isogamous haplodiplontic microalgae and provides a window into the phylogenomic complexity of marine unicellular eukaryotes. [ABSTRACT FROM AUTHOR]

Coccolithophores are major producers of ocean biogenic calcite, 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 voltagegated 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. [ABSTRACT FROM AUTHOR]

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 paleo‐temperature 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. [ABSTRACT FROM AUTHOR]

Phytoplankton are exposed to different concentrations of nutrients in different waters along with changing light levels during diurnal and seasonal cycles. We grew the coccolithophorid Gephyrocapsa oceanica semi-continuously at different nitrate levels under indoor low and outdoor high light conditions, and found that reduced nitrate availability significantly increased its production of particulate inorganic carbon (PIC), with its growth being reduced. High light treatment suppressed the growth of nitrate-limited cells and their efficiency of N assimilation by up to 63% compared to low light treatment. The combination of high light and nitrate limitation increased contents of PIC per cell due to sustained photochemical energy transfer, resulting in faster sinking rates by up to 82% in comparison with nitrate-repleted cells. Additionally, the sinking rates were positively correlated with ratios of PIC to particulate organic carbon (POC). These results imply that coccolithophores distributed in oligotrophic waters could be more effective as the ballast in aggregates, facilitating particulate organic carbon flux to deeper waters.

Carbon isotope records from alkenone biomarkers (εp37:2) produced by haptophyte algae are frequently used for atmospheric CO2 paleobarometry, but this method has yielded inconsistent results during periods where CO2 variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that εp37:2 indeed records CO2 information: εp37:2 increases as aqueous CO2 concentrations increase relative to carbon demand. However, interpretations of εp37:2 are complicated by irradiance, where higher irradiance yields higher εp37:2. Here we examine the roles of physiology and environment in setting εp37:2 in the ocean. We compile water‐column and sediment core‐top εp37:2 data and add new core‐top measurements, including estimates of cell sizes and growth rates of the alkenone‐producing population. In support of culture studies, we find irradiance to be a key control on εp37:2 in the modern ocean. We test a culture‐derived model of εp37:2 and find that the quantitative relationships calibrated in culture experiments can be used to predict εp37:2 in sediment samples. In water‐column samples, the model substantially overestimates εp37:2, largely resulting from higher irradiance at the depth of sample collection than the integrated light conditions under which the sampled biomass was produced and vertically mixed to the collection depth. We argue that the theory underpinning the conventional diffusive alkenone carbon isotope fractionation model, including the "b" parameter, is not supported by field data and should not be used to reconstruct past CO2 changes. Future estimates of CO2 from εp37:2 should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations. Key Points: Irradiance is a primary control on alkenone carbon isotope fractionation in the natural environmentCulture‐calibrated model can predict alkenone carbon isotope fractionation in core‐top sediment samplesDiffusive model "b" value is not correlated with growth rate: diffusive model should not be used for paleobarometry [ABSTRACT FROM AUTHOR]

Elemental contents change with shifts in macromolecular composition of marine phytoplankton. Recent studies focus on the responses of elemental contents of coccolithophores, a major calcifying phytoplankton group, to changing carbonate chemistry, caused by the dissolution of anthropogenically derived CO2 into the surface ocean. However, the effects of changing carbonate chemistry on biomacromolecules, such as protein and carbohydrate of coccolithophores, are less documented. Here, we disentangled the effects of elevated dissolved inorganic carbon (DIC) concentration (900 to 4,930μmolkg−1) and reduced pH value (8.04 to 7.70) on physiological rates, elemental contents, and macromolecules of the coccolithophore Emiliania huxleyi. Compared to present DIC concentration and pH value, combinations of high DIC concentration and low pH value (ocean acidification) significantly increased pigments content, particulate organic carbon (POC), and carbohydrate content and had less impact on growth rate, maximal relative electron transport rate (rETR max), particulate organic nitrogen (PON), and protein content. In high pH treatments, elevated DIC concentration significantly increased growth rate, pigments content, rETR max, POC, particulate inorganic carbon (PIC), protein, and carbohydrate contents. In low pH treatments, the extents of the increase in growth rate, pigments and carbohydrate content were reduced. Compared to high pH value, under low DIC concentration, low pH value significantly increased POC and PON contents and showed less impact on protein and carbohydrate contents; however, under high DIC concentration, low pH value significantly reduced POC, PON, protein, and carbohydrate contents. These results showed that reduced pH counteracted the positive effects of elevated DIC concentration on growth rate, rETR max, POC, PON, carbohydrate, and protein contents. Elevated DIC concentration and reduced pH acted synergistically to increase the contribution of carbohydrate–carbon to POC, and antagonistically to affect the contribution of protein–nitrogen to PON, which further shifted the carbon/nitrogen ratio of E. huxleyi. [ABSTRACT FROM AUTHOR]

Abstract Carbon isotope records from alkenone biomarkers (εp37:2) produced by haptophyte algae are frequently used for atmospheric CO2 paleobarometry, but this method has yielded inconsistent results during periods where CO2 variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that εp37:2 indeed records CO2 information: εp37:2 increases as aqueous CO2 concentrations increase relative to carbon demand. However, interpretations of εp37:2 are complicated by irradiance, where higher irradiance yields higher εp37:2. Here we examine the roles of physiology and environment in setting εp37:2 in the ocean. We compile water‐column and sediment core‐top εp37:2 data and add new core‐top measurements, including estimates of cell sizes and growth rates of the alkenone‐producing population. In support of culture studies, we find irradiance to be a key control on εp37:2 in the modern ocean. We test a culture‐derived model of εp37:2 and find that the quantitative relationships calibrated in culture experiments can be used to predict εp37:2 in sediment samples. In water‐column samples, the model substantially overestimates εp37:2, largely resulting from higher irradiance at the depth of sample collection than the integrated light conditions under which the sampled biomass was produced and vertically mixed to the collection depth. We argue that the theory underpinning the conventional diffusive alkenone carbon isotope fractionation model, including the “b” parameter, is not supported by field data and should not be used to reconstruct past CO2 changes. Future estimates of CO2 from εp37:2 should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations.

Elemental contents change with shifts in macromolecular composition of marine phytoplankton. Recent studies focus on the responses of elemental contents of coccolithophores, a major calcifying phytoplankton group, to changing carbonate chemistry, caused by the dissolution of anthropogenically derived CO2 into the surface ocean. However, the effects of changing carbonate chemistry on biomacromolecules, such as protein and carbohydrate of coccolithophores, are less documented. Here, we disentangled the effects of elevated dissolved inorganic carbon (DIC) concentration (900 to 4,930μmolkg−1) and reduced pH value (8.04 to 7.70) on physiological rates, elemental contents, and macromolecules of the coccolithophore Emiliania huxleyi. Compared to present DIC concentration and pH value, combinations of high DIC concentration and low pH value (ocean acidification) significantly increased pigments content, particulate organic carbon (POC), and carbohydrate content and had less impact on growth rate, maximal relative electron transport rate (rETRmax), particulate organic nitrogen (PON), and protein content. In high pH treatments, elevated DIC concentration significantly increased growth rate, pigments content, rETRmax, POC, particulate inorganic carbon (PIC), protein, and carbohydrate contents. In low pH treatments, the extents of the increase in growth rate, pigments and carbohydrate content were reduced. Compared to high pH value, under low DIC concentration, low pH value significantly increased POC and PON contents and showed less impact on protein and carbohydrate contents; however, under high DIC concentration, low pH value significantly reduced POC, PON, protein, and carbohydrate contents. These results showed that reduced pH counteracted the positive effects of elevated DIC concentration on growth rate, rETRmax, POC, PON, carbohydrate, and protein contents. Elevated DIC concentration and reduced pH acted synergistically to increase the contribution of carbohydrate–carbon to POC, and antagonistically to affect the contribution of protein–nitrogen to PON, which further shifted the carbon/nitrogen ratio of E. huxleyi.

Phytoplankton stoichiometry and cell size could result from both phenology and environmental change. Zooplankton graze on primary producers, and this drives both the balance of the ecosystem and the biogeochemical cycles. In this study, we performed incubations with copepods and coccolithophores including different prey sizes and particulate carbon contents by considering phytoplankton biovolume concentration instead of chlorophyll a level (Chl a) as is usually performed in such studies. The egestion of fecal pellet and ingestion rates were estimated based on a gut fluorescence method. The latter was calibrated through the relationship between prey Chl a level and the biovolume of the cell. Chl a/biovolume ratio in phytopkanton has to be considered in the copepod gut fluorescent content method. Both coccolithophore biovolume and particulate inorganic/organic carbon ratios affect the food foraging by copepods. Finally, we observed a non-linear relationship between ingestion rates and fecal pellet egestion, due to the presence of calcite inside the copepod’s gut. These results illustrate that both prey size and stoichiometry need to be considered in copepod feeding dynamics, specifically regarding the process leading to the formation of fecal pellets.

Abstract The carbon isotope fractionation in algal organic matter (εp), including the long‐chain alkenones produced by the coccolithophorid family Noelaerhabdaceae, is used to reconstruct past atmospheric CO2 levels. The conventional proxy linearly relates εp to changes in cellular carbon demand relative to diffusive CO2 supply, with larger εp values occurring at lower carbon demand relative to supply (i.e., abundant CO2). However, the response of Gephyrocapsa oceanica, one of the dominant alkenone producers of the last few million years, has not been studied closely. Here, we subject G. oceanica to various CO2 levels by increasing pCO2 in the culture headspace, as opposed to increasing dissolved inorganic carbon (DIC) and alkalinity concentrations at constant pH. We note no substantial change in physiology, but observe an increase in εp as carbon demand relative to supply decreases, consistent with DIC manipulations. We compile existing Noelaerhabdaceae εp data and show that the diffusive model poorly describes the data. A meta‐analysis of individual treatments (unique combinations of lab, strain, and light conditions) shows that the slope of the εp response depends on the light conditions and range of carbon demand relative to CO2 supply in the treatment, which is incompatible with the diffusive model. We model εp as a multilinear function of key physiological and environmental variables and find that both photoperiod duration and light intensity are critical parameters, in addition to CO2 and cell size. While alkenone carbon isotope ratios indeed record CO2 information, irradiance and other factors are also necessary to properly describe alkenone εp.

Summary: The development of calcification by the coccolithophores had a profound impact on ocean carbon cycling, but the evolutionary steps leading to the formation of these complex biomineralized structures are not clear. Heterococcoliths consisting of intricately shaped calcite crystals are formed intracellularly by the diploid life cycle phase. Holococcoliths consisting of simple rhombic crystals can be produced by the haploid life cycle stage but are thought to be formed extracellularly, representing an independent evolutionary origin of calcification.We use advanced microscopy techniques to determine the nature of coccolith formation and complex crystal formation in coccolithophore life cycle stages.We find that holococcoliths are formed in intracellular compartments in a similar manner to heterococcoliths. However, we show that silicon is not required for holococcolith formation and that the requirement for silicon in certain coccolithophore species relates specifically to the process of crystal morphogenesis in heterococcoliths.We therefore propose an evolutionary scheme in which the lower complexity holococcoliths represent an ancestral form of calcification in coccolithophores. The subsequent recruitment of a silicon‐dependent mechanism for crystal morphogenesis in the diploid life cycle stage led to the emergence of the intricately shaped heterococcoliths, enabling the formation of the elaborate coccospheres that underpin the ecological success of coccolithophores. See also the Commentary on this article by Mock, 231: 1663–1666. [ABSTRACT FROM AUTHOR]

The carbon isotope fractionation in algal organic matter (εp), including the long‐chain alkenones produced by the coccolithophorid family Noelaerhabdaceae, is used to reconstruct past atmospheric CO2 levels. The conventional proxy linearly relates εp to changes in cellular carbon demand relative to diffusive CO2 supply, with larger εp values occurring at lower carbon demand relative to supply (i.e., abundant CO2). However, the response of Gephyrocapsa oceanica, one of the dominant alkenone producers of the last few million years, has not been studied closely. Here, we subject G. oceanica to various CO2 levels by increasing pCO2 in the culture headspace, as opposed to increasing dissolved inorganic carbon (DIC) and alkalinity concentrations at constant pH. We note no substantial change in physiology, but observe an increase in εp as carbon demand relative to supply decreases, consistent with DIC manipulations. We compile existing Noelaerhabdaceae εp data and show that the diffusive model poorly describes the data. A meta‐analysis of individual treatments (unique combinations of lab, strain, and light conditions) shows that the slope of the εp response depends on the light conditions and range of carbon demand relative to CO2 supply in the treatment, which is incompatible with the diffusive model. We model εp as a multilinear function of key physiological and environmental variables and find that both photoperiod duration and light intensity are critical parameters, in addition to CO2 and cell size. While alkenone carbon isotope ratios indeed record CO2 information, irradiance and other factors are also necessary to properly describe alkenone εp. Key Points: Alkenone carbon isotope fractionation primarily influenced by CO2, cell size, and irradiance, with a smaller effect from growth rateDiffusive model, the basis for conventional alkenone paleobarometer, of carbon isotope fractionation does not describe culture dataIrradiance is necessary to accurately describe alkenone carbon isotope fractionation and should be considered in the natural environment [ABSTRACT FROM AUTHOR]

Through a combination of selective extractions and molecular characterization techniques including Isoelectric Focusing Chromatography and Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass spectrometry, molecular structures of diatom (Phaeodactylum tricornutum) and coccolithophore (Emiliania huxleyi)-associated biopolymers that are responsible for the distinct partitioning behavior between 210Pb and 210Po were determined. Our results show that diatom-derived biopolymers have distinctive elemental grouping distributions as compared to those excreted by the coccolithophore, with the former consisting of more heterogeneous elements (i.e., nitrogen, sulfur and phosphorus-containing organic compounds). For the coccolithophore culture, two 210Pb-enriched biopolymers (non-attached exopolymeric substances and coccosphere shell-associated biopolymers) have a higher abundance of CHO-type compounds, suggesting CHO-only-type compounds as the main binding moieties for 210Pb. In contrast, such association was not evident in the diatom culture. Different with 210Pb, 210Po enrichment in coccolithophore-derived attached exopolymeric substances and Fe-Mn-associated metabolites coincided with the higher abundance of nitrogen/sulfur-containing organic compounds in these two biopolymer fractions, suggesting the strong parallel of Po with the production of nitrogen-rich organic matter as well as sulfur-containing amino acids. These different associations between 210Pb/210Po and organic functional groups were further explored by separating 210Pb or 210Po-labeled coccolithophore-derived biopolymers via isoelectric focusing. This technique suggests that phosphate group-containing molecules but not the other molecules that contain heterogeneous elements (e.g., CHONS, CHON, and CHOS) as the strongest binding agents for 210Pb, while the more hydrophobic (high protein to carbohydrate ratio) nitrogen/sulfur-enriched organic moieties acted as the main 210Po-binding ligands. It is concluded that the deficiency of 210Po with respect to 210Pb can be influenced by the relative abundance of nitrogen/sulfur-enriched organic moieties to the nitrogen/sulfur-depleted organic compounds in the water column. This behavior constrains the application of 210Po-210Pb approach to quantify the particulate organic carbon (POC) export flux in the ocean. It also explains that differences in chemical binding of the 210Po as compared to those of other radionuclides (e.g., thorium-234) as the main factor. That suggests that differences in decay half-lives or physical factors are less important when these nuclides are applied to estimate the POC flux in the ocean.

Coccolithophores are one of the major contributors to the pelagic production of calcium carbonate and their fossilized remains are a key component of the biogeochemical cycles of calcium (Ca), magnesium (Mg), and other divalent cations present in the intracellular precipitated calcitic structures (coccoliths). The geochemical signature of coccoliths (e.g., Sr/Ca and Mg/Ca ratios) is used as paleoproxy to reconstruct past environmental conditions and to understand the underlying physiological precipitation kinetics. Here, we present the elemental fractionation of Sr and Mg in calcite of the coccolithophore Gephyrocapsa oceanica from controlled laboratory experiments applying an extended temperature gradient (12 to 27°C). The physiological reaction norm of G. oceanica, in terms of growth rate, exhibited optimum behavior while the partition coefficient of Sr (DSr) was linearly correlated with temperature and DMg indicated no specific trend. Our results indicate: (1) a presumably secondary physiological control of DSr, and (2) the importance of calibrating coccolithophore-based proxies using experiments that include the full physiological reaction norms (i.e., a possible non-linear response) to environmental drivers (e.g., temperature, salinity, and pH, etc.). The presented results contribute to an improved understanding of the underlying physiological kinetics involved in regulating coccolith elemental fractionation and give additional implications for designing future laboratory experiments to calibrate and apply coccolithophore based paleoproxies on the fossil sediment record.

Coccolithophores are unique primary producers in the ocean with the ability to calcify. They are known to produce calcareous scales, which form the significant part of calcite oozes or chalk deposits on the seafloor. Coccolithophores are very noteworthy and they are explored to a great extent as nannofossils to reconstruct the past climate. Calcite plates in coccolithophores make them a vital tool in global climate change studies specifically with ocean acidification. These microscopic plants are the major contributor of the carbonate rain that controls the inorganic carbon pump in the ocean, which in turn influences both carbon and carbonate cycles. The emergence of advanced techniques enables us to study the biological aspects of this pelagic calcifier with improved precision. But still, they are understudied world over compared to any other phytoplankton groups. The northern Indian Ocean, being landlocked in three sides and vulnerable to climate change and ocean acidification, severely lacks focused studies on coccolithophores, though the US JGOFS in the 1990s have outlined the ecological significance of coccolithophores in the Arabian Sea. This paper reviews and outlines our understanding of coccolithophores as well as the nix in the northern Indian Ocean. [ABSTRACT FROM AUTHOR]

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. 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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Coccolithophores are a group of phytoplankton widely distributed in the ocean, which secrete extracellular calcite plates termed coccoliths. Coccoliths have been increasingly employed as an archive for geochemical, ecological and paleoclimate studies in recent years. A robust application of coccolith-based geochemical proxies relies on understanding the carbon acquisition strategies and the pathways of carbon supply for calcification. Carbonic anhydrase (CA) plays important roles in the carbon concentrating mechanism s of aquatic algae and potentially also in calcification. However, it is difficult to independently assess the role of CA in carbon supply for photosynthesis versus calcification. To fill this gap, we explored a new method to detect the CA activity inside coccolithophore. To achieve this, coccolithophores were cultured with oxygen and carbon isotope labeled dissolved inorganic carbon (DIC). By exploiting the different behavior of oxygen and carbon isotopes with (sea)water, this double label method can elucidate the significance of CA activity in the calcification pathway. Application of this method to Emiliania huxleyi shows that CA is present in the calcification pathway, and that there is no significant difference in the CA activity between a high and low CO 2 treatment. However, under low CO 2 treatment E. huxleyi enhanced the bicarbonate pumping rate on both cell and chloroplast membranes. This novel method could be performed on other species of coccolithophores in the future and have a potential to extend our knowledge on coccolith oxygen isotope vital effects. [ABSTRACT FROM AUTHOR]

Hydrogen isotope ratios (δ 2 H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of 2 H/ 1 H fractionation in algal lipids by systematically manipulating temperature, light, and CO 2 (aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica . We analyze the hydrogen isotope fractionation in alkenones (α alkenone ), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the α alkenone with increasing CO 2 (aq) and confirm α alkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ 2 H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with 2 H-richer intracellular water, increasing α alkenone . Higher chloroplast CO 2 (aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of α alkenone to CO 2 (aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO 2 at the Rubisco site, but rather that chloroplast CO 2 varies with external CO 2 (aq). The pervasive inverse correlation of α alkenone with CO 2 (aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, α alkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis., Competing Interests: Competing interests statement:The authors declare no competing interest.

Phytoplankton play a central role in the regulation of global carbon and nutrient cycles, forming the basis of the marine food webs. A group of biogeochemically important phytoplankton, the coccolithophores, produce calcium carbonate scales that have been hypothesized to deter or reduce grazing by microzooplankton. Here, a meta-analysis of mesocosm-based experiments demonstrates that calcification of the cosmopolitan coccolithophore, Emiliania huxleyi, fails to deter microzooplankton grazing. The median grazing to growth ratio for E. huxleyi (0.56 ± 0.40) was not significantly different among non-calcified nano- or picoeukaryotes (0.71 ± 0.31 and 0.55 ± 0.34, respectively). Additionally, the environmental concentration of E. huxleyi did not drive preferential grazing of non-calcified groups. These results strongly suggest that the possession of coccoliths does not provide E. huxleyi effective protection from microzooplankton grazing. Such indiscriminate consumption has implications for the dissolution and fate of CaCO3 in the ocean, and the evolution of coccoliths.

While seawater acidification induced by elevated CO2 is known to impact coccolithophores, the effects in combination with decreased salinity caused by sea ice melting and/or hydrological events have not been documented. Here we show the combined effects of seawater acidification and reduced salinity on growth, photosynthesis and calcification of Emiliania huxleyi grown at 2 CO2 concentrations (low CO2 LC:400 μatm; high CO2 HC:1000 μatm) and 3 levels of salinity (25, 30, and 35‰). A decrease of salinity from 35 to 25‰ increased growth rate, cell size and photosynthetic performance under both LC and HC. Calcification rates were relatively insensitive to salinity though they were higher in the LC-grown compared to the HC-grown cells at 25‰ salinity, with insignificant differences under 30 and 35‰. Since salinity and OA treatments did not show interactive effects on calcification, changes in calcification:photosynthesis ratios are attributed to the elevated photosynthetic rates at lower salinities, with higher ratios of calcification to photosynthesis in the cells grown under 35‰ compared with those grown at 25‰. In contrast, photosynthetic carbon fixation increased almost linearly with decreasing salinity, regardless of the pCO2 treatments. When subjected to short-term exposure to high light, the low-salinity-grown cells showed the highest photochemical effective quantum yield with the highest repair rate, though the HC treatment enhanced the PSII damage rate. Our results suggest that, irrespective of pCO2, at low salinity Emiliania huxleyi up-regulates its photosynthetic performance which, despite a relatively insensitive calcification response, may help it better adapt to future ocean global environmental changes, including ocean acidification, especially in the coastal areas of high latitudes.

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. Key Points: Long‐term evolutionary coccolith size trends are found globally in low and middle latitudes with minima at 27 and 23 MaRegional differences in size exist when paleoproductivity drives the presence of large coccoliths [ABSTRACT FROM AUTHOR]

Rapid changes on nutrient supply and CO2 concentration that occurred in the northern South China Sea (SCS) during the Early Oligocene, provides an ideal natural laboratory, allowing us to peer into the coccolithophores' physiology in the geological records. In this study, we established a new nannofossil assemblage index, termed as E* ratio, which is calculated by the relative abundance of eutrophic taxa and meso-oligotrophic taxa ( E ∗ = e e + c × 100 , where e is eutrophic taxa, and c is meso-oligotrophic taxa). Eutrophic taxa include small Reticulofenestra, Reticulofenestra lockeri group, Reticulofenestra bisecta group and Coccolithus pelagicus group, while meso-oligotrophic taxa include Cyclicargolithus spp. The E* ratio is well correlated with nutrient proxy during the Early Oligocene, while with different covarying patterns under the higher and lower CO2 condition. By comparing the assemblage changes to the published data, we suggest that coccolithophores may change the way they use carbon source and nutrient with the decline of CO2. Furthermore, this implies a possible initiation of the carbon concentrating mechanism. [ABSTRACT FROM AUTHOR]

Coccolithophores are important contributors to global calcium carbonate through their species-specific production of calcite coccoliths. Nannofossil coccolith calcite remains an important tool for paleoreconstructions through geochemical analysis of isotopic and trace element incorporation including Sr, which is a potential indicator of past surface ocean temperature and productivity. Scyphosphaera apsteinii (Zygodiscales) exhibits an unusually high Sr/Ca ratio and correspondingly high partitioning coefficient (D Sr = 2.5) in their two morphologically distinct types of coccoliths: flat muroliths and barrel-like lopadoliths. Whether or not this reflects mechanistic differences in calcification compared to other coccolithophores is unknown. We therefore examined the possible role of Sr in S. apsteinii calcification by growing cells in deplete (0.33 mmol/mol Sr/Ca), ambient (9 mmol/mol Sr/Ca), and higher than ambient Sr conditions (36 and 72 mmol/mol Sr/Ca). The effects on growth, quantum efficiency of photosystem II (F v /F m), coccolith morphology, and calcite D Sr were evaluated. No effect on S. apsteinii growth rate or F v /F m was observed when cells were grown in Sr/Ca between 0.33–36 mmol/mol. However, at 72 mmol/mol Sr/Ca growth rate was significantly reduced, although F v /F m was unaffected. Reducing the Sr/Ca from ambient (9 mmol/mol) did not significantly alter the frequency of malformed and aberrant muroliths and lopadoliths, but at higher than ambient Sr/Ca conditions coccolith morphology was significantly disrupted. This implies that Sr is not a critical determining factor in normal coccolith calcite morphology in this dimorphic species. Using energy dispersive spectroscopy (EDS) we observed an increase in [Sr] and decrease in D Sr of coccoliths as the Sr/Ca of the growth medium increased. Interestingly, muroliths had significantly lower Sr/Ca than lopadoliths at ambient and elevated [Sr], and lopadolith tips had lower Sr than bases in ambient conditions. In summary, the Sr fractionation behavior of S. apsteinii is distinct from other coccolithophores because of an unusually high D Sr and inter- and intra-coccolith variability in Sr/Ca. These observations could be explained by mechanistic differences in the selectivity of the Ca2+ transport pathway or in the Sr-and Ca-binding capacity of organic components, such as polysaccharides associated with coccolithogenesis. [ABSTRACT FROM AUTHOR]

Emiliania huxleyi is a globally important coccolithophore and one of the most successful eukaryotic organisms in the modern oceans. Despite a large body of work on this organism, including the sequencing of its genome, the tools required for forward and reverse functional genetic studies are still undeveloped. Here we present an optimized method for the clonal isolation of E. huxleyi by plating on solid medium. We demonstrate the utility of this method for a variety of strains including haploid, calcifying‐diploid, and noncalcifying diploid strains. We show that, in contrast to previous studies, no changes in cell ploidy status occur when the cells are plated. Our method will greatly aid attempts to elucidate the genetic basis of the remarkable physiology of E. huxleyi by forward and reverse genetic approaches. [ABSTRACT FROM AUTHOR]

Emiliania huxleyi is a globally important coccolithophore and one of the most successful eukaryotic organisms in the modern oceans. Despite a large body of work on this organism, including the sequencing of its genome, the tools required for forward and reverse functional genetic studies are still undeveloped. Here we present an optimized method for the clonal isolation of E. huxleyi by plating on solid medium. We demonstrate the utility of this method for a variety of strains including haploid, calcifying‐diploid, and noncalcifying diploid strains. We show that, in contrast to previous studies, no changes in cell ploidy status occur when the cells are plated. Our method will greatly aid attempts to elucidate the genetic basis of the remarkable physiology of E. huxleyi by forward and reverse genetic approaches. [ABSTRACT FROM AUTHOR]