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Coccolithophores and other haptophyte algae acquire the carbon required for metabolic processes from the water in which they live. Whether carbon is actively moved across the cell membrane via a carbon concentrating mechanism, or passively through diffusion, is important for haptophyte biochemistry. The possible utilization of carbon concentrating mechanisms also has the potential to over-print one proxy method by which ancient atmospheric CO.sub.2 concentration is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 µmol L.sup.-1 . I compile published alkenone-based CO.sub.2 reconstructions from multiple sites over the Pleistocene and recalculate them using a common methodology, which allows comparison to be made with ice core CO.sub.2 records. Interrogating these records reveals that the relationship between proxy CO.sub.2 and ice core CO.sub.2 breaks down when local aqueous CO.sub.2 concentration falls below 7 µmol L.sup.-1 . The recognition of this threshold explains why many alkenone-based CO.sub.2 records fail to accurately replicate ice core CO.sub.2 records, and it suggests the alkenone proxy is likely robust for much of the Cenozoic when this threshold was unlikely to be reached in much of the global ocean.

The coccolith sedimentary and micropalaeontological archive has fostered great interest in palaeoclimate applications. Indeed, the geochemistry of coccolith calcite has the potential to reconstruct both palaeo-CO.sub.2 concentrations and palaeo-temperature of seawater. Studying coccolith geochemistry aims at better understanding the changes in the vital effect of coccoliths with changes in environmental parameters, especially the carbonate chemistry of seawater. To this aim, we need to deconvolve the biological imprint from the environmental signals recorded in the composition of coccoliths. We have undertaken culture experiments of four coccolithophore strains with various sizes and growth rates, grown under eight CO.sub.2 /pH conditions typifying the CO.sub.2 evolution of the Cenozoic era. We propose an assessment of the expression of the vital effects for Emiliania huxleyi, Gephyrocapsa oceanica, Helicosphaera carteri and Coccolithus braarudii with simultaneous changes in dissolved inorganic carbon (DIC) and pH in the medium resulting in variations in dissolved CO.sub.2 (CO.sub.2 aq) availability to the cells. We have identified a distinct isotopic response of C. braarudii to pCO.sub.2 levels on both sides of the 600 ppmv (pH 7.89) condition. We propose that this discrepancy is the result of a modification of the proton efflux across the plasma membrane through voltage-dependent proton channels. We further show that as the CO.sub.2 level rises and pH decreases (from 200 to 500 ppmv and from 8.29 to 7.96 pH units, respectively), a significant increase in [delta]13Ccoccolith of C. braarudii is expressed, along with a coeval decrease in [delta]13Corg. The constant physiological parameters of C. braarudii (growth rate, particulate inorganic carbon (PIC) and particulate organic carbon (POC)) across the 200 to 500 ppmv interval support the idea that the change in [delta]13Ccoccolith is the consequence of a lower fractionation between dissolved CO.sub.2 and organic matter. Meanwhile, the small cells of E. huxleyi and G. oceanica are less carbon-limited and do not exhibit any change in their carbon vital effects with changes in carbonate chemistry of the environment across the whole CO.sub.2 spectrum. Using this biogeochemical framework, we have established a calibration between CO.sub.2 aq concentration and the differential vital effect (Î[delta].sup.13 C) between isotopically invariant small G. oceanica and large coccoliths C. braarudii, whose vital effect is CO.sub.2 -dependent at low CO.sub.2 . The CO.sub.2 -Î[delta].sup.13 C transfer equation allows palaeo-pCO.sub.2 reconstructions based on isotope changes explained by physiological processes, especially at low and medium CO.sub.2 levels.

Beyond the pCO.sub.2 records provided by ice core measurements, the quantification of atmospheric CO.sub.2 concentrations and changes thereof relies on proxy data, the development of which represents a foremost challenge in paleoceanography. In the paleoceanographic toolbox, the coccolithophores occupy a notable place, as the magnitude of the carbon isotopic fractionation between ambient CO.sub.2 and a type of organic compounds that these photosynthetic microalgae synthesize (the alkenones) represents a relatively robust proxy to reconstruct past atmospheric CO.sub.2 concentrations during the Cenozoic. The isotopic composition of coeval calcite biominerals found in the sediments and also produced by the coccolithophores (the coccoliths) have been found to record an ambient CO.sub.2 signal through culture and sediment analyses. These studies have, however, not yet formalized a transfer function that quantitatively ties the isotopic composition of coccolith calcite to the concentrations of aqueous CO.sub.2 and, ultimately, to atmospheric CO.sub.2 levels. Here, we make use of a microseparation protocol to compare the isotopic response of two size-restricted coccolith assemblages from the North Atlantic to changes in surface ocean CO.sub.2 during Termination II (ca. 130-140 ka). Performing paired measurements of the isotopic composition ([delta].sup.13 C and [delta].sup.18 O) of relatively large and small coccoliths provides an isotopic offset that can be designated as a \"differential vital effect\". We find that the evolution of this offset follows that of aqueous CO.sub.2 concentrations computed from the ice core CO.sub.2 curve and an independent temperature signal. We interpret this biogeochemical feature to be the result of converging carbon fixation strategies between large and small cells as the degree of carbon limitation for cellular growth decreases across the deglaciation. We are therefore able to outline a first-order trend between the coccolith differential vital effects and aqueous CO.sub.2 in the range of Quaternary CO.sub.2 concentrations. Although this study would benefit from further constraints on the other controls at play on coccolith geochemistry (growth rate, air-sea gas exchange, etc.), this test of the drivers of coccolith Î[delta].sup.13 C and Î[delta].sup.18 O in natural conditions is a new step in the development of a coccolith paleo-CO.sub.2 probe.

Studies on CO.sub.2 effects on coccolithophores, unicellular calcifying phytoplankton, show species-specific responses, although only fewer than 5 % of the ⼠280 living species have been tested so far. Helicosphaera carteri significantly contributes to carbon fluxes and CaCO.sub.3 storage due to its size and high calcite production. Despite its importance, few studies have examined H. carteri under experimental conditions, and only one has addressed the effects of rising CO.sub.2 /decreasing pH. H. carteri being a large-sized, obligated calcifier species, an important aspect to understand is how changes in seawater carbonate chemistry may affect its morphology. It has already been suggested for other coccolithophores species that the presence of malformed coccoliths may represent a disadvantage for these organisms. Moreover, an alteration in coccolith morphology may affect their contribution to CaCO.sub.3 sedimentation and ballasting. As for H. carteri, it has also been suggested that due to its high PIC:POC ratio, the species could show a high sensitivity to CO.sub.2 rise. In this study, we investigate for the first time whether high pCO2/low pH does affect the morphology of H. carteri coccoliths, by culturing this species under pre-industrial CO.sub.2 levels (⼠295 µatm) and ⼠600 µatm, i.e., the SSP 2-4.5 scenario for 2100 (IPCC, 2021). We also analyzed cellular PIC and POC quotas using morphometric data, roundness, and protoplast and coccosphere size to observe the pCO.sub.2 influence on the calcification and photosynthesis ratio.

• 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.

Coccolithophores are well-known haptophytes that produce small calcium carbonate coccoliths, which in turn contribute to carbon sequestration in the marine environment. Despite their important ecological role, only two of eleven haptophyte plastid genomes are from coccolithophores, and those two belong to the order Isochrysidales. Here, we report the plastid genomes of two strains of Ochrosphaera neapolitana (Coccolithales) from Spain (CCAC 3688 B) and the USA (A15,280). The newly constructed plastid genomes are the largest in size (116,906 bp and 113,686 bp, respectively) among all the available haptophyte plastid genomes, primarily due to the increased intergenic regions. These two plastid genomes possess a conventional quadripartite structure with a long single copy and short single copy separated by two inverted ribosomal repeats. These two plastid genomes share 110 core genes, six rRNAs, and 29 tRNAs, but CCAC 3688 B has an additional CDS (ycf55) and one tRNA (trnL-UAG). Two large insertions at the intergenic regions (2 kb insertion between ycf35 and ycf45; 0.5 kb insertion in the middle of trnM and trnY) were detected in the strain CCAC 3688 B. We found the genes of light-independent protochlorophyllide oxidoreductase (chlB, chlN, and chlL), which convert protochlorophyllide to chlorophyllide during chlorophyll biosynthesis, in the plastid genomes of O. neapolitana as well as in other benthic Isochrysidales and Coccolithales species, putatively suggesting an evolutionary adaptation to benthic habitats.

The genus Acropora comprises the most diverse and abundant scleractinian corals (Anthozoa, Cnidaria) in coral reefs, the most diverse marine ecosystems on Earth. However, the genetic basis for the success and wide distribution of Acropora are unknown. Here, we sequenced complete genomes of 15 Acropora species and 3 other acroporid taxa belonging to the genera Montipora and Astreopora to examine genomic novelties that explain their evolutionary success. We successfully obtained reasonable draft genomes of all 18 species. Molecular dating indicates that the Acropora ancestor survived warm periods without sea ice from the mid or late Cretaceous to the Early Eocene and that diversification of Acropora may have been enhanced by subsequent cooling periods. In general, the scleractinian gene repertoire is highly conserved; however, coral- or cnidarian-specific possible stress response genes are tandemly duplicated in Acropora. Enzymes that cleave dimethlysulfonioproprionate into dimethyl sulfide, which promotes cloud formation and combats greenhouse gasses, are the most duplicated genes in the Acropora ancestor. These may have been acquired by horizontal gene transfer from algal symbionts belonging to the family Symbiodiniaceae, or from coccolithophores, suggesting that although functions of this enzyme in Acropora are unclear, Acropora may have survived warmer marine environments in the past by enhancing cloud formation. In addition, possible antimicrobial peptides and symbiosis-related genes are under positive selection in Acropora, perhaps enabling adaptation to diverse environments. Our results suggest unique Acropora adaptations to ancient, warm marine environments and provide insights into its capacity to adjust to rising seawater temperatures.

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.

Millimeter-sized plastics are abundant in most marine surface waters, and known to carry fouling organisms that potentially play key roles in the fate and ecological impacts of plastic pollution. In this study we used scanning electron microscopy to characterize biodiversity of organisms on the surface of 68 small floating plastics (length range = 1.7-24.3 mm, median = 3.2 mm) from Australia-wide coastal and oceanic, tropical to temperate sample collections. Diatoms were the most diverse group of plastic colonizers, represented by 14 genera. We also recorded 'epiplastic' coccolithophores (7 genera), bryozoans, barnacles (Lepas spp.), a dinoflagellate (Ceratium), an isopod (Asellota), a marine worm, marine insect eggs (Halobates sp.), as well as rounded, elongated, and spiral cells putatively identified as bacteria, cyanobacteria, and fungi. Furthermore, we observed a variety of plastic surface microtextures, including pits and grooves conforming to the shape of microorganisms, suggesting that biota may play an important role in plastic degradation. This study highlights how anthropogenic millimeter-sized polymers have created a new pelagic habitat for microorganisms and invertebrates. The ecological ramifications of this phenomenon for marine organism dispersal, ocean productivity, and biotransfer of plastic-associated pollutants, remains to be elucidated.

Here, we investigated the response of the coccolithophore Pleurochrysis carterae (P. carterae) isolated from the Bohai Sea to ocean acidification under different irradiance levels. A full factorial matrix of two pCO[sub.2] conditions (400 ppm and 800 ppm) and irradiance levels of 50, 200, 500 and 800 μmol photons m[sup.−2] s[sup.−1] were examined. The results suggest that ocean acidification suppressed the photosynthesis and increased the saturation irradiance for growth of the coccolithophore Pleurochrysis carterae. Further comparison with previously published results reveals that the physiological processes of the coastal coccolithophore specie Pleurochrysis carterae are less sensitive to ocean acidification than the smaller-sized species Emiliania huxleyi and Gephyrocapsa oceanica, and the saturation irradiance for the growth, particulate organic carbon (POC) and particulate inorganic carbon (PIC) production of Pleurochrysis carterae are much lower than those of the other two species. The ecologically important marine phytoplankton group coccolithophores have a global distribution. The impacts of ocean acidification on the cosmopolitan species Emiliania huxleyi have received much attention and have been intensively studied. However, the species-specific responses of coccolithophores and how these responses will be regulated by other environmental drivers are still largely unknown. To examine the interactive effects of irradiance and ocean acidification on the physiology of the coastal coccolithophore species Pleurochrysis carterae, we carried out a semi-continuous incubation experiment under a range of irradiances (50, 200, 500, 800 μmol photons m[sup.−2] s[sup.−1]) at two CO[sub.2] concentration conditions of 400 and 800 ppm. The results suggest that the saturation irradiance for the growth rate was higher at an elevated CO[sub.2] concentration. Ocean acidification weakened the particulate organic carbon (POC) production of Pleurochrysis carterae and the inhibition rate was decreased with increasing irradiance, indicating that ocean acidification may affect the tolerating capacity of photosynthesis to higher irradiance. Our results further provide new insight into the species-specific responses of coccolithophores to the projected ocean acidification under different irradiance scenarios in the changing marine environment.