Multispecies expression of coccolithophore vital effects with changing CO<sub>2</sub> concentrations and pH in the laboratory with insights for reconstructing CO<sub>2</sub> levels in geological history
<p>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-<span class="inline-formula">CO<sub>2</sub></span&...
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| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-05-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/22/2287/2025/bg-22-2287-2025.pdf |
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| Summary: | <p>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-<span class="inline-formula">CO<sub>2</sub></span> 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 <span class="inline-formula">CO<sub>2</sub></span>/pH conditions typifying the <span class="inline-formula">CO<sub>2</sub></span> evolution of the Cenozoic era. We propose an assessment of the expression of the vital effects for <i>Emiliania huxleyi</i>, <i>Gephyrocapsa oceanica</i>, <i>Helicosphaera carteri</i> and <i>Coccolithus braarudii</i> with simultaneous changes in dissolved inorganic carbon (DIC) and pH in the medium resulting in variations in dissolved <span class="inline-formula">CO<sub>2</sub></span> (<span class="inline-formula">CO<sub>2 aq</sub></span>) availability to the cells. We have identified a distinct isotopic response of <i>C. braarudii</i> to <span class="inline-formula"><i>p</i></span><span class="inline-formula">CO<sub>2</sub></span> 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 <span class="inline-formula">CO<sub>2</sub></span> level rises and pH decreases (from 200 to 500 ppmv and from 8.29 to 7.96 pH units, respectively), a significant increase in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">δ</mi><msub><mrow class="chem"><msup><mi/><mn mathvariant="normal">13</mn></msup><mi mathvariant="normal">C</mi></mrow><mtext>coccolith</mtext></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="ed793b00b07727321b91f02da8ec5393"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-2287-2025-ie00001.svg" width="58pt" height="16pt" src="bg-22-2287-2025-ie00001.png"/></svg:svg></span></span> of <i>C. braarudii</i> is expressed, along with a coeval decrease in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">δ</mi><msub><mrow class="chem"><msup><mi/><mn mathvariant="normal">13</mn></msup><mi mathvariant="normal">C</mi></mrow><mtext>org</mtext></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="38pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="422bba6f2947eec7dd38c1f05f000d14"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-2287-2025-ie00002.svg" width="38pt" height="17pt" src="bg-22-2287-2025-ie00002.png"/></svg:svg></span></span>. The constant physiological parameters of <i>C. braarudii</i> (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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">δ</mi><msub><mrow class="chem"><msup><mi/><mn mathvariant="normal">13</mn></msup><mi mathvariant="normal">C</mi></mrow><mtext>coccolith</mtext></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="c4eb374327484c2bb1b9d7cf8329e108"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-2287-2025-ie00003.svg" width="58pt" height="16pt" src="bg-22-2287-2025-ie00003.png"/></svg:svg></span></span> is the consequence of a lower fractionation between dissolved <span class="inline-formula">CO<sub>2</sub></span> and organic matter. Meanwhile, the small cells of <i>E. huxleyi</i> and <i>G. oceanica</i> 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 <span class="inline-formula">CO<sub>2</sub></span> spectrum. Using this biogeochemical framework, we have established a calibration between <span class="inline-formula">CO<sub>2 aq</sub></span> concentration and the differential vital effect (<span class="inline-formula">Δ<i>δ</i><sup>13</sup>C</span>) between isotopically invariant small <i>G. oceanica</i> and large coccoliths <i>C. braarudii</i>, whose vital effect is <span class="inline-formula">CO<sub>2</sub></span>-dependent at low <span class="inline-formula">CO<sub>2</sub></span>. The <span class="inline-formula">CO<sub>2</sub></span>–<span class="inline-formula">Δ<i>δ</i><sup>13</sup>C</span> transfer equation allows palaeo-<span class="inline-formula"><i>p</i></span><span class="inline-formula">CO<sub>2</sub></span> reconstructions based on isotope changes explained by physiological processes, especially at low and medium <span class="inline-formula">CO<sub>2</sub></span> levels.</p> |
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| ISSN: | 1726-4170 1726-4189 |