Low sensitivity of a heavily calcified coccolithophore under increasing CO₂: the case study of <i>Helicosphaera carteri</i>

<p>Studies on CO<span class="inline-formula">₂</span> effects on coccolithophores, unicellular calcifying phytoplankton, show species-specific responses, although only fewer than 5 % of the <span class="inline-formula">∼</span> 280 living species have been tested so far. <i>Helicosphaera carteri</i> significantly contributes to carbon fluxes and CaCO<span class="inline-formula">₃</span> storage due to its size and high calcite production. Despite its importance, few studies have examined <i>H. carteri</i> under experimental conditions, and only one has addressed the effects of rising CO<span class="inline-formula">₂</span>/decreasing pH. <i>H. carteri</i> 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<span class="inline-formula">₃</span> sedimentation and ballasting. As for <i>H. carteri</i>, it has also been suggested that due to its high <span class="inline-formula">PIC:POC</span> ratio, the species could show a high sensitivity to CO<span class="inline-formula">₂</span> rise. In this study, we investigate for the first time whether high <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mn mathvariant="normal">2</mn></msub><mo>/</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3893fa1895987b3f7aa0444298f8eafe"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-1821-2025-ie00001.svg" width="13pt" height="14pt" src="bg-22-1821-2025-ie00001.png"/></svg:svg></span></span>low pH does affect the morphology of <i>H. carteri</i> coccoliths, by culturing this species under pre-industrial CO<span class="inline-formula">₂</span> levels (<span class="inline-formula">∼</span> 295 <span class="inline-formula">µ</span>atm) and <span class="inline-formula">∼</span> 600 <span class="inline-formula">µ</span>atm, 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 <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span> influence on the calcification and photosynthesis ratio.</p> <p>Our results indicate that <i>H. carteri</i> morphology is not significantly affected by increasing CO<span class="inline-formula">₂</span>, in contrast to other heavily calcified species. The protoplast volume and coccosphere shape of <i>Helicosphaera carteri</i> did not vary with changes in CO<span class="inline-formula">₂</span>, and neither did its particulate inorganic carbon (PIC) and particulate organic carbon (POC) quotas, nor the <span class="inline-formula">PIC:POC</span> ratio, indicating unaltered physiological state.</p> <p>The low <span class="inline-formula">PIC:POC</span> ratio found in this work for <i>H. carteri</i> compared to ratios previously measured in the same strain under different experimental conditions, and compared to other highly calcified species, could explain the observed low sensitivity of <i>H. carteri </i>to CO<span class="inline-formula">₂</span>. Moreover, the observation of a stable ratio between calcification and photosynthesis in <i>H. carteri</i> under increasing CO<span class="inline-formula">₂</span> might suggest a constant contribution to the rain ratio under climate change. However, further studies comparing experimental and field data from past ocean acidification events will be required to confirm the conclusions drawn here.</p>