The influence of irradiance and interspecific differences on <i>δ</i>¹¹B, <i>δ</i>¹³C and elemental ratios in four coralline algae complexes from Aotearoa, New Zealand

<p>Coralline algae are a cosmopolitan group of important foundational species. The calcium carbonate they produce is increasingly being used as paleoenvironmental archives, as well as used to trace physiological responses of these important macroalgae to environmental change. In this context, evaluating the effect of oceanic change and photo-physiological parameters on geochemical proxies is critical, as such gaps may lead to erroneous paleoenvironmental reconstructions, misattributed drivers of calcification responses, and ultimately compromise conservation strategies. Here we address the impact of light (irradiance) on four species complexes of coralline red algae including two morphologies; geniculate (branching) and non-geniculate (encrusting). The four complexes up-regulated their <span class="inline-formula"><i>δ</i>¹¹B</span> derived pH<span class="inline-formula">_CF</span> relative to seawater by 0.6 to 0.8 pH unit. <span class="inline-formula"><i>δ</i>¹¹B</span> was not measurably affected by varying irradiance despite evidence of increasing photosynthesis. All complexes were able to maintain and elevate their pH<span class="inline-formula">_CF</span> relative to seawater for all treatments. Non-geniculate and geniculate complexes had distinct geochemical signatures of <span class="inline-formula"><i>δ</i>¹¹B</span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" 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>mineral</mtext></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="53pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="beb10e485aba230d7f2d8812b62ee16e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-7669-2025-ie00001.svg" width="53pt" height="16pt" src="bg-22-7669-2025-ie00001.png"/></svg:svg></span></span> and trace elements. These differences in geochemical signatures indicate a variety of calcification mechanisms exist within coralline algae. We propose that different sources of dissolved inorganic carbon (DIC) are necessary to explain the observed <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" 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>mineral</mtext></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="53pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="d0f4b724b30144769dbb7d7bae10ad2a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-7669-2025-ie00002.svg" width="53pt" height="16pt" src="bg-22-7669-2025-ie00002.png"/></svg:svg></span></span>. As geniculate species have higher photosynthetic activity (i.e. gross photosynthesis), the DIC sources allocated to calcification might be limited due to greater <span class="inline-formula">CO₂</span> drawdown. This is supported by <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">B</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c0f64f4592e8dde807a00769c8a08c80"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-7669-2025-ie00003.svg" width="30pt" height="14pt" src="bg-22-7669-2025-ie00003.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">U</mi><mo>/</mo><mi mathvariant="normal">Ca</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ad1503912e26592770e55071db30cf63"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-7669-2025-ie00004.svg" width="30pt" height="14pt" src="bg-22-7669-2025-ie00004.png"/></svg:svg></span></span> ratios suggesting modulation of carbonate chemistry and especially lower DIC<span class="inline-formula">_CF</span> in geniculate relative to non-geniculate complexes. DIC sources might come from direct <span class="inline-formula">CO₂</span> diffusion or better recycling of metabolic <span class="inline-formula">CO₂</span> which would explain the depleted <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" 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>mineral</mtext></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="53pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="7b95f6b55e9179f4f6107320d9a8faf2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-7669-2025-ie00005.svg" width="53pt" height="16pt" src="bg-22-7669-2025-ie00005.png"/></svg:svg></span></span>. This strategy likely arises from the different energy needs of the organisms, with non-geniculate using relatively more energy to support calcification. We suggest the different calcification mechanisms between morphologies are linked to different interactions between photosynthesis and carbon allocation. While photosynthesis can provide energy to geniculate complexes to maintain their metabolic needs, their calcification may be limited by DIC. In contrast, non-geniculate forms may benefit from more limited DIC drawdown due to lower photosynthetic activity, therefore maintaining higher internal DIC concentrations ultimately supporting faster calcification.</p>