Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> One of the great challenges in biogeochemical research over the past half a century has been to quantify and understand the mechanisms underlying stable carbon isotope fractionation (<span class="inline-formula"><i>ε</i>_p)</span> in phytoplankton in response to changing CO<span class="inline-formula">₂</span> concentrations. This interest is partly grounded in the use of fossil photosynthetic organism remains as a proxy for past atmospheric CO<span class="inline-formula">₂</span> levels. Phytoplankton organic carbon is depleted in <span class="inline-formula">¹³</span>C compared to its source because of kinetic fractionation by the enzyme RubisCO during photosynthetic carbon fixation, as well as through physiological pathways upstream of RubisCO. Moreover, other factors such as nutrient limitation, variations in light regime as well as phytoplankton culturing systems and inorganic carbon manipulation approaches may confound the influence of aquatic CO<span class="inline-formula">₂</span> concentrations [CO<span class="inline-formula">₂</span>] on <span class="inline-formula"><i>ε</i>_p</span>. Here, based on experimental data compiled from the literature, we assess which underlying physiological processes cause the observed differences in <span class="inline-formula"><i>ε</i>_p</span> for various phytoplankton groups in response to C-demand/C-supply, i.e., particulate organic carbon (POC) production <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="880d1b22cfae9b4167ff115d05c6894c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-19-3305-2022-ie00001.svg" width="8pt" height="14pt" src="bg-19-3305-2022-ie00001.png"/></svg:svg></span></span> [CO<span class="inline-formula">₂</span>]) and test potential confounding factors. Culturing approaches and methods of carbonate chemistry manipulation were found to best explain the differences in <span class="inline-formula"><i>ε</i>_p</span> between studies, although day length was an important predictor for <span class="inline-formula"><i>ε</i>_p</span> in haptophytes. Extrapolating results from culturing experiments to natural environments and for proxy applications therefore require caution, and it should be carefully considered whether culture methods and experimental conditions are representative of natural environments.