Carbon isotope fractionation in phytoplankton as a potential proxy for pH rather than for [CO₂(aq)]: Observations from a carbonate lake

Abstract

The method of reconstructing paleoatmospheric CO2 levels using carbon isotope signatures of organic matter buried in sediments has been questioned due to the dubious foundation that carbon isotope fractionation during phytoplankton photosynthesis (εp) is controlled primarily by aquatic CO2 concentration ([CO2(aq)]). Consequently, what carbon isotope data from bulk sedimentary organic matter reflects is a puzzle. In this study, we determined the carbon isotope compositions of dissolved inorganic carbon and particulate organic carbon in a lake located in a carbonate area. Partial correlation analysis was employed to distinguish between direct and indirect factors in controlling εp. The results show that εp is more closely, and more steadily related with pH than with [CO2(aq)], which is in accordance with recent advances in our understanding of the physiology of carbon utilization by phytoplankton for CO2 and HCO3−. Therefore, we propose that carbon isotope fractionation in phytoplankton is more suitable as a proxy of pH than of [CO2(aq)]. One advantage of this amendment is that information on HCO3−, the main species of carbon uptake by phytoplankton, is likewise included. In the future, culture experiments aiming at revealing the relationship between pH and cellular carbon isotope signatures is necessary to construct a new isotope fractionation formula to couple the different effects of CO2 and HCO3−, which is of critical importance to improve the understanding of carbon isotope fractionation, and to more precisely model pH and CO2.

Effects of CO2 and RuBisCO concentration on cyanobacterial growth and carbon isotope fractionation

“…The positive relationship between CO2 concentration and carbon isotope fractionation has been observed empirically for a variety of autotrophs (Freeman & Hayes, 1992;Hinga et al, 1994;Schubert & Jahren, 2012;Wilkes et al, 2018), including for cyanobacteria in particular (Eichner et al, 2015;Hurley et al, 2021). Other studies showed that photoautotrophs that grew under a lower pH, and thus a higher proportion of CO2, also exhibited higher carbon fractionation (Mizutani & Wada, 1982;Roeske & O'Leary, 1984;Wang, Yeager, & Lu, 2016;Yoshioka, 1997).…”

CO2 limited conditions favor cyanobacteria in a hypereutrophic lake: An empirical and theoretical stable isotope study

“…If relationships between δ 13 C POC and [CO 2 ] aq are to be established in surface waters, then extended to the sediments as a paleo‐atmospheric tool, variations on the diel scale should be acknowledged and accounted for if possible. Furthermore, it may be that C acquisition by cyanobacteria may be more sensitive to pH, rather than CO 2 availability (Mangan et al ; Wang et al ), an argument that is supported by the fact that modeled ε p was most sensitive to F HCO3 , which is a function of pH. Care should thus be taken when interpreting sedimentary δ 13 C POC in the context of atmospheric CO 2 .…”

“…Headspace and ambient samples were analyzed for CO 2 and CH 4 by GC at the Nanjing Institute of Geography and Limnology (NIGLAS), Chinese Academy of Sciences. In situ partial pressures were calculated from headspace concentrations using Henry's Law, along with a temperature dependent solubility according to Wanninkhof (1992). Ambient air passed through the soda lime scrubber was found to contain a small amount of CO 2 .…”

Stable Carbon Isotopes of Phytoplankton as a Tool to Monitor Anthropogenic CO2 Submarine Leakages

“…Indeed, during the experiments, the ε CO2-POC values were not constant either in the ASW or in the NAT. Moreover, the presence of CCMs in diatoms allows the uptake of both CO , no fractionation occurs during the process of intracellular dehydration, making the substrate for photosynthesis 9-10‰ enriched than CO 2(aq) , thus affecting the resulting phytoplankton isotopic ε CO2-POC values [112]. In this process, pH was identified as an important controlling factor, since it regulates the level of CA activity, the induction of CCMs and affects carbon leakage from the cell in relation to the CO 2(aq) and HCO 3 − uptake and conversion [112].…”

“…Moreover, the presence of CCMs in diatoms allows the uptake of both CO , no fractionation occurs during the process of intracellular dehydration, making the substrate for photosynthesis 9-10‰ enriched than CO 2(aq) , thus affecting the resulting phytoplankton isotopic ε CO2-POC values [112]. In this process, pH was identified as an important controlling factor, since it regulates the level of CA activity, the induction of CCMs and affects carbon leakage from the cell in relation to the CO 2(aq) and HCO 3 − uptake and conversion [112]. The increase in pH detected in both conditions during diatom growth has clearly moved the carbonate system equilibrium towards higher concentrations of bicarbonate; thus, both in the ASW and NAT, HCO 3 − should have played an important role as a photosynthetic substrate.…”