Critical switch towards bicarbonate uptake during cyanobacteria blooms at much higher than expected pH values as shown by multiple carbon isotope investigations

Cyanobacteria blooms can affect turnover of aqueous carbon, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). We investigated them in a pond for 23 months. During these events POC isotope values (d¹³CPOC) increased up to −17.4 ‰, after aqueous CO2 (CO2(aq)) fell below an atmospheric equilibration value of 412 μatm. Additionally, carbon isotope enrichment between CO2(aq) and POC (eCO2-phyto) ranged of between values of 2.0 and 21.5 ‰ with lowest fractionations observed at pH values above 8.9. The increase of d¹³CPOC and decrease of εCO2-phyto values at low pCO2 and high pH was plausibly caused by the activation of the carbon concentrating mechanism (CCM). This mechanism correlated with prevalent assimilation of ¹³C-enriched HCO3^- species. Surprisingly, CO2(aq) still contributed more than 50% to the POC pool down to pCO2 values of around 150 μatm. Only after this threshold, the reduced εCO2-phyto suggested incorporation of ¹³C-enriched HCO3^-. This work is based on a publication by (Piatka et al., 2022).

Piatka, D. R., Frank, A. H., Köhler, I., Castiglione, K., van Geldern, R., & Barth, J. A. C. (2022). Balance of carbon species combined with stable isotope ratios show critical switch towards bicarbonate uptake during cyanobacteria blooms. Science of the Total Environment, 807. doi:10.1016/j.scitotenv.2021.151067