Photosynthetic Carbon Isotope Fractionation by Cyanobacteria

Photosynthetic carbon isotope fractionation (ɛ_P) forms the basis of both paleoenvironmental records including atmospheric carbon dioxide concentrations and the interpretation of biomarker records related to primary productivity. The factors governing ɛ_P values in eukaryotic algae, namely those that control the carbon supply such as growth rate and external CO₂ concentration, have been well studied over the past half century. Yet ɛ_P values in photosynthetic cyanobacteria surprisingly seem to differ from the relationships established in eukaryotic algae, lacking a systematic response to CO₂ concentrations and exhibiting less overall fractionation [1].

We use controlled batch culture experiments to characterize photosynthetic carbon isotope fractionation by a diverse suite of beta cyanobacteria in response to variations in carbon supply. Specific strains do exhibit a linear ɛ_P response to changes in CO₂ concentrations, similar to eukaryotic algae. We also observe a single relationship between ɛ_P values and growth rate across all strains, suggesting that fractionation is controlled by underlying physiology. In concert, we develop quantitative bio-isotopic model to gain a mechanistic understanding this fractionation, including the influence of CO₂ concentrating mechanisms (CCMs). This work provides an updated framework for interpreting cyanobacterial contributions to biomarker records [2, 3].

[1] Popp BN, et al. (1998) Effect of Phytoplankton Cell Geometry on Carbon Isotopic Fractionation. Geochimica et Cosmochimica Acta 62(1).

[2] Close HG, et al. (2011) Inverse carbon isotope patterns of lipids and kerogen record heterogeneous primary biomass. Geobiology 9(3).

[3] Witkowski CR, et al. (2018). Molecular fossils from phytoplankton reveal secular pCO₂ trend over the Phanerozoic. Science Advances 4(11).