Isotopic imprints of algal carbon dynamics

Phytoplankton convert dissolved CO2 into biomass through the process of photosynthesis, providing a fundamental link between the biosphere and climate. The fractionation of carbon isotopes accompanying photosynthesis (εP) is reflected in marine organic molecules, preserved in marine sediments, and used to study the evolution of global carbon cycling—including variations in atmospheric CO2 levels. While classical models for interpreting these signals are based on the balance between diffusion of CO2 and its fixation into biomass by the enzyme RubisCO, the details of phytoplankton carbon dynamics in reality are more complex. Phytoplankton employ a diversity of RubisCO types, and they also use carbon concentrating mechanisms to enhance intracellular CO2 concentrations. It is essential to understand the significance of these physiological features as controls on εp, as they may play important roles in explaining marine archives. In this talk, I will discuss chemostat culture experiments with phytoplankton, including a dinoflagellate employing Form II RubisCO—a structurally and catalytically unique form of the enzyme among phytoplankton. These experiments underscore a broader conundrum: as more data become available, RubisCO isotope fractionation factors inferred from culture studies (in vivo) consistently disagree with measurements on the purified enzyme (in vitro). I will present a revised model which may account for these differences in prominent eukaryotic phytoplankton groups and inform applications in different environments. Additionally, I will introduce a promising avenue for extracting more detailed ecophysiological information from the modern water column and the carbon isotope record by probing the natural variations in 13C/12C at specific atomic positions within marine organic molecules.