EAPS DLS – Elise Wilkes (Caltech)

“Isotopic Imprints of Algal Carbon Dynamics”

Through the process of photosynthesis, phytoplankton fix CO2 into biomass that is depleted in 13C relative to ambient dissolved CO2. This photosynthetic carbon isotope fractionation (εP) is reflected in the carbon isotope ratios of marine organic molecules, preserved in sediments, and used to study the evolution of global carbon cycling — including variations in atmospheric CO2 levels. Carbon isotope ratios also provide a window into the underlying physiology of phytoplankton cells. In this talk, I will discuss two complementary efforts to interpret these organic archives with greater constraints and specificity. First, I will present results from cultures and numerical modeling studies to revisit the fundamental factors controlling phytoplankton εP values. 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. We developed a revised theoretical model to address these missing and important features of carbon isotope fractionation across several dominant algal groups, enabling taxon- and environment-specific predictions. Second, I will discuss recent efforts to measure carbon isotope patterns within organic molecules (i.e., at different atomic positions) using high-resolution Orbitrap mass spectrometry. Position-specific carbon isotope ratios may serve as sensitive tracers of biogeochemical processes but are largely unexplored in environmental samples. I will present the first Orbitrap-based measurements of natural-abundance, position-specific carbon isotope variation in an amino acid isolated from a biological matrix, paving the way for new organic proxy approaches and interpretations.

About this Series:
The Department Lecture Series at EAPS at MIT is a series of weekly talks given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. For more information and Zoom password please contact Madelyn Musick: mmusick@mit.edu