FRISO (v1.0): A physiological carbon flux model for simulating algal ¹³C-fractionation

Abstract. The CO₂-dependaency of stable carbon isotope fractionation (ε_p) during marine algal carbon acquisition is widely used as a proxy for past atmospheric pCO₂. However, a mechanistic model linking ε_p and pCO₂ that explains observations in the modern ocean, culture experiments and across Pleistocene glacial-interglacial cycles is still lacking. Here, we present a multi-compartment model of an algal cell that simulates environmental control on passive CO₂ diffusion, active HCO_3⁻ transport, and ¹³C fractionation during C-fixation. Relative to previous models, our main new implementations include light-sensitive HCO_3⁻ acquisition and calcification, and daylength-dependent pyrenoid permeability, based on physiological expectations. The model is fitted to and tested with culture data of multiple dinoflagellate species and the coccolithophore Gephyrocapsa huxleyi. This shows robust performance across a wide range of parameter values from culture experiments, implying an appropriate representation of the mechanistic control of ¹³C-fractionation in algal organic matter, potentially paving the way for sedimentary work.