Investigating the carbon concentrating mechanism of the marine diatom Phaeodactylum tricornutum through kinetic modeling and gene expression analysis

Marine diatoms play a critical role in the global carbon cycle, where they are responsible for 20% of all primary production. RubisCO, the rate limiting enzyme in carbon fixation, has a half-saturation constant many times higher than oceanic CO2 concentrations. In order to overcome this limitation, diatom species have evolved a diverse array of highly efficient carbon concentrating mechanisms. Increasing our understanding of these mechanisms provides a foundation to improve genetic engineering of these organisms for biofuel production and increased carbon sequestration, as well as a basis to potentially improve the efficiency of photosynthesis in terrestrial plants.In this study, the carbon concentrating mechanism of the diatom species Phaeodactylum tricornutum is investigated through kinetic modeling of potential pathways and analysis of differential gene expression of CCM-related genes. An existing kinetic model is rebuilt and expanded to explore proposed carbon-concentrating mechanisms and provide predictive values of carbon fluxes through the system at varying external conditions. The feasibility of each of three potential mechanisms is evaluated. Through gene expression analysis, potential major regulators of the carbon concentrating mechanism are identified.This investigation finds that the presence of bicarbonate transporters on the plasmalemma, CER, and chloroplast membranes is needed for the CCM to achieve expected carbon uptake and photosynthetic fluxes, but a bicarbonate transporter on the PPC membrane is not necessary. Additionally, the down-regulation of a chloroplast-membrane targeted bicarbonate transporter at sub-atmospheric CO2 indicates that a secondary CCM pathway takes over under low carbon stress.