Proteomic responses under differing pH and pCO ₂ levels in the diatom Thalassiosira pseudonana are consistent with a hybrid carbon concentrating mechanism

The effects of ongoing anthropogenic climate change are not well known in marine diatoms, a key group of primary producers. In particular, detailed characterizations of their carbon concentrating mechanisms (CCM) are lacking, which limits the understanding of how changing ocean carbonate chemistry will impact global primary production. While the model diatom Thalassiosira pseudonana has been widely studied, contrasting results have prevented the clear elucidation of its CCM. A quantitative proteomic analysis was therefore performed across three experimental treatments (low pCO ₂ /high pH, high pCO ₂ /low pH, low pCO ₂ /low pH) to discern the specific roles of proteins that can be involved in CCMs as well as other cellular processes (e.g. pH and redox regulation). This analysis revealed a hybrid CCM consisting of both biophysical and biochemical steps that facilitate increased CO ₂ diffusion into the cell, the formation and transport of an organic carbon intermediate into the chloroplast, the subsequent decarboxylation of this intermediate, and the facilitated diffusion of inorganic carbon from the stroma into the pyrenoid-penetrating thylakoid. No evidence supporting alternative roles for the identified CCM proteins was found. As several aspects of this CCM require further investigation, common challenges inherent to CCM research are discussed and strategies to overcome them are suggested.