Proteomic responses under differing pH and pCO2 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. Specifically, detailed characterizations of their carbon-concentrating mechanisms (CCMs) 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 pCO2/high pH, high pCO2/low pH, low pCO2/low pH) to discern the specific roles of proteins that can be involved in both CCMs and other cellular processes (e.g. pH regulation). The results suggest a hybrid CCM consisting of both biophysical and biochemical steps that facilitate increased CO2 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 into the pyrenoid-penetrating thylakoid. No evidence supporting roles for candidate CCM proteins in pH regulation, cyclic electron transport, or excess energy dissipation was found. As each CCM step still requires functional validation, common challenges inherent to CCM research are discussed and strategies to overcome them are suggested.