Carbon concentrating mechanism and growth response of the diatom P. tricornutum to changes in Zn and carbonate chemistry

The efficiency of marine diatom carbon concentrating mechanisms (CCMs) play a critical role in photosynthesis and enable cells to maintain rapid growth rates under a variety of environmental conditions. To assess the plasticity of the model diatom P. tricornutums CCMs, specifically carbonic anhydrase (CA) enzymes and bicarbonate transporters, we measured growth response, bulk CA activity, and corresponding shifts in the proteome under a range of Zn and pCO2 conditions in culture. CA activity increased with Zn availability and decreased with pCO2. A positive growth effect was observed due to Zn addition and increasing pCO2 from 200 to 400 ppm, however growth rate decreased as pCO2 further increased to 750 ppm. Across the six treatments, the protein abundance of ISIP2A, which functions to bring Fe into the cell via a FeCO3 complex and is used as a biomarker for Fe stress, demonstrated an inverse relationship with [CO32-], consistent with its role as a phytotransferrin. Under conditions of Zn limitation ([Zn2+] = 0.3 pM), the cell appeared to allocate this metal away from CA, instead relying on a Mn-CA with a 100-fold lower intrinsic activity than that of the primary Zn-CA, as calculated using paired abundance-activity measurements. We further observed a continued increase in bicarbonate transport protein abundance after CA activity plateaued at 1.2×10-6 (reactions sec-1cell-1), suggesting any deficit in DIC required to maintain high growth rates is accomplished through HCO3- uptake. We hypothesize that bicarbonate uptake and CO2 diffusion operate in tandem via CA enzymatic activity to supply adequate CO2 for photosynthesis.