Subhas, A. | Search bioRxiv

bioRxiv2026

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 1.2x10-6 (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.

biochemistry↗

bioRxiv2026

The effects of elevated seawater pH and total alkalinity following dosing of sodium hydroxide in Calanus finmarchicus

Ocean Alkalinity Enhancement (OAE) is a marine carbon dioxide removal (mCDR) strategy that involves adding alkaline substances to surface waters to enhance CO2 uptake and storage. The dispersal of alkaline materials such as sodium hydroxide (NaOH) into seawater can cause rapid increases in pH and total alkalinity (TA) that substantially exceeds natural variability in marine environments. Such fluctuations may negatively affect marine life, especially small animals like copepods who cannot avoid OAE plumes and whose physiological processes could be disrupted by large and rapid shifts in seawater pH. To address knowledge gaps regarding potential biological impacts of OAE, we studied these effects in Calanus finmarchicus, a keystone copepod species in the Northwest Atlantic Ocean. We exposed C. finmarchicus from the late juvenile copepodite stages and adult females to NaOH-dosed seawater at pH 10.5 ([~]5,000 {micro}mol kg-1 TA) and pH 9.0 ([~]3,150 {micro}mol kg-1 TA) for durations that reflect expected short-term exposure times during field OAE deployments (pH 10.5: 1, 5, 10 minutes; pH 9.0: 1, 15, 30 minutes). None of the treatment combinations resulted in mortality immediately after the initial exposure. Individuals were monitored for survival for 72 hours post-exposure (hpe), and only one treatment group (juveniles exposed to pH 10.5 for 10 minutes) showed a significant reduction in final survival; no other pH-duration combination showed increased mortality. Effects on the ability to initiate an escape response were more substantial. Adult females treated with pH 10.5 for 5 or 10 minutes showed a significant reduction in escape response immediately after exposure. In contrast, juveniles showed no immediate change in escape response following exposure to pH 10.5 or pH 9.0, although juveniles exposed to pH 10.5 for 10 minutes exhibited reduced escape response at 24 hpe. Using microrespirometry, we measured oxygen consumption following a 10-minute exposure to pH 10.5 and detected no effect on routine metabolic rate immediately post-exposure or at 12 hpe. Overall, our results suggest that C. finmarchicus is relatively tolerant to short-term exposures to very high pH and alkalinity. Future work should prioritize longer-term exposure under more moderate ocean OAE conditions.

ecology↗