Functional responses of smaller and larger diatoms to gradual CO₂ rise

Diatoms and other phytoplankton groups are exposed to abrupt changes in pCO₂, in waters in upwelling areas, near CO₂ seeps, or during their blooms; or to more gradual pCO₂ rise through anthropogenic CO₂ emissions. Gradual CO₂ rises have, however, rarely been included in ocean acidification (OA) studies. We therefore compared how small (Thalassiosira pseudonana) and larger (Thalassiosira weissflogii) diatom cell isolates respond to gradual pCO₂ rises from 180 to 1000 μatm in steps of ~40 μatm with 5-10 generations at each step, and whether their responses to gradual pCO₂ rise differ when compared to an abrupt pCO₂ rise imposed from ambient 400 directly to 1000 μatm. Cell volume increased in T. pseudonana but decreased in T. weissflogii with an increase from low to moderate CO₂ levels, and then remained steady under yet higher CO₂ levels. Growth rates were stimulated, but Chl a, particulate organic carbon (POC) and cellular biogenic silica (BSi) decreased from low to moderate CO₂ levels, and then remained steady with further CO₂ rise in both species. Decreased saturation light intensity (I_k) and light use efficiency (α) with CO₂ rise in T. pseudonana indicate that the smaller diatom becomes more susceptible to photoinhibition. Decreased BSi/POC (Si/C) in T. weissflogii indicates the biogeochemical cycles of both silicon and carbon may be more affected by elevated pCO₂ in the larger diatom. The different CO₂ modulation methods resulted in different responses of some key physiological parameters. Increasing pCO₂ from 180 to 400 μatm decreased cellular POC and BSi contents, implying that ocean acidification to date has already altered diatom contributions to carbon and silicon biogeochemical processes.