Functional responses of smaller and larger diatoms to gradual CO2 rise

Diatoms and other phytoplankton groups are exposed to abrupt changes in pCO(2), in waters in upwelling areas, near CO2 seeps, or during their blooms; or to more gradual pCO(2) 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 pCO(2) rises from 180 to 1000 palm in steps of similar to 40 mu atm with 5-10 generations at each step, and whether their responses to gradual pCO(2) rise differ when compared to an abrupt pCO(2) rise imposed from ambient 400 directly to 1000 palm. 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 (alpha) 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 pCO(2) in the larger diatom. The different CO2 modulation methods resulted in different responses of some key physiological parameters. Increasing pCO(2) from 180 to 400 mu atm decreased cellular POC and BSi contents, implying that ocean acidification to date has already altered diatom contributions to carbon and silicon biogeochemical processes. (C) 2019 Elsevier B.V. All rights reserved.