Functional responses of smaller and larger diatoms to gradual CO rise.

Diatoms and other phytoplankton groups are exposed to abrupt changes in pCO, in waters in upwelling areas, near CO seeps, or during their blooms; or to more gradual pCO rise through anthropogenic CO emissions. Gradual CO 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 rises from 180 to 1000 μatm in steps of ~40 μatm with 5-10 generations at each step, and whether their responses to gradual pCO rise differ when compared to an abrupt pCO 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 CO levels, and then remained steady under yet higher CO levels. Growth rates were stimulated, but Chl a, particulate organic carbon (POC) and cellular biogenic silica (BSi) decreased from low to moderate CO levels, and then remained steady with further CO rise in both species. Decreased saturation light intensity (I) and light use efficiency (α) with CO 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 in the larger diatom. The different CO modulation methods resulted in different responses of some key physiological parameters. Increasing pCO 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.