Physiological and biochemical responses of Thalassiosira weissflogii (diatom) to seawater acidification and alkalization

Increasing atmospheric pCO₂ leads to seawater acidification, which has attracted considerable attention due to its potential impact on the marine biological carbon pump and function of marine ecosystems. Alternatively, phytoplankton cells living in coastal waters might experience increased pH/decreased pCO₂ (seawater alkalization) caused by metabolic activities of other photoautotrophs, or after microalgal blooms. Here we grew Thalassiosira weissflogii (diatom) at seven pCO₂ levels, including habitat-related lowered levels (25, 50, 100, and 200 μatm) as well as present-day (400 μatm) and elevated (800 and 1600 μatm) levels. Effects of seawater acidification and alkalization on growth, photosynthesis, dark respiration, cell geometry, and biogenic silica content of T. weissflogii were investigated. Elevated pCO₂ and associated seawater acidification had no detectable effects. However, the lowered pCO₂ levels (25 ∼100 μatm), which might be experienced by coastal diatoms in post-bloom scenarios, significantly limited growth and photosynthesis of this species. In addition, seawater alkalization resulted in more silicified cells with higher dark respiration rates. Thus, a negative correlation of biogenic silica content and growth rate was evident over the pCO₂ range tested here. Taken together, seawater alkalization, rather than acidification, could have stronger effects on the ballasting efficiency and carbon export of T. weissflogii.