The Response of Thalassiosira pseudonana to Long-Term Exposure to Increased CO2 and Decreased pH

The effect of ocean acidification conditions has been investigated in cultures of the diatom Thalassiosira pseudonana CCMP1335. Expected end-of-the-century pCO(2) (aq) concentrations of 760 mu atm (equivalent to pH 7.8) were compared with present-day condition (380 matm CO2, pH 8.1). Batch culture pH changed rapidly because of CO2 (aq) assimilation and pH targets of 7.8 and 8.1 could not be sustained. Long-term (similar to 100 generation) pH-auxostat, continuous cultures could be maintained at target pH when cell density was kept low (<2X10(5) cells mL(-1)). After 3 months continuous culture, the C:N ratio was slightly decreased under high CO2 conditions and red fluorescence per cell was slightly increased. However, no change was detected in photosynthetic efficiency (F-v/F-m) or functional cross section of PS II (sigma(PSII)). Elevated pCO(2) has been predicted to be beneficial to diatoms due to reduced cost of carbon concentration mechanisms. There was reduced transcription of one putative delta-carbonic anhydrase (CA-4) after 3 months growth at increased CO2 but 3 other delta-CAs and the small subunit of RUBISCO showed no change. There was no evidence of adaptation or clade selection of T. pseudonana after similar to 100 generations at elevated CO2. On the basis of this long-term culture, pH change of this magnitude in the future ocean may have little effect on T. pseudonana in the absence of genetic adaption.