| Summary: | 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 matm (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 (,100 generation) pH-auxostat, continuous cultures could be maintained at target pH when cell density was kept low (,2610 5 cells mL 21). After 3 months continuous culture, the C:N ratio was slightly decreased under high CO 2 conditions and red fluorescence per cell was slightly increased. However, no change was detected in photosynthetic efficiency (Fv/Fm) or functional cross section of PS II (sPSII). Elevated pCO2 has been predicted to be beneficial to diatoms due to reduced cost of carbon concentration mechanisms. There was reduced transcription of one putative d-carbonic anhydrase (CA-4) after 3 months growth at increased CO2 but 3 other d-CAs and the small subunit of RUBISCO showed no change. There was no evidence of adaptation or clade selection of T. pseudonana after,100 generations at elevated CO2. On the basis of this long-term culture, pH change of this magnitude in the future ocean
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