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

Katharine J Crawfurd,Glen L Wheeler,Emily J Baxter,Ian Joint,John A Raven,Howard Browman

doi:10.1371/journal.pone.0026695

Katharine J Crawfurd, Glen L Wheeler + Show 4 more

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https://doi.org/10.1371/journal.pone.0026695

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Abstract

The effect of ocean acidification conditions has been investigated in cultures of the diatom Thalassiosira pseudonana CCMP1335. Expected end-of-the-century pCO2 (aq) concentrations of 760 µatm (equivalent to pH 7.8) were compared with present-day condition (380 µatm 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 (<2×105 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 (Fv/Fm) or functional cross section of PS II (σPSII). 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 δ-carbonic anhydrase (CA-4) after 3 months growth at increased CO2 but 3 other δ-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 may have little effect on T. pseudonana in the absence of genetic adaption.

Highlights

Anthropogenic emissions of carbon dioxide (CO2) are leading to rapid changes of ocean pH. This is commonly referred to as ocean acidification [1] because, as CO2 dissolves in the surface ocean, aqueous carbon dioxide (CO2) reacts with water to form carbonic acid (H2CO3), which is a weak acid. This change in the surface ocean is potentially important for phytoplankton because the pH of the growing medium changes, and because there will be an increase in the concentration of CO2 – the substrate for RUBISCO
Results pH control in aerated batch cultures The ability of T. pseudonana cultures to change the carbonate system within the culture media was monitored by measuring pH in batch cultures (f/2 medium)
We concluded that the addition of organic buffers was not an appropriate methodology to study the longterm effects of ocean acidification on T. pseudonana

Summary

Introduction

Anthropogenic emissions of carbon dioxide (CO2) are leading to rapid changes of ocean pH This is commonly referred to as ocean acidification [1] because, as CO2 dissolves in the surface ocean, aqueous carbon dioxide (CO2 (aq)) reacts with water to form carbonic acid (H2CO3), which is a weak acid. This change in the surface ocean is potentially important for phytoplankton because the pH of the growing medium (seawater) changes, and because there will be an increase in the concentration of CO2 (aq) – the substrate for RUBISCO. A global study of individual coccolith mass in ocean sediments from the past forty thousand years broadly supports the hypothesis that calcification is likely to decline with a decrease in ocean CO322 concentrations [5]

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