Antarctic Phytoplankton down-regulate Their Carbon-Concentrating Mechanisms under High CO2 with no Change in Growth Rates

High-latitude oceans, in particular the coastal Western Antarctic Peninsula (WAP) region of the Southern Ocean, are experiencing a rapidly changing environment due to rising surface ocean temperatures and CO₂ concentrations. However, the direct effect of increasing CO₂ on polar ocean primary production is unclear, with a number of experiments showing conflicting results. It has been hypothesized that increased CO₂ may cause a reduction of the energy-intensive carbon concentrating mechanism (CCM) in phytoplankton, and these energy savings may lead to increased productivity. To test this hypothesis, we incubated natural phytoplankton communities in the WAP under high (800 ppm), current (400 ppm) and low (100 ppm) CO₂ for 2 to 3 wk during the austral spring-summer of 2012/2013. In 2 incubations with diatom-dominated phytoplankton assemblages, high CO₂ led to a clear down-regulation of CCM activity, as evidenced by an increase in half-saturation constants for CO₂, a decrease in external carbonic anhydrase activity and a higher biological fractionation of stable carbon isotopes. In a third incubation, there was no observable regulation of the CCM. We did not observe a significant effect of CO₂ on growth rates or community composition in the diatom-dominated communities. The lack of a measureable effect on growth despite CCM down-regulation is likely explained by a very small energetic requirement to concentrate CO₂ and saturate Rubisco at low temperatures.