Photochemical responses of the diatom Skeletonema costatum grown under elevated CO2 concentration to acute pH drops.

The variability of pH is a common occurrence in many aquatic environments, due to physical, chemical and biological processes. In coastal waters, lagoons, estuaries and inland waters, pH can change very rapidly (in seconds to hrs), in addition to undergoing daily and seasonal changes. At the same time, progressive ocean acidification caused by anthropogenic CO2 emission is superimposed on these spatial and temporal pH changes. Therefore, photosynthetic organisms are unavoidably subject to conspicuous pH variations at the surface of their cells. Whether the fact that cells already experience a large range of high frequency pH changes will affect their response to long term ocean acidification is still not known nor is it known whether the short term sensitivity to pH changes is affected by the pCO2 to which the cells are acclimated. We posed this latter open question as our experimental hypothesis: acclimation to seawater acidification affects response of phytoplankton to acute pH variations. The diatom Skeletonema costatum, which is commonly found in coastal and estuarine waters, where substantial short term pH changes occur, was selected to test the hypothesis. When the diatom was grown at both 390 (pH 8.2, low CO2, LC) and 1000 (pH 7.9, high CO2, HC) µatm CO2 for at least 20 generations, its photosynthetic responses to acute changes of pH (between 8.2 and 7.6) were investigated. The effective quantum yield of LC-grown cells decreased appreciably about 70% only when exposed to pH 7.6, which was not observed in pH 7.9 or 8.2 treatments, whereas it did not show significant changes in the HC-grown cells in any pH treatment. The non-photochemical quenching showed opposite trends. In general, our results indicated that while LC-grown cells are rather sensitive to the acidification, HC-grown cells are relatively unresponsive to it in terms of its photochemical performance.