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Variability in pH is a common occurrence in many aquatic environments, due to physical, chem- ical and biological processes. In coastal waters, la- goons, estuaries and inland waters, pH can change very rapidly (within seconds or hours) in addition to daily and seasonal changes. At the same time, pro- gressive ocean acidification caused by anthropogenic CO2 emissions is superimposed on these spatial and temporal pH changes. Photosynthetic organisms are therefore unavoidably subject to significant pH varia- tions at the cell surface. Whether this will affect their response to long-term ocean acidification is still un- known, nor is it known whether the short-term sensi- tivity to pH change is affected by the pCO2 to which the cells are acclimated. We posed the latter open question as our experimental hypothesis: Does accli- mation to seawater acidification affect the response of phytoplankton to acute pH variations? The diatom Skeletonema costatum, commonly found in coastal and estuarine waters where short-term acute changes in pH frequently occur, was selected to test the hypo thesis. Diatoms were 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, and photosynthetic re- sponses to short-term and acute changes in pH (be- tween 8.2 and 7.6) were investigated. The effective quantum yield of LC-grown cells decreased by ca. 70% only when exposed to pH 7.6; this was not ob- served when exposed to pH 7.9 or 8.2. HC-grown cells did not show significant responses in any pH treat- ment. Non-photochemical quenching showed opposite trends. In general, our results indicate that while LC- grown cells are rather sensitive to acidification, HC- grown cells are relatively unresponsive in terms of photochemical performance.

Photochemical responses of the diatom Skeletonema costatum grown under elevated CO2 concentrations to short-term changes in pH