Abstract
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.
Highlights
The concentration of H+ in water are subjected to dynamic changes that occur over time scales ranging from seconds to seasons
Superimposed on such ‘natural’ variations, the average pH of the world’s oceans is declining at a rate of about 0.002 yr−1, which could lead to a potential drop of 0.3 to 0.4 units by 2100, as a consequence of the increasing atmospheric CO2 concentration associated with anthropogenic CO2 emissions (IGBP et al 2013)
Acute exposures to acidification at pH 7.6 had different impacts on the cells that were acclimated to present conditions (LC) than from those acclimated to elevated CO2 (HC) concentrations, with the former being much more susceptible to changes in their quantum yield and Non-photochemical quenching (NPQ)
Summary
The concentration of H+ in water (and its negative log, the pH) are subjected to dynamic changes that occur over time scales ranging from seconds to seasons. Aquat Biol 23: 109–118, 2015 oligotrophic open oceans, daily and yearly pH variations are typically limited to a few tens of a unit (Dore et al 2009) Superimposed on such ‘natural’ variations, the average pH of the world’s oceans is declining at a rate of about 0.002 yr−1, which could lead to a potential drop of 0.3 to 0.4 units by 2100, as a consequence of the increasing atmospheric CO2 concentration associated with anthropogenic CO2 emissions (IGBP et al 2013). Typical changes linked to ocean acidification (OA) are increased concentrations of pCO2, H+ and HCO3−, decreased concentrations of CO32− and decreases in the CaCO3 saturation state (Gattuso et al 2010) These changes will lead to a decrease in the alkalinity of seawater, and the consequent decrease in buffering capacity will increase the magnitude of short-term pH changes in the future. In combination with eutrophication and hypoxia events, anthropogenic acidification may occur earlier and at a more accelerated rate in coastal and estuarine waters than in pelagic environments (Cai et al 2011)
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