Effects of elevated pCO2 on physiological performance of marine microalgae Dunaliella salina (Chlorophyta, Chlorophyceae

The present study was conducted to determine the effects of elevated pCO(2) on growth, photosynthesis, dark respiration and inorganic carbon acquisition in the marine microalga Dunaliella salina. To accomplish this, D. salina was incubated in semi-continuous cultures under present-day CO2 levels (390 mu atm, pH(NBS): 8.10), predicted year 2100 CO2 levels (1 000 mu atm, pH(NBS): 7.78) and predicted year 2300 CO2 levels (2 000 mu atm, pH(NBS): 7.49). Elevated pCO(2) significantly enhanced photosynthesis (in terms of gross photosynthetic O-2 evolution, effective quantum yield (Delta F/F'(m)), photosynthetic efficiency (alpha), maximum relative electron transport rate (rETR(max)) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity) and dark respiration of D. salina, but had insignificant effects on growth. The photosynthetic O-2 evolution of D. salina was significantly inhibited by the inhibitors acetazolamide (AZ), ethoxyzolamide (EZ) and 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), indicating that D. salina is capable of acquiring HCO3- via extracellular carbonic anhydrase and anion-exchange proteins. Furthermore, the lower inhibition of the photosynthetic O-2 evolution at high pCO(2) levels by AZ, EZ and DIDS and the decreased carbonic anhydrase showed that carbon concentrating mechanisms were down-regulated at high pCO(2). In conclusion, our results show that photosynthesis, dark respiration and CCMs will be affected by the increased pCO(2)/low pH conditions predicted for the future, but that the responses of D. salina to high pCO(2)/low pH might be modulated by other environmental factors such as light, nutrients and temperature. Therefore, further studies are needed to determine the interactive effects of pCO(2), temperature, light and nutrients on marine microalgae.