Alkalization of the Medium by Unicellular Green Algae during Uptake Dissolved Inorganic Carbon

摘要

WhenChlorella vulgaris11h,Chlorella vulgarisC-l,Chlamydomonas reinhardtii,Chlamydomonas moewusii,Scenedesmus obliquus, orDunaliella tertiolectawere illuminated in with 0.5 mM NaHCO3, the pH of the medium increased in a few minutes from 6 to about 9 or 10. The alkalization, which was accompanied by O2evolution, was dependent on light, external dissolved inorganic carbon (DIC) as HCO-3, and algae grown or adapted to a low, air-level CO2in order to develop a DIC concentrating mechanism. There was little pH increase by algae without a DIC concentrating process from growth on 3%CO2in air. Photosynthetic O2evolution without alkalization occurred using either internal DIC or external CO2at acidic pH. The PH increase stopped between pH 9 to 10, but the alkalization would restart upon re-acidification between pH 6 and 8. Alkalization was suppressed by the carbonic anhydrase inhibitors, acetazolamide, ethoxyzolamide or carbon oxysulfide. The pH increase appeared to be the consequence of the external conversion of HCO−3into CO2plus OH−during photosynthesis by cells with a high affinity for CO2uptake. Cells grown on high CO2to suppress the DIC pump, when given low levels of HCO−3in the light, acidified the medium from pH 10 to 7. Air adaptedScenedesmuscells with a HCO−3pump, as well as a CO2pump, alkalized the medium very rapidly in the light to a pH of over 10, as well as slower in the dark or in the light with DCMU or without external DIC and O2evolution. Alkalization of the medium during photosynthetic DIC uptake by algae has been considered to be part of the global carbon cycle for converting H2CO3to HCO−3and for the formation of carbonate salts by calcareous algae from the alkaline conversion of bicarbonate to carbonate. These processes seem to be a consequence of the algal CO2concentratin