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

When Chlorella vulgaris 11h, Chlorella vulgaris C-l, Chlamydomonas reinhardtii, Chlamydomonas moewusii, Scenedesmus obliquus, or Dunaliella tertiolecta were 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 O2 evolution, was dependent on light, external dissolved inorganic carbon (DIC) as HCO-3, and algae grown or adapted to a low, air-level CO2 in order to develop a DIC concentrating mechanism. There was little pH increase by algae without a DIC concentrating process from growth on 3% CO2 in air. Photosynthetic O2 evolution without alkalization occurred using either internal DIC or external CO2 at 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−3 into CO2 plus OH− during photosynthesis by cells with a high affinity for CO2 uptake. Cells grown on high CO2 to suppress the DIC pump, when given low levels of HCO−3 in the light, acidified the medium from pH 10 to 7. Air adapted Scenedesmus cells with a HCO−3 pump, as well as a CO2 pump, 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 O2 evolution. Alkalization of the medium during photosynthetic DIC uptake by algae has been considered to be part of the global carbon cycle for converting H2CO3 to HCO−3 and 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 CO2 concentrating process.