Effect of nitrogen source and growth rate on phytoplankton‐mediated changes in alkalinity1

Continuous cultures of the marine chrysophyte Dunaliella tertiolecta were grown on four nitrogen sources: NO3 NO2−, NH4+, and urea. Alkalinity changes were consistent with a simple stoichiometric model in which OH production is balanced by NO3− and NO2− uptake, H+ production is balanced by NH4+ uptake and no change occurs when the uncharged species urea is assimilated. Neither the influent N concentration nor the growth rate had any effect on the 1:1 stoichiometry between N uptake and alkalinity change. These results preclude the possibility of excretion of an organic acid (e.g. glycolic acid) stronger than carbonic acid. However, excretion of a weak organic acid or a salt of a strong organic acid cannot be ruled out. In general, the results are consistent with the notion that excretion of glycolic acid by healthy marine phytoplankton cells is minimal.