Effect of CO2 and phosphate deprivation on the control of nitrate, nitrite and ammonium metabolism in Chlorella

Chlorella vulgaris Beijerinck, strain 211/12, uses nitrate, nitrite and ammonium at pH 8.2 but not at pH 6.4 when kept under conditions of CO2‐deprivation, as observed in cell suspensions aerated with CO2‐free air during a 20–30. h period Most of the nitrate absorbed at pH 8.2, however, was not assimilated but was released into the external medium as nitrite and ammonium. Cells of Chlorella previously grown in phosphate‐limited continuous cultures were unable to absorb nitrate, nitrite or ammonium under conditions of phosphate starvation at either pH 6.4 or 8.2 in cell suspensions flushed with air containing 5% CO2, However, in cell suspensions flushed with CO2‐free air, the capacity of the alga to absorb and reduce nitrate and to excrete nitrite and ammonium at pH 8.2 was restored. It is hypothesized that in Chlorella the metabolism of nitrate, nitrite and ammonium is influenced by the availability of other nutrients and controlled by the cell's carbon status at the level of ion entry into the cell. With respect to nitrate this carbon‐dependent control is distinct and works independently of that triggered by the cell's nitrogen status.