Abstract
In Ankistrodesmus braunii, in the absence of CO(2), i.e. in CO(2)-free air or N(2), photosynthetic nitrate uptake and nitrate reduction were inhibited, especially at low pH. Under such conditions, glucose stimulated nitrate uptake and reduction to almost the same level in the pH range between 6 and 8.5. CO(2) at 0.03% effected an intermediate pH dependence of nitrate uptake; saturating CO(2) concentration (more than 1%) eliminated the pH dependence, as did glucose, but the rates were enhanced compared with glucose. Glucose and, even more, CO(2), drastically reduced the release of nitrite and ammonia to the medium, the stoichiometry between alkalinization of the medium and nitrate uptake (OH(-)/NO(3) (-)) approached 1.Due to the lack of storage vacuoles in Ankistrodesmus, nitrate uptake and nitrate reduction were closely coupled processes whose experimental separation is difficult. The relieving effect of glucose and CO(2) suggests a carrier-mediated nitrate uptake which is more limiting than nitrate reduction and is sensitive to low pH, but which is stabilized by some intermediate originating from an active carbon metabolism.