Species-specific influence of Pi-status on inorganic carbon acquisition in microalgae (Chlorophyceae)

Inorganic phosphorus (Pi) is often the primary limiting nutrient in freshwater ecosystems. Since Pi-limitation affects energy transduction, and inorganic carbon (Ci) acquisition can be energy demanding, Ci-acquisition strategies were compared in four species of green algae grown under Pi-replete and Pi-limited conditions predominantly at low and partly at high CO2. Although Pi-limitation was evident by the 10-fold higher cellular C:P ratio and enhanced phosphatase activity, it only decreased Ci-acquisition to a small extent. Nonetheless, the effects of Pi-limitation on both CO2 and HCO3- acquisition were demonstrated. Decreased CO2 acquisition under conditions of Pi limitation was mainly visible in the maximum uptake rate (Vmax) and, for the neutrophile Scenedesmus vacuolatus, in the affinity for CO2 acquisition. Discrimination against 13C was higher under Pi-limited, high CO2 conditions, compared with Pi-replete, high CO2 conditions, in Chlamydomonas acidophila and S. vacuolatus. In the pH-drift experiments, HCO3- acquisition was reduced in Pi-limited C. reinhardtii. In general, energy demanding bicarbonate uptake was indicated by the less strong discrimination against 13C under low CO2 conditions in the neutrophiles (HCO3- users), separating them from the acidophilic or acidotolerant species (CO2 users). The high variability of the influence of Pi supply among different green algal species is linked to their species-specific Ci-acquisition strategies.