INORGANIC CARBON ACQUISITION BY CHRYSOPHYTES

Twelve species, representing 12 families of the chrysophytes sensu lato, were tested for their ability to take up inorganic carbon. Using the pH-drift technique, CO₂ compensation points generally varied between 1 and 20 μmol · L⁻¹ with a mean concentration of 5 μmol · L⁻¹. Neither pH nor alkalinity affected the CO₂ compensation point. The concentration of oxygen had a relatively minor effect on CO₂-uptake kinetics, and the mean CO₂ compensation point calculated from the kinetic curves was 3.6 μmol · L⁻¹ at 10–15 kPa starting oxygen partial pressure and 3.8 μmol · L⁻¹ at atmospheric starting oxygen partial pressure (21 kPa). Similarly, uptake kinetics were not affected by alkalinity, and hence concentration of bicarbonate. Membrane inlet mass spectrometry (MIMS) in the presence and absence of acetazolamide suggested that external carbonic anhydrase in Dinobryon sertularia Ehrenb. and Synura petersenii Korschikov was either very low or absent. Rates of net HCO₃⁻ uptake were very low (∼5% of oxygen evolution) using MIMS and decreased rather than increased with increasing HCO₃⁻ concentration, suggesting that it was not a real uptake. The CO₂ compensation points determined by MIMS for CO₂ uptake and oxygen evolution were similar to those determined in pH-drift and were >1 μmol · L⁻¹. Overall, the results suggest that chrysophytes as a group lack a carbon-concentrating mechanism (CCM), or an ability to make use of bicarbonate as an alternative source of inorganic carbon. The possible evolutionary and ecological consequences of this are briefly discussed.