• carbon dioxide;
  • carbon-concentrating mechanisms;
  • freshwater algae;
  • pH-drift;
  • photosynthesis;
  • pyrenoids

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.