Variability of the pyrenoid-based CO₂ concentrating mechanism in hornworts (Anthocerotophyta)

Hornworts (Anthocerotophyta) are the only group of land plants with pyrenoid-containing chloroplasts. CO₂ exchange and carbon isotope discrimination values (Δ¹³C) values have previously demonstrated the presence of a CO₂ concentrating mechanism (CCM) in some pyrenoid-containing species. We have examined hornwort CCM function by using a combined fluorometer/mass spectrometer based technique to compare pyrenoid-containing (Phaeoceros Prosk. and Notothylas Sull.) and pyrenoid-lacking (Megaceros Campbell) hornworts, with the liverwort Marchantia polymorpha L. that has standard C₃ photosynthesis and a thalloid growth form similar to hornworts. We found that Notothylas has more CCM activity than Phaeoceros, and that Megaceros has the least CCM activity. Notothylas and Phaeoceros had compensation points from 11-13 parts per million (ppm) CO₂, lower K_0.5(CO₂) than Marchantia, negligible photorespiration, and they accumulate a pool of dissolved inorganic carbon (DIC) between 19-108 nmol mg^-1 chlorophyll. Megaceros had an intermediate compensation point of 31 ppm CO₂ (compared with 64 ppm CO₂ in Marchantia), a lower K_0.5(CO₂) than Marchantia, and some photorespiration, but no DIC pool. We also determined the catalytic rate of carboxylation per active site of Rubisco for all four species (Marchantia, 2.6 s^-1; Megaceros, 3.3 s^-1; Phaeoceros, 4.2 s^-1; Notothylas 4.3 s^-1), and found that Rubisco content was 3% of soluble protein for pyrenoid-containing species, 4% for Megaceros and 8% for Marchantia.