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 (PhaeocerosProsk. and Notothylas Sull.) and pyrenoid-lacking (Megaceros Campbell) hornworts, with the liverwort Marchantia polymorphaL. 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 K0.5(CO2) than Marchantia, negligible photorespiration, and they accumulate a pool of dissolved inorganic carbon (DIC) between 19-108 nmol mg^-1 chlorophyll. Megaceroshad an intermediate compensation point of 31 ppm CO2 (compared with 64 ppm CO2 in Marchantia), a lower K0.5(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.