Andrews Hanson and Badger (Functional Plant Biology Volume 29 Number 2 & 3 2002) 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 polymorphaL. that has standard C3 photosynthesis and a thalloid growth form similar to hornworts. They 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, with 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 K0.5 (CO2) than Marchantia, and some photorespiration, but no DIC pool. They 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.
