The role of carbonic anhydrase in photosynthesis and the activity of the carbon-concentrating-mechanism in bryophytes of the class Anthocerotae

The role of carbonic anhydrase in the carbon-concentrating-mechanism of bry ophytes of the class Anthocerotae was investigated by comparing the gas-exc hange characteristics of material which had been incubated in the membrane- permeable Carbonic Anhydrase inhibitor ethoxyzolamide, with those of untrea ted material and material which had been incubated in buffer solution. In P haeoceros laevis (Anthocerotae), incubation in ethoxyzolamide caused a depr ession in the rate of gross assimilation and a decrease in CO2 affinity bey ond that which could be attributed to increased diffusion limitation. A ran ge of liverworts and mosses, in which a carbon-concentrating-mechanism is a bsent, were also investigated. These showed no depression of rates of gross assimilation after incubation in ethoxyzolamide relative to those of untre ated material. The CO2 compensation point and CO2 uptake characteristics of Phaeoceros laevis were significantly affected by incubation in ethoxyzolam ide. Values of CO2 compensation point for Phaeoceros laevis rose from 2.5 P a, after incubation in buffer, to 20 Pa after incubation in ethoxyzolamide. The CO2 compensation point for the liverworts Pellia epiphylla and Marchan tia polymorpha was not significantly affected by incubation in ethoxyzolami de. Measurements of the release of CO2 at the end of a short (15 min) perio d of illumination revealed that, after suppression of carbonic anhydrase ac tivity, the rapid release of a CO2 pool occurred in Phaeoceros laevis but n ot in the liverworts. There were also significant differences between value s for fractionation measured in units per mil (parts per thousand), measure d instantaneously, for Phaeoceros laevis incubated in ethoxyzolamide, compa red with fractionation values for this species after incubation in buffer. Incubation in ethoxyzolamide caused fractionation values to rise from 12.4- 22.7 parts per thousand, indicating that the carbon-concentrating-mechanism of this species had been inactivated. Incubation in ethoxyzolamide had no effect on fractionation values for the liverworts. The convexity of the lig ht saturation curves of liverworts and Phaeoceros laevis was also investiga ted, but there were no differences between groups before or after the two t reatments. The data indicate an important role for carbonic anhydrase in th e functioning of the carbon-concentrating-mechanism in the Anthocerotae.