Transport and assimilation of inorganic carbon byLichina pygmaeaunder emersed and submersed conditions

SUMMARY Photosynthetic O2evolution by the upper littoral lichen,Lichina pygmaea(Lightf.) C.Ag., under light‐saturated conditions at 5 °C is saturated by the 2 mol m−3inorganic C found in seawater at pH 8.0. Photosynthesis is not reduced when pH is increased to pH 9.4, and is slightly reduced at pH 10.0, when submersed in seawater with 2 mol m−3inorganic C. The rate of photosynthesis at pH 10 greatly exceeds the rate of uncatalysed conversion of HCO3−. It is concluded that HCO3−is used in photosynthesis. Since extracellular carbonic anhydrase is present, it is possible that CO2enters the photobiont (Calothrix) cells even during HCO3use. pH drift experiments support the notion of HCO3−use. Emersed photosynthesis at 5 °C is more than half‐saturated by 35 Pa (normal atmospheric) CO2; the light‐ and CO2‐saturated emersed photosynthetic rate is not significantly different from the light and inorganic C‐saturated photosynthetic rate when submersed. Inorganic C diffusion from the thallus surface to the photobiont needs, at least under some conditions, carbonic anhydrase activity which permits HCO3−fluxes to supplement CO2movement. The CO2compensation partial pressure at 5 °C is 0.83 Pa, i.e. at the low range of values found for terrestrial cyanobacterial lichens. Dark14C‐inorganic C assimilation when submersed is a small fraction of the dark respiratory rate, consistent with the observed absence of diel CAM‐like variation in intracellular titratable acidity. The high value (−11.5%) of δ13C, the low CO2compensation partial pressure, and the relatively high affinity for inorganic C., are consistent with the operation of an inorganic C concentrating mechanism such as occurs in free‐living cyanobacteria and probably occurs in terrestrial cyanobacterial lichens and in most intertidal algae.