UTILIZATION OF INORGANIC CARBON BY ULVA-LACTUCA

Thalli discs of the marine macroalga Ulva lactuca were given inorganic carbon in the form of HCO3-, and the progression of photosynthetic O2 evolution was followed and compared with predicted O2 evolution as based on calculated external formation of CO2 (extracellular carbonic anhydrase was not present in this species) and its carboxylation (according to the K(m)(CO2) of ribulose-1,5-bisphosphate carboxylase/oxygenase), at two different pHs, assuming a photosynthetic quotient of 1. The K(m)(inorganic carbon) was some 2.5 times lower at pH 5.6 than at the natural seawater pH of 8.2, whereas V(max) was similar under the two conditions, indicating that the unnaturally low pH per se had no adverse effect on U. lactuca's photosynthetic performance. These results, therefore, could be evaluated with regard to differential CO2 and HCO3- utilization. The photosynthetic performance observed at the lower pH largely followed that predicted, with a slight discrepancy probably reflecting a minor diffusion barrier to CO2 uptake. At pH 8.2, however, dehydration rates were too slow to supply CO2 for the measured photosynthetic response. Given the absence of external carbonic anhydrase activity, this finding supports the view that HCO3- transport provides higher than external concentrations of CO2 at the ribulose-1,5-bisphosphate carboxylase/oxygenase site. Uptake of HCO3- by U. lactuca was further indicated by the effects of potential inhibitors at pH 8.2. The alleged band 3 membrane anion exchange protein inhibitor 4,4'-diisothiocyanostilbene-2,2'disulphonate reduced photosynthetic rates only when HCO3- (but not CO2) could be the extracellular inorganic carbon form taken up. A similar, but less drastic, HCO3--competitive inhibition of photosynthesis was obtained with Kl and KNO3. It is suggested that, under ambient conditions, HCO3- is transported into cells at defined sites either via facilitated diffusion or active uptake, and that such transport is the basis for elevated internal [CO2] at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation.