CARBONATE IONS APPEAR TO NEITHER INHIBIT NOR STIMULATE USE OF BICARBONATE IONS IN PHOTOSYNTHESIS BY ULVA-LACTUCA

Rates of photosynthesis by the marine macroalga Ulva lactuca were measured in a factorial experiment at five concentrations of HCO3- and CO3(2-) between 0.20 and 1.26 mol m-3, but very low concentrations of CO2. The results demonstrated that HCO3- was available for use, but an analysis of variance showed that CO3(2-) had neither an inhibiting nor a stimulating effect on rates of photosynthesis over this concentration range. Over the experiment, pH varied from 8.46 to 10.06 and this also had no significant effect on rates of photosynthesis. The lack of a stimulatory effect of high concentrations of CO3(2-) on the rate of photosynthesis at low concentrations of HCO3- was taken as circumstantial evidence for direct uptake of HCO3- rather than proton extrusion and external production of CO2. In the rockpools in which U. lactuca grows, pH values up to 10.35 have been recorded, and for much of the time, CO3(2-) was the major form of inorganic carbon available. The apparent lack of an ability to use CO3(2-) under these conditions suggests that direct use of CO3(2-) as a source of inorganic carbon for photosynthesis is unlikely to be widespread.