Photosynthesis of Ulva fasciata. IV. pH, carbonic anhydrase and inorganic carbon conversions in the unstirred layer

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Abstract
The common marine macroalga Ulva was found to have a surface pH of about 10 during photosynthesis. Under such a condition, the equilibrium CO2 concentration within the unstirred layer would be below reported CO2 compensation points, and dehydration of HCO3 could not occur. Even at a compensation point approaching zero, uncatalysed rates of HCO3 to CO2 conversion within the unstirred layer volume could not support photosynthetic rates as measured under stirred conditions in the presence of a carbonic anhydrase inhibitor. Based on this, it is concluded that Ulva takes up HCO3. It is likely that HCO3 uptake leads to high internal CO2 levels which, in turn, suppress photorespiration and thus cause this plant's efficient gas exchange features. Carbonic anhydrase activity was measurable only in plant extracts. However, inhibitor studies suggest the presence of a surface enzyme. The possible functions of extracellular carbonic anhydrase in Ulva are assessed in terms of CO2 hydration during emergence and a possible HCO3, transport system.