Regulation of the mechanism for HCO3− use by the inorganic carbon level in Porphyra leucosticta Thur. in Le Jolis (Rhodophyta)
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The capacity for HCO3− use by Porphyra leucosticta Thur. in Le Jolis grown at different concentrations of inorganic carbon (Ci) was investigated. The use of HCO3− at alkaline pH by P. leucosticta was demonstrated by comparing the O2 evolution rates measured with the O2 evolution rates theoretically supported by the CO2 spontaneously formed from HCO3−. Both external and internal carbonic anhydrase (CA; EC 126.96.36.199) were implied in HCO3− use during photosynthesis because O2 evolution rates and the increasing pH during photosynthesis were inhibited in the presence of azetazolamide and ethoxyzolamide (inhibitors for external and total CA respectively). Both external and internal CA were regulated by the Ci level at which the algae were grown. A high Ci level produced a reduction in total CA activity and a low Ci level produced an increase in total CA activity. In contrast, external CA was increased at low Ci although it was not affected at high Ci. Parallel to the reduction in total CA activity at high Ci is a reduction in the affinity for Ci, as estimated from photosynthesis versus Ci curves, was found. However, there was no evident relationship between external CA activity and the capacity for HCO3− use because the presence of external CA became redundant when P. leucosticta was cultivated at high Ci. Our results suggest that the system for HCO3− use in P. leucosticta is composed of different elements that can be activated or inactivated separately. Two complementary hypotheses are postulated: (i) internal CA is an absolute requirement for a functioning Ci-accumulation mechanism; (ii) there is a CO2 transporter that works in association with external CA.
Key wordsCarbonic anhydrase Inorganic carbon Macroalga pH Photosynthesis Porphyra
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