Inorganic Carbon Utilization And Environmental Regulation Of Carbonic Anhydrase In Nitzschia Closterium Minutissima

Marine diatom is a major group of coastal phytoplankton, which had a higher photosynthetic rate under low CO₂ concentration. This mainly owed to its carbon concentrating mechanism and carbonic anhydrase. Inorganic carbon utilization and environmental regulation of carbonic anhydrase in Nitzschia closterium minutissima were investigated in this paper, and the main results were as follows:The algae had higher extracellular and intracellular CA activity, and extracellurlar CA activity was 1.2 times of intracellular CA activity; pH compensation point was 10.2; Under alkaline conditions, the actual photosynthetic O₂ exchange rates was 1.6 times of the theoretical calculated maximum rates supported solely by the CO₂ supply from the spontaneous dehydration of HCO₃^-. The P-C curve showed that K_0.5(CO₂) had a value of 71.0μM and 0.7μM at pH6.0 and pH8.2, respectively. Under alkaline conditions, photosynthesis was significantly inhibited by AZ, EZ and DIDS.The specific growth rate increased with the incremental N and P concentration. The extracellular CA activity was not detected at low N concentration, but had a significant increase under high N concentration. P concentration had insignificant effect on extracellular CA activity, however resulted in the rise in intracellular CA activity. V_max and K_0.5(CO₂) were elevated in high N and P concentration. Both extracellular and intracellular CA were significantly affected by light intensity. V_max and K_0.5(CO₂) decreased with the increase of light intensity or temperature. The specific growth rate reached the maximal value at 20℃.CA activity increased with the temperature rise.ABS/RC,DI_o/RC and Vj obviously declined, and ET_o/RC,φ_Eo,ψ_o andφ_Po increased in high N concentration(882μmolL^-1)compared with natural seawater (2.1μmolL-1 N). ABS/RC,DI_o/RC,TRoRC and Vj significantly declined, andφ_Eo andψ_o increased in high P concentration (18.1μmolL-1), different P concentrations had no significant effect onφ_Po. High light intensity resulted in the decrease of TRo/RC,φ_Eo,ψ_o andφ_Po, and the increase in ABS/RC,DI_o/RC,TRo/RC and Vj . ABS/RC,DI_o/RC and TRo/RC firstly decreased and then increased from 15℃to 25℃. Temperature had no significant effect onφ_Po, but increasedφ_Eo andψ_o at 20℃and 25℃.In conclusion, the indirect HCO₃^- uptake mediated by external carbonic anhydrase was the main mode of inorganic carbon utilization, and direct uptake of HCO₃^- also existed in Nitzschia closterium minutissima. Environmental effects on inorganic carbon utilization in Nitzschia closterium minutissima were achieved by regulating CO₂ affinity and CA activity. Photosynthesis adapted to change of temperature,light intensity and nutrient levels through modulating energy cascade from light absorption to electron transport.