Abstract: Periplasmic carbonic anhydrase (pCA) is a key enzyme in CCM and the photosynthetic carbon fixation of algae. When algae in high CO2 concentration are transferred to low CO2 concentration condition, perplasmic carbonic anhydrase is induced in them. We studied the pCA molecular localization and activity inducement, using the gold immunolocalization technology and strict pH regulation. The results showed that pCAs were mainly located in the periplasmic space (between cell wall and plasmatic membrane), with some pCAs on the cell walls and a little PCAs outside cell walls, implying that pCAs could penetrate from periplasmic space into outside of cell walls. With both CO2 inducement and strict pH regulation, pCA activity was increased. The higher the pH increased, the higher the pCA activity became, indicating that the CO2 inducement of pCA was closely related to the regulation of pH.