<p>Previous studies have shown that the carbon concentrating mechanism (CCM) of the red alga <i>Pyropia haitanensis</i> may be involved in the high-temperature stress response, but the detailed mechanism remains unclear. To this end, we selected a high-temperature-tolerant strain, W28, and a high-temperature-sensitive strain, WO14-1, as experimental materials and investigated the roles played by the key components of the CCM in the high-temperature (30&#xa0;°C) response in both strains. The results showed that high temperature significantly suppressed the net photosynthesis rate of <i>P. haitanensis</i> and that the rate was further decreased by the addition of CCM inhibitors. The extracellular carbonic anhydrases (eCAs) were more important than the intracellular carbonic anhydrases (iCAs) and bicarbonate transporters in the high-temperature responses of <i>P. haitanensis</i>. In addition, high temperature substantially reduced the growth rate of both strains and acetazosulfanilamide (AZ), an inhibitor of eCAs, further decreased the growth of both strains; the growth of WO14-1 was decreased earlier and to a greater degree than that of W28. These results indicate that the high-temperature tolerance of <i>P. haitanensis</i> was substantially weakened by the inhibition of eCAs. However, the biochemical and transcriptomic results of the present study suggest no clear trends regarding the effects of AZ on the activities of two important antioxidant enzymes and the content of H₂O₂. Taken together, we inferred that eCAs play an important role in the high-temperature response of <i>P. haitanensis</i>, but the specific mechanism needs to be further investigated.</p>