pH changes differently affect carbonic anhydrases (Ap-CAs) transcriptions in the marine toxic dinoflagellate Alexandrium pacificum.

Carbonic anhydrases (CAs) involve carbon dioxide (CO2) concentration in photosynthetic microalgae. They are known to function under low CO2 level in seawater, possibly affected by pH conditions. In the present study, we determined four novel CA genes, namely Ap-αCA, -βCA, -δCA1, and -δCA2, from the toxic marine dinoflagellate Alexandrium pacificum. The Ap-CAs were divided into three classes, alpha (α), beta (β), and delta (δ) via their phylogeny and motif features. Individual Ap-CA might be differently located to chloroplast, cytoplasm, mitochondria, or extracellular as judged by signal peptides. In addition, we evaluated the photosynthetic responses and transcriptional levels of the CAs and three photosynthesis genes (rbcL, atpB, and psbA) in the cells exposed to various pH conditions. The relative gene expressions of Ap-αCA and Ap-βCA were upregulated at higher pH conditions (pH 8.5 and 8.9) compared to the control (pH 8.1), but showed no significant changes in lower pH conditions (pH 7.3 and 7.7). In contrast, the Ap-δCAs expression was little changed. Expression patterns of rbcL, a carbon fixation gene marker, were similar to those of Ap-αCA and Ap-βCA, while there were no significant changes in atpB and psbA. These results suggest that photosynthetically available CO2 levels may decrease with increasing pH conditions, and the CAs function as a carbon concentration mechanism in marine environments.