Carbonic Anhydrase Localization in Charophycean Green Algae: Ecological and Evolutionary Significance

Two carbonic anhydrase (CA) antibodies, one to the primarily periplasmic alpha‐type enzyme of the chlorophycean Chlamydomonas and another to pea (Pisum sativum) beta‐type CA, which is primarily located in the chloroplast stroma, were applied to the charophycean green alga Mougeotia sp., shoot cells of the later divergent charophycean, Chara zeylanica, and control Chlamydomonas cells and pea leaf tissues. Immunolocalizations were quantitatively assessed at the ultrastructural level; nonimmune labeling was negligible in all cases. The Chlamydomonas antibody intensely labeled inner cell wall, periplasmic region, and peripheral cytoplasm of Chlamydomonas, Mougeotia, and Chara. The concentration of periplasmic label did not significantly differ among these green algae. This is the first immunolocalization evidence for occurrence of a periplasmic CA in Chara. Chloroplasts of Mougeotia, Chara, and pea were also labeled by the Chlamydomonas antibody, suggesting that plants inherited a plastid thylakoid‐based, alpha‐type CA from charophycean ancestors. The pea antibody labeled Chara chloroplasts but not those of Chlamydomonas or Mougeotia. The concentration of pea antibody stromal label did not significantly differ between pea and Chara. This is the first immunolocalization evidence for occurrence of a higher‐plant, beta‐type stromal CA in green algal plastids.