THE ROLE OF CARBONIC ANHYDRASE IN THE FORMATION OF MOUGEOTIA (CHAROPHYCEAE) BLOOMS IN ACIDIC ENVIRONMENTS

Abstract

Mougeotia sp, a dominant component of a metaphytic algal bloom‐forming alga in an experimentally acidified lake (Little Rock Lake, Vilas Co. WI) was isolated to analyze pH effects on carbonic anhydrase (CA) activity, localization, and algal morphology. A potentiometric method was used to assess internal and external CA activities using two CA inhibitors, ethoxyzolamide (EZA) and acetazolamide (AZA) in cultures grown at pH 5 and pH 8. External and total CA activities at pH 8 were significantly greater than those at pH 5, but internal CA activities were not significantly different at both pH levels. The non‐penetrating inhibitor AZA inhibited external CA activity at pH 8, whereas EZA, which inhibits internal CA activity in some green algae, did not inhibit Mougeotia CA activity, possibly because of permeability problems. Ultrastructural immunogold labeling with a polyclonal antibody for Chlamydomonas external CA suggested that an antigenically‐similar enzyme was located in the periplasmic space and inflated end walls of Mougeotia grown at both pHs. This antibody, labeled the Mougeotia chloroplast, but not the pyrenoid and cytoplasmic peripheral regions. Activity measurements, inhibitor results, and localization data were consistent with the operation of a carbon concentrating mechanism (CCM) at both pH levels. Growth form, cell dimensions, chloroplast morphology, and cell wall ultrastructure were significantly different in cultures grown at both pH levels. These features probably contributed to the ability of Mougeotia to form blooms in acidified waters. A polyclonal antibody to pea chloroplast CA did not label Mougeotia, suggesting evolutionarily significant differences in the plastid CA systems of this alga and higher land plants.