Effects of pH on cell morphology and carbonic anhydrase activity and localization in bloom-forming Mougeotia (Chlorophyta, Charophyceae)

A species of Mougeotia (C.A. Agardh) that was the dominant component of a metaphytic bloom-forming filamentous algal assemblage in an experimentally acidified lake (Little Rock Lake, Vilas County, Wisconsin, U.S.A.) was isolated into unialgal culture for analysis of pH effects on cell morphology and carbonic anhydrase activity and localization. External and total carbonic anhydrase activities at pH 8 were significantly greater than those at pH 5, but internal carbonic anhydrase activities were not significantly different at the two pH levels, as determined by use of a potentiometric technique. Ultrastructural immunogold labeling with a polyclonal antibody to Chlamydomonas periplasmic carbonic anhydrase suggested that an antigenically similar protein was located in the periplasmic space, inflated end walls, chloroplast, and peripheral cytoplasm of Mougeotia grown at both pH 8 and 5. Activity measurements and localization data were consistent with the hypothesis that a carbon concentration mechanism operates in this Mougeotia species at both high and low pH. Growth form, cell dimensions, chloroplast morphology, and cell wall ultrastructure were significantly different in cultures grown at pH 5 and pH 8. These structural and carbon acquisition features may contribute to Mougeotia's ability to form conspicuous metaphytic blooms in acidified waters.Key words: carbonic anhydrase, acidification, algal blooms, Mougeotia.