MEPS 260:33-41 (2003)  -  doi:10.3354/meps260033

Effects of high pH on the growth and survival of six marine heterotrophic protists

Maria Fenger Pedersen, Per Juel Hansen*

Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark
*Corresponding author. Email:

ABSTRACT: The pH tolerance of the ciliates Balanion comatum, Favella ehrenbergii, Rimostrombidium caudatum and R. veniliae and the dinoflagellates Gyrodinium dominans and Oxyrrhis marina was studied using laboratory cultures at specific pH levels and prey concentrations. The results of these experiments divided the tested species into 2 groups: pH-tolerant species and pH-non-tolerant species. The tolerant group consisted of B. comatum, which experienced a reduction in growth when pH exceeded 9.5, and O. marina, which maintained its maximum growth within the pH limit of the experiment (pH 9.9). The pH-non-tolerant group consisted of 3 ciliates and 1 dinoflagellate. The most pH sensitive species were F. ehrenbergii, R. caudatum and R. veniliae. Their growth rate became affected at pH 8.8 to 8.9 and they did not grow when pH exceeded 9.0. The more tolerant species of this group, G. dominans, experienced a reduction in its growth when pH exceeded 9.2, and negative growth when pH exceeded 9.4. In a different set of experiments with the same species, the algae were allowed to grow and thereby raise the pH. In these experiments, the pH-sensitive species F. ehrenbergii, R. caudatum and R. veniliae all died within 24 h when pH exceeded 9.3, whereas some cells of the more tolerant dinoflagellate G. dominans were able to survive at pH values around 10 for up to 5 d. Thus, heterotrophic protists differ in their pH limits for growth and in their survival response when exposed to pH exceeding their limits for growth. In nature, algal blooms may lead to elevated pH (>9). Our results suggest that such pH levels will kill many, but not all, heterotrophic protists. This may, at least temporarily, lead to a reduction in grazing control of such algal blooms, thereby further allowing their growth and persistence.

KEY WORDS: pH tolerance · Ciliates · Dinoflagellates · Balanion comatum · Rimostrombidium caudatum · Rimostrombidium veniliae · Gyrodinium dominans · Favella ehrenbergii · Oxyrrhis marina

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