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Physiological and cytological effects of sodium fluoride additions to cultures of euryhaline phytoplankters with emphasis on a sensitive dinoflagellate Klut, Maria Emilia de Andrade Alves de Sá


The effects of 50-200 mg/L fluoride (F) additions on the growth of five axenic phytoplankter species in nutrient-enriched half-salinity seawater (14-15‰) were examined. The chlorophyte Dunalieila tertiolecta and the diatom Thalassiosira weissfloqii were virtually unaffected in both growth rate and maximum growth density, whereas another diatom Chaetoceros gracilis appeared to be growth-stimulated by all the fluoride concentrations tested. The growth of the prymnesiophyte Pavlova lutheri was 35-50 % inhibited, at 150-200 mg/L F concentration. However, this inhibition was partially overcome upon repeated transfers at these fluoride levels. The dinoflagellate Amphidinium carterae was the most sensitive to the highest fluoride concentration, with growth inhibited by 20-25% at 150 mg/L and by more than 90% at 200 mg/L F. When this species was exposed to gradual increases in F concentration, or was allowed to adapt to the highest F level over an extended period of time, normal growth was resumed. Interestingly, the F-resistant strain of Amphidinium did not require additional fluoride for growth. However, during adaptation this dinoflagellate was apparently undergoing major metabolic changes which were accompanied by biochemical, physiological and ultrastructural alterations. Although chlorophyll biosynthesis of Amphidinium seemed to be arrested at the time of F-inhibition, normal biosynthetic rates were resumed after F-adaptation, while the total carotenoid content increased. Concurrent with the development of F-resistance, there was an enhancement in dark respiration and possibly in photorespiration. Although the normal photosynthetic rate of this dinoflagellate was restored during the adaptation period, it appears that this recovery was impaired by a reduction in the photosynthetic efficiency due to increased photorespiration. Ultrastructural studies of F-adapted Amphidinium revealed abnormal features in the chloroplast (especially in the pyrenoid), mitochondria, and the nucleus. Thylakoid formation was greatly affected by fluoride, leading to the appearance of a prolamellar-like configuration in the pyrenoid matrix. This unexpected appearance of prolamellar-like structures in the F-adapted cells suggests that the pyrenoid may be a center for thylakoid assembly. These cells also showed large osmiophilic inclusions in the mitochondria. Large microbodies were found in close association with the mitochondria and chloroplast, suggesting an increased metabolic dependence on photorespiration. The nuclei of F-adapted cells showed dark and light concentric rings in the nucleolar region, accompanied by signs of mitotic activity, which were not observed in the F-inhibited cells. It is inferred that F-adaptation may have entailed some form of genetic change, which appears to be permanent and expressed as a characteristic phenotypic mutant.

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