RESPONSES OF CERTAIN FRESHWATER PLANKTONIC ALGAE TO FLUORIDE1

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Abstract
The effects of dissolved fluoride supplied as NaF at up to 150 p.p.m. F− (7.9 mM) on growth, photosynthesis, dark respiration, enolase activity and fluoride uptake were determined for six phytoplankters: Synechococcus leopoliensis (Racib.) Komarek (Cyanophyta), Oscillatoria limnetica Lemmermann (Cyanophyta), Ankistrodesmus braunii Brun (Chlorophyta), Scenedesmus quadricauda (Turp.) Bréb. (Chlorophyta), Cyclotella meneghiniana Kützing (Bacillariophyta) and Stephanodiscus minutus Grun. ex Cleve et Moll (Bacillariophyta). Growth (determined by absorbance at 660 nm or by cell‐numbers) was unaffected by fluoride at up to 50 p.p.m. (2.6 mM) in all algae except S. leopoliensis, in which growth ceased transiently followed by resumption of growth at reduced rate. These effects showed a threshold at ca. 25 p.p.m. (1.3 mM) F− and increased with increasing F− concentration above this threshold. Photosynthetic O2 evolution in the chlorophytes was unaffected by F− at up to 50 p.p.m., whereas in S. leopoliensis F− above ca. 25 p.p.m. caused a concentration‐dependent inhibition of photosynthesis which was most pronounced at saturating irradiance. Dark O2 uptake was unaffected at up to 50 p.p.m. in chlorophytes but was stimulated in S. leopoliensis. Enolase in clarified cell‐extracts of all six algae was inhibited by F−, with Ki values ranging from 27 to 319 μM. Fluorine (measured by proton‐induced gamma‐ray emission) could not be detected in chlorophytes exposed during growth to up to 50 p.p. m. F−, but was detected in S. leopoliensis, O. limnetica and C. meneghiniana. Fluorine associated with cells of these algae increased as the external F− concentration increased.