pH Tolerance and Metal Toxicity in Populations of the Planktonic Diatom Asterionella: Influences of Synthetic and Natural Dissolved Organic Carbon

Two clones of Asterionella ralfsii cf americana Körn. were more tolerant of acidic pH with respect to batch culture growth rate than their congener, A. formosa Hass. The relative inability of A. formosa to satisfy trace metal requirements at lower pH appeared to be a major factor in its reduced tolerance of acidic conditions. The pH tolerance of A. formosa, and at times A. ralfsii, also could be modified when grown in media containing different concentrations of artificial or natural forms of dissolved organic carbon (DOC). Mixed trace metal additions (Cu, Zn, Mn, Fe, and Al) further affected pH tolerance in A. ralfsii both by alleviating limitation and by inducing toxicity, depending on the nature and concentration of DOC. Overall, our results were consistent with pH classification schemes for both A. ralfsii and A. formosa as they are commonly used in paleolimnological pH reconstruction models. However, apparent pH classifications for both Asterionella species could be altered by changes in DOC-mediated trace metal bioavailability. Their assignment to a specific pH category or distribution thus may not represent an inherent biological property of that taxon, but may strongly depend on chemical conditions which often vary in humic lakes subject to acid deposition.