Seasonal and diurnal patterns of NH,+ and NO,-uptake were determined for Lake Kinneret phytoplankton. Nanoplankton generally, but not always, had a higher uptake of NH,+ and NO,-than did net plankton. Ammonium was always taken up preferentially and the phytoplankton had lower apparent affinity constants (K,) for this ion than for N03-. However, during the annual dinoflagellate bloom of Peridinium (February-May), when ambient levels of NH,+ were low and those of N03-were high, a considerable portion of the total N flux was derived from NO,-. The observed daily fluctuations of specific uptake rates for both NH;' and NOJ-can be partially explained by changes in light intensity. The finding that N03-was utilized by the dinoflagellates implies that control of the amounts of this nutrient, which is predominantly supplied from watershed sources, could be important in limiting eutrophication in Lake Kinneret.
A comprehensive antioxidative mechanism was found in the freshwater dinoflagellate Peridinium gatunense Lemm. during the spring bloom in Lake Kinneret. Ascorbate was present throughout the bloom period and was responsible, together with catalase, for the elimination of photosynthetically produced H,O,. As glutathione concen tra tions and ascorba te regenerative enzymes were negligible during mid-spring, ascorbate was presumably biosynthesized during the photosynthetically active period. Antioxidative activity increased overall at the end of the spring in conjunction with elevated ambient stress conditions, for example high light. Under such circumstances, ascorbate was regenerated. Ascorbate levels doubled when cells were exposed to a n increase in irradiance from 60 to 600 pmol photons.m-2.s-1, and on addition of H202, concentrations increased a further 20-fold. SigniJcant antioxidative activity was also noted i n the dark, although this was dependent on the presence of H202. Diurnal changes i n antioxidants and their regenerative enzymes were obseroed. The activities of monodehydroascorbate reductase, glutathione reductase, and ascorbate concentrations showed ultraradian periodicity and were completely in phase throughout the day/ night petiod. Dehydroascorbate reductase activity and glutathione concentrations were also in phase but showed aperiodic variation, as did ascorbate peroxidase activity. Superoxide dismutase and catalase activities were generally out of phase during the 24-h period but did show ultraradian periodicity. Lake samples entrained under constant light revealed a n inate 12-h rhythm for catalase activity, during at least 34 h.
A modified assay for alkaline or acid phosphatases associated with microorganisms in aquatic environments has been developed. This is based on collecting microplankton on filters and subsequently determining the enzyme activities spectrophotometrically or fluorometrically, using p-nitrophenyl-phosphate or 4-methylumbelliferyl phosphate, respectively as substrates. The assay is simple and rapid, and has the further advantage of permitting phosphatase activities to be assigned to specific size fractions of the natural microplankton.In samples taken from Lake Kinneret and a nearby reservoir, a consistently high proportion of the total alkaline or acid phosphatase activity was associated with the size fraction x0.8 pm > 0.2 pm indicating the potentially high contribution of bacteria to these activities. This approach can also be used to examine the enzymatic potential of microplankton to release orthophosphate from other organo-phosphate substrates.
Acid and alkaline phosphatases have been isolated from Peridinium cinctum f. westii (Dinophyceae) during an algal bloom in Lake Kinneret. Acid phosphatase activity was fairly constant over the entire period of the bloom, although fluctuations in activity appeared to correlate with the chlorophyll content of the cells. Histochemical studies showed that the enzyme was localized inside the cell. Alkaline phosphatase activity was very low until May, a month after the peak of the bloom, when it increased sharply. Polyacrylamide gel electrophoresis revealed one or two bands of alkaline phosphatase that increased in intensity as the bloom progressed. However, the highest activity of the enzyme (in the last sample collected) corresponded to a new, very intense band on the gels. Similarly to acid phosphatase, alkaline phosphatase was also localized inside the cell. The appearance of alkaline phosphatase is probably related to the available phosphate concentration in the lake, although the influence of other factors that may contribute to the induction of the enzyme cannot be ruled out.
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