| Summary: | Bibliography: pages 132-140. === Exudation patterns of organically bound nitrogen and polyphenol substances, as well as the uptake of nitrate, ammonium and phosphate were studied in the kelp Ecklonia maxima (Phaeophyceae) in ex-situ experiments. It was found that exudation of organic materials only rarely followed a straight accumulatory trend ; fluctuations of kelp-derived substances occurred as a net result of pulsating production patterns coupled with differential rates of utilization by marine heterotrophic organisms. Pulsing took place at irregular intervals in different plants, precluding statistical treatment of exudation. The observed initial pulsing liberation of DON and polyphenols, affected the quantification of exudation rates which could only be tentatively determined. Antibiotic agents used in one experiment did not succeed in controlling free-living bacteria in the water column without affecting the kelp DOM production was diminished in the presence of antibiotics, leading to the conclusion that they affected the physiology of the epidermal cells. This provides indirect evidence that exudation is an energy-dependent, active process. Measurements of uptake velocities of ambient and enriched nitrogen established that there was a higher affinity for ammonium as a nitrogen source than for nitrate at enriched N levels. At ambient nutrient levels, nitrate uptake velocities were 1,5 times higher than those for ammonium, making nitrate the primary nitrogen resource of E. maxima. However, the uptake potential for either source was not competitively inhibited by the other. Ammonium and nitrate uptake velocities still increased from enriched levels of 30-40 uM N to approximately 100 uM N. Both exudation and uptake phenomena were subject to seasonality: During summer, the rates for exudation and uptake were found to be higher than in winter, which is known to be a dormant phase in the growth of E. maxima. A comparative analysis of dissolved free amino acids in the water surrounding the kelp and a tissue analysis of free amino acids obtained under enriched conditions showed that alanine was prominent b6th in the thallus and in the water. Glutamic acid was detected in the water only after the kelp had been introduced during a nitrogen feeding experiment and appeared to be GS/GOGAT derived. Aspartine was al so detected at high relative concentrations. Polyphenolic substances (phlorotannins) accumulated in exudates of E. maxima more readily due to the low biodegradability of these compounds and contributed to the yellow colouration and uvabsorption characteristics (Gelbstoff) of the surrounding water.
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