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Title: The inorganic nitrogen metabolism of marine dinoflagellates
Author: Dixon, G. K.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1987
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Nitrogen-replete cells of Amphidinium carterae took up ammonium in the light at a rate 5 - 6 times that of nitrate even though exponential growth rates were similar on these two N-sources. A. carterae exhibited a capability for enhanced initial ammonium uptake, particularly when deprived of nitrogen. Enhanced initial rates of ammonium uptake were also observed in a natural population of Gyrodinium aureolum. Initially ammonium accumulated within the cells of A. carterae but was assimilated into organic-N within a matter of hours; increases in total cellular-N, total free amino acids, glutamine and cellular protein were observed 4 h after an ammonium addition. In comparison, very little nitrate was accumulated. Ammonium (250 μM) inhibited reversibly the uptake of nitrate; the rapidity of the response suggests a direct effect on uptake. Prior nitrogen deprivation of the cells did not affect this inhibition. Rates of ammonium uptake were similar in the light and dark but nitrate uptake was completely inhibited by darkness in nitrogen replete cells of A. carterae and in a natural population of G. aureolum. Dark uptake of nitrate was stimulated by a period of nitrogen deprivation. Ammonium uptake in darkness by A. carterae was accompanied by the utilization of cellular polysaccharide, mainly glucose polysaccharide. Most of this carbon was unavailable for the assimilation of nitrate in the dark. It is suggested that a control mechanism is in operation, via a product of ammonium assimilation, on one or more of the enzymes concerned with polysaccharide breakdown, e.g. α-amylase or phosphorylase. Ammonium addition caused a marked enhancement of dark CO2 fixation in several nitrogen-replete dinoflagellates. Nitrate addition produced little enhancement in comparison. The amount of enhancement was dependent on species, age of culture and period of diel cycle. Nitrogen deprivation caused a 2-3 fold increase in enhancement in all species tested. The measurement of dark 14CO2 fixation shows promise as a technique for determining the nitrogen status of phytoplankton in both the laboratory and in the field. A natural population of Gyrodinium aureolum appeared to be slightly N-limited using this technique, an observation supported by other field data. The use of this technique as a tool to determine the nitrogen status of phytoplankton in culture and in the field is discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available