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Title: Guanine uptake and metabolism by Phaeodactylum and other marine microalgae
Author: Shah, N.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1983
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Purines were, generally, good nitrogen sources for growth of the diatom Phaeodactylum, growth being similar to that on ammonium or nitrate. However, adenine inhibited growth. All the pyrimidines tested were poor nitrogen sources for growth. Nitrate grown cells of Phaeodactylum did not take up guanine until after a period of nitrogen-deprivation. Development of uptake ability was inhibited by ammonium, nitrate and cycloheximide and had a requirement for concurrent photosynthesis and sodium ions. Cells with a developed uptake ability took up guanine under dark aerobic conditions but not under dark anaerobiosis. Uptake was inhibited completely by 10-M CCCP and by low temperature. Ammonium and all the purines (except adenine) inhibited guanine uptake. Hypoxanthine was a competitive, and xanthine a non-competitive inhibitor of guanine uptake. The Ks value for guanine uptake was 0.44um. Uptake of guanine by Phaeodactylum was sodium-dependent with a concentration of 8.25mM Na allowing half the maximum rate of uptake; it was inhibited by increasing concentration of potassium ions. Autoradiographs of chromatograms of cell extracts revealed that all the 14C- taken up was confined to one compound, and the chromatographic characteristics of this compound were substantially different from those of all the free purine bases and very similar to those of methylhypoxanthine. It is concluded that uptake of guanine probably occurs by a group-translocation mechanism. The enzymes uricase, allantoinase and allantoicase could not be detected in nitrate grown Phaeodactylum and did not appear during N-deprivation but were induced in the presence of guanine or other purines. In contrast methyl transferase which converts guanine in the presence of S-adenosyl-(14CH3) methionine, to methylhypoxanthine was found in both I-replete and N-deprived cells of Phaeodactylum. A survey to determine the presence of a guanine uptake system amongst a number of freshwater and marine microalgae revealed that all, except Porphiridium cruentum, possessed a guanine uptake mechanism. Detailed studies of a number of marine microalgae showed that the guanine uptake systems were similar to that of Phaeodactylum in that the development was inhibited by ammonium and cycloheximide and required concurrent photosynthesis. Unlike Phaeodactylum, all the chlorophytes tested were found to accumulate guanine as a free purine base whereas another diatom (Amphora) accumulated the same compound as was found in Phaeodactylum. Uptake in all showed Michaelis-Mentenlike kinetics and was found to be sodium-dependent. It is concluded that purines, such as guanine, could be significant nitrogen source for natural phytoplankton growth.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available