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Physiological, ultrastructural and cytochemical studies on the utilization of various intermediates of the purine catabolism pathway as sole sources of nitrogen by marine phytoplankters

University of British Columbia

Eleven species of marine microalgae belonging to six different taxonomic divisions were tested for their ability to grow on allantoin, allantoic acid, hypoxanthine and urea as sole sources of nitrogen. All species were able to utilize the nitrogen atoms of urea but only six of these were able to grow on allantoic acid, while five showed moderate to good growth in hypoxanthine. None was able to utilize allantoin. The study of nickel requirements for the growth of these microalgae on the different sources of nitrogen, together with the results of inhibitor tests suggest that those species capable of utilizing both hypoxanthine and allantoic acid catabolize purines through the standard pathway of purine oxidation described in other microorganisms and higher plants. This pathway leads to the production of urea and its subsequent conversion to utilizable ammonium. In the case of one species, Pavlova lutheri, growth on urea is inhibited by urease inhibitors, while growth in allantoic acid or hypoxanthine occurs in the presence, of urease inhibitors. The results suggest that in this case the catabolic oxidation of purines and their derivatives does not involve urea production and occurs through a pathway different from that observed in the other species. Cells of Amphidinium carterae grown on hypoxanthine undergo major ultrastructural changes. These affect the perichromatinic granules, the dictyosomes and dictyosome-derived vesicles, the distribution of the endoplasmic reticulum, the number of mitochondria and microbodies, and the size and distribution of the vacuolar compartment. Some of these ultrastructural changes, such as increase in endoplasmic reticulum and the number of microbodies, along with the cytochemical demonstration of both uricase and catalase activities within microbodies, support the occurrence in these microalgae of the standard pathway for the catabolic degradation of purines. Cells of both Dunaliella tertiolecta and Pavlova lutheri grown on hypoxanthine also undergo major ultrastructural changes. These affect mainly the endoplasmic reticulum, mitochondria and vacuoles. The effect on mitochondria is particularly interesting since cytochemical tests reveal the presence of both uricase and catalase activities in these organelles. When one takes into consideration that no microbodies are observed in these microalgae and that uricase controls the key step of the formation of allantoin and H₂O₂ through the oxidation of urate, it becomes apparent that in these microalgae mitochondria participate in the oxidative degradation of purines and their derivatives and play a major role in the organic N-budget of these microorganisms.

Text

2010-08-18

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http://hdl.handle.net/2429/27491

Botany

University of British Columbia

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