Studies on the nitrogen utilization of endosymbiotic algae isolated from Japanese Paramecium bursaria

Endosymbionts of Japanese Paramecium bursaria show novel nitrogen utilization. Japanese endosymbiont F36-ZK grew very slowly in a medium that contained ammonium nitrate as a sole nitrogen source, suggesting poor ammonia assimilation. Activities of ammonia assimilation pathway enzymes, glutamine synthetase (GS), glutamate 2-oxoglutarate amido transferase (GOGAT) and glutamate dehydrogenase (GDH), were measured but no GS and low GDH activities were observed. Furthermore, F36-ZK could not utilize l-glutamic acid, which is one of the two amino acids generated by ammonia assimilation pathways. Thus, F36-ZK seemed to have an unknown ammonia assimilation pathway. F36-ZK could utilize some basic and neutral amino acids as nitrogen sources, such as l-serine, l-alanine, l-arginine, l-glutamine, l-asparagine, l-proline and glycine. Initial amino acid uptake rates were measured using [14C]-amino acids, which showed that the endosymbiont F36-ZK was able to take up all amino acids. In contrast, free-living Chlorella vulgaris NIES-227, which is phylogenetically close to F36-ZK [R. Hoshina, S. Kamako, N. Imamura, Phylogenetic position of endosymbiotic green algae in Paramecium bursaria Ehrenberg from Japan. Plant Biol. 6 (2004) 447–453], took up only l-arginine. These findings indicate that there is no parallel relationship between utilization and uptake of amino acids in F36-ZK, suggesting that some amino acid metabolic pathways may degenerate as a result of symbiosis.