Nitrate Assimilation Underlying Kleptoplasty
While photoautotrophic organisms utilize inorganic nitrogen as the nitrogen source, heterotrophic organisms utilize organic nitrogen and thus do not generally have an inorganic nitrogen assimilation pathway. Here we focused on the nitrogen metabolism of Rapaza viridis, a unicellular eukaryote exhibiting kleptoplasty. Although belonging to the lineage of essentially heterotrophic flagellates, R. viridis exploits the photosynthetic products of the kleptoplasts and was therefore suspected to potentially utilize inorganic nitrogen. From the transcriptome data of R. viridis, we identified the gene RvNaRL, which had sequence similarity to nitrate reductases found in plants. Phylogenetic analysis revealed that RvNaRL was acquired by a horizontal gene transfer event. To verify its function of the protein product RvNaRL, we established a RNAi mediated knockdown and a CRISPR-Cas9-mediated knockout experiments for the first time in R. viridis and applied them to this gene. The RvNaRL knockdown and knockout cells exhibited significant growth only when ammonium was supplied but, in contrast to the wild-type cells, no substantial growth when nitrate was supplied. Such arrested growth in absence of ammonium was attributed to impaired amino acid synthesis due to the deficiency of nitrogen supply from the nitrate assimilation pathway; this in turn resulted in the accumulation of excess photosynthetic products in the form of cytosolic polysaccharide grains as observed. These results indicate that RvNaRL is certainly involved in nitrate assimilation by R. viridis. Thus, we infer that R. viridis achieved its advanced kleptoplastic strategy owing to a posteriori acquisition of the nitrate assimilation pathway the horizontal gene transfer.