Influence of nitrate starvation on transcript levels of nitrate transporter (Nrt2) and β-subunit of phycoerythrin (Cpeb) genes in Rhodomonas sp.

Abstract

Rhodomonas species are promising aquaculture feeds that provide high nutritional value. However, the degradation of intracellular phycoerythrin (PE) under nitrogen starvation negatively affects their nutritional value and growth. The present study aimed to identify the Nrt2 and Cpeb genes, which encode the high-affinity nitrate transporter system and the β-subunit of PE in Rhodomonas sp., respectively, and to investigate the effect of nitrate starvation on their expression. First, we screened RhNrt2 and RhCpeb by analysis of sequencing data from the transcriptome library of Rhodomonas sp. Blast analysis revealed that the amino acid sequences of RhNRT2 and RhCPEB with other organisms shared 29.5–53.3 % and 84.3–100 % identity, respectively. Additionally, their amino acid sequences exhibited distinctive features in their primary structure. Secondly, a batch culture experiment was designed under nitrate-replete and -starved conditions. Consistent with previous findings, nitrate-starved conditions had a detrimental effect on cell growth and intracellular PE content. The maximum cell density was 3.4 × 106 cells mL−1 under nitrate-starved condition, whereas a higher maximum cell density of 4.4 × 106 cells mL−1 was achieved under the nitrate-replete condition. Nitrate-starved conditions resulted in an over 80 % reduction in intracellular PE content. Relative RhNrt2 expression was consistently higher under nitrate-starved conditions compared to under nitrate-replete conditions, increasing 60.9-fold from the beginning to the end of the test. Conversely, relative RhCpeb expression was significantly suppressed and barely detectable under nitrate-starved conditions. Notably, the intracellular PE content decreases following RhCpeb suppression. The regulated expression of RhNrt2 and RhCpeb may allow these genes to be used as molecular indicators of nitrate status and intracellular PE biosynthesis. This study represents the first identification of RhNrt2 and RhCpeb genes and characterizes their expression in Rhodomonas species.