Optimal nitrogen source for the recovery of phycoerythrin in discolored Rhodomonas sp.

Rhodomonas species are of interest as excellent aquaculture feed because of their ideal biochemical composition, especially their high protein content, resulting from the presence of intracellular phycoerythrin (PE). One of the most crucial issue with this species as a feed source is PE degradation under ambient nitrogen starvation, which leads to unpredictable growth defects and reduced nutritional value associated with cell discoloration. The aim of this study was to propose an optimal nitrogen source for the recovery of intracellular PE in discolored Rhodomonas sp. We supplied three different nitrogen sources, i.e., nitrate, ammonium, and urea, to PE-degraded cells and monitored changes in PE content and the expression of related genes. Supplementation with each nitrogen source increased cell density, with nitrate exhibiting the highest specific growth rate of 0.30 ± 0.02 day−1. The maximum PE content in each nitrogen group at 48 h after N supplementation was about 3.8 pg cell−1 with no major difference between groups. In contrast, the highest relative PE recovery rate of 5.73 ± 0.18-fold and the highest PE productivities of 0.32 ± 0.01 μg mL−1 h−1 were obtained in the nitrate group. Nitrogen transporter genes, including RhNrt2, RhAmt1, and RhDur3, were upregulated under nitrogen-starvation conditions, demonstrating the ability of this species to utilize each of the tested nitrogen sources. Furthermore, RhCpeb, which serves as a molecular indicator of PE biosynthesis, was significantly upregulated in all nitrogen-supplemented groups, with the strongest and fastest increase in expression in the nitrate group, emphasizing the potential of nitrate as the most favorable nitrogen source for PE recovery in this species. Our findings pave the way for informed strategies for the stable management and production of Rhodomonas species as high-protein feed in aquaculture.