Novel Evidence for Nitrogen-Dependent Regulation of Nitrate Reductase in Coral Symbiosis

The nutritional symbiosis between corals and their photosynthetic dinoflagellate partners underpins the ecological success of reef-building corals in nutrient-poor environments. Although coral holobionts can assimilate the abundant yet highly variable environmental nitrate, direct insight into how nitrate reductase is regulated in these symbiotic algae has been lacking. Here, we provide the first characterization of nitrate reductase protein and gene expression in cultured Symbiodiniaceae exposed to different nitrogen sources and light regimes, revealing the multifactorial nature of its regulation. We demonstrate that nitrate reductase operates as a substrate-induced enzyme: nitrate stimulates protein synthesis in nitrogen-starved cultures, whereas ammonium actively suppresses its expression in a concentration-dependent manner. Light availability and photosynthetic electron transport further modulate protein abundance, suggesting that while nitrate reductase synthesis depends on nitrate availability, its stability may rely on photosynthesis. We also show that nitrate reductase is synthesized within hours of nitrate exposure by symbionts from nitrogen-starved corals, demonstrating that nitrate reduction can occur within the host environment. However, this response is transient and diminished relative to free-living cells, indicating that nitrate reduction is a facultative pathway activated when preferred nitrogen sources are limited. Finally, gene expression measurements and pharmacological inhibition confirm that nitrate reductase regulation is predominantly post-transcriptional, enabling this rapid and reversible control of nitrate assimilation. Together, these findings reveal a tightly regulated and responsive nitrate reduction system in coral symbionts that provides a flexible mechanism contributing to nitrogen homeostasis under fluctuating nutrient regimes.