Nitrogen-Dependent Regulation of Nitrate Reductase in Coral Symbioses.

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, nitrate reductase protein abundance and gene expression were characterized in cultured Symbiodiniaceae exposed to different nitrogen regimes and light conditions, revealing the multifactorial nature of its regulation. The results indicate that nitrate reductase behaves 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 although nitrate reductase synthesis depends on nitrate availability, its stability may rely on photosynthesis. In hospite, nitrate reductase is synthesized within hours of nitrate exposure by symbionts from nitrogen-starved corals, indicating that nitrate reduction can occur within the host environment. However, this response is transient and reduced relative to free-living cells, suggesting that nitrate reduction may function as a facultative pathway activated when preferred nitrogen sources become limited. Finally, gene expression measurements and pharmacological inhibition experiments are consistent with predominant post-transcriptional regulation of nitrate reductase protein expression, enabling rapid and reversible control of nitrate assimilation. Together, these findings describe a tightly regulated and responsive nitrate reduction system in coral symbionts that provides a flexible mechanism contributing to nitrogen homeostasis under fluctuating nutrient regimes.