Physiological and molecular responses to urea environment in Cladocopium goreaui (Symbiodiniaceae)

Nitrogen is an essential nutrient for photosynthetic productivity, and its enrichment in coral reef ecosystems due to anthropogenic activities has raised concerns about ecological impacts. Urea is a readily available nitrogen source that can influence nitrogen dynamics in coral reef ecosystems, but the underlying mechanisms of its assimilation and utilization by coral symbionts remain unclear. This study investigates the physiological and molecular responses of Cladocopium goreaui to urea and nitrate, highlighting key differences in nitrogen assimilation. Although there was no significant difference in the expression of urease genes and proteins under urea and nitrate conditions, the form of nitrogen source did not affect urease activity; instead, nitrogen concentration was the primary factor influencing urease expression. Moreover, the regulation of C. goreaui gene expression by light intensity was more pronounced than the influence of nitrogen source type, suggesting that environmental light plays a more substantial role in gene regulation than the form of nitrogen available. In addition, transcriptomic analysis revealed that the response time of gene expression to nitrogen availability occurred approximately 2 h later than expected, emphasizing the delayed nature of the C. goreaui response. A total of 7786 differentially expressed genes (DEGs) were identified, including 2209 DEGs specific to urea treatment and 2675 DEGs specific to nitrate treatment. Proteomic analysis confirmed these findings, further detailing distinct nitrogen regulatory pathways, including stable metabolic responses to urea and dynamic shifts under nitrate treatment. Additionally, isotopic analyses showed that urea conditions resulted in higher δ¹³C and δ¹⁵N enrichment, indicating more efficient nitrogen and carbon assimilation. These results highlight the advantages of urea as an energetically favorable nitrogen source for C. goreaui, leading to stable metabolic responses and more efficient assimilation of both nitrogen and carbon. The findings underscore the metabolic flexibility of C. goreaui and its ability to adapt to varying nitrogen sources, with a greater impact from light conditions than nitrogen source type.