Synergistic/antagonistic effects of nitrate/ammonium enrichment on fatty acid biosynthesis and translocation in coral under heat stress

Superimposed on ocean warming, nitrogen enrichment caused by human activity puts corals under even greater pressure. Biosynthesis of fatty acids (FA) is crucial for coral holobiont survival. However, the responses of FA biosynthesis pathways to nitrogen enrichment under heat stress in coral hosts and Symbiodiniaceae remain unknown, as do FA translocation mechanisms in corals. Herein, we used the thermosensitive coral species Acropora hyacinthus to investigate changes in FA biosynthesis pathways and polyunsaturated FA translocation of coral hosts and Symbiodiniaceae with respect to nitrate and ammonium enrichment under heat stress. Heat stress promoted pro-inflammatory FA biosynthesis in coral hosts and inhibited FA biosynthesis in Symbiodiniaceae. Nitrate enrichment inhibited antiinflammatory FA biosynthesis in Symbiodiniaceae, and promoted pro-inflammatory FA biosynthesis in coral hosts and translocation to Symbiodiniaceae, leading to bleaching after 14 days of culture. Intriguingly, ammonium enrichment promoted anti-inflammatory FA biosynthesis in Symbiodiniaceae and translocation to hosts, allowing corals to better endure heat stress. We constructed schematic diagrams of the shift in FA biosynthesis and translocation in and between A. hyacinthus and its Symbiodiniaceae under heat stress, heat and nitrate co-stress, and heat and ammonium co-stress. The fndings provide insight into the mechanisms of coral bleaching under environmental stress from a fatty acid perspective.

Automated summary

Human-induced nitrogen enrichment, combined with ocean warming, increases the pressure on coral reefs. This study investigated the changes in fatty acid biosynthesis pathways and translocation mechanisms in coral hosts and Symbiodiniaceae under heat stress and nitrogen enrichment. The results revealed that heat stress promoted pro-inflammatory fatty acid biosynthesis in coral hosts and inhibited it in Symbiodiniaceae. Nitrate enrichment inhibited anti-inflammatory fatty acid biosynthesis in Symbiodiniaceae, while ammonium enrichment promoted it. These findings provide insights into the mechanisms of coral bleaching under environmental stress, from a fatty acid perspective.