Effects Of Nitrogen Enrichment And Feeding On Autotrophic And Heterotrophic Nitrogen Flow In Coral Symbionts

Corals can acquire their nitrogen sources either from inorganic nitrogen uptake by the endosymbiotic dinoflagellates(Zooxanthellae),or through praying on the exogenous organics.nitrogen enrichment caused by intensive human activities has become a main threaten to the health of coral ecosystem,however,the underlying mechanism remains poorly constrained.Exploring the relative contribution of inorganic and organic nitrogen flow to coral symbiont under different nutrient condition holds the key to better elucidate and predict the impacts of nitrogen enrichment on coral system.In this study,Galaxea fascicularis,the dominant coral specie of Luhuitou Coral Reserve in Sanya,is selected to conduct the research.Corals were cultured either under 20 μmol L-1 NH4Cl or fed once a day for one month.The corals were then transferred into 20 μmol L-1 15N-NH4+ or fed by 15N labeled Artemia,the isotopic composition of the coral tissue,zooxanthellae and surrounding water were measured to trace the flow of 15N flux in these treatments.Our results showed distinct distribution of 15N between Ammonium derived nitrogen(ADN)and Feeding derived nitrogen(FDN)in the coral symbionts,demonstrating the fate of most autotrophic and heterotrophic nitrogen was coral tissue.The 67.3±6.6%ADN from the zooxanthellae is rapidly transferred to the coral in 3h.and 75.2%±3.1%of FDN(feeding derived nitrogen)are still preserved in coral tissue.the flow rate of ADN is much higher than that of FDN.Secondly,nutrient enrichment can improve the nitrogen storage and reduce the distribution of autotrophic and heterotrophic nitrogen.The C/N ratio of the NH4+enrichment group was significantly lower than that of the control group,and the ADN assimilation rate(1.7±1.0 μg cm-2 h-1)and transfer rate(1.7±1.0 μg cm-2 h-1)of NH4+ enrichment group was only 50%of the control group.Tthe distribution of FDN in zooxanthellae(8.2±1.5%)was 1/3 of the control group.The ADN assimilation rate(6.3±0.6 μg cm-2 h-1)and ADN transfer rate(4.1 ±0.4 μg cm-2 h-1)of NH4+enrichment group were twice of those in the control group.Finally,the combined effects of NH4+ enrichment and feeding on autotrophic and heterotrophic nitrogen flow processes were consistent with the effects of NH4+ enrichment.The trend of autotrophic and heterotrophic nitrogen flow in the NH4+ enrichment+feeding groups was similar to that in the NH4+enrichment group.Together,our study demonstrated that autotrophic nitrogen can flow efficiently between symbionts,and heterotrophic nitrogen tends to be preserved in coral host.Meanwhile,the nitrogen flow pathway of coral symbionts is easily affected by nitrogen enrichment environment.Coral symbionts can adjust the symbiotic relationship by changing the flow process of autotrophic nitrogen and heterotrophic nitrogen,maintaining homeostasis in the coral system to the nitrogen enrichment.