[M-IS14] Biogeochemistry

[MIS14-01] Factors controlling the nitrogen isotope ratios of corals: the effect of autotrophic and heterotrophic nutrition

*Jincen Li¹, Yoshito Chikaraishi¹ (1.Hokkaido University)

Keywords:Amino acids, Stable isotope, Mixotrophy

Many coral species host dinoflagellates (called zooxanthellae) in their polyps, and the symbiotic algae provide corals with photosynthate. These corals assimilate organic resources via not only heterotrophic nutrition but also autotrophic nutrition for the growth and energy in their life. The corals are thus apparently functioning between autotrophy and heterotrophy (which is sometimes called mixotrophy) in marine ecosystems. However, because it is difficult to trace the assimilation balance between autotrophic and heterotrophic nutrition via observation and/or conversional chemical analysis, we have not yet quantified the nutrient cycles of corals and their symbiotic systems. Here, we carried out compound-specific isotope analysis of nitrogen within amino acids to evaluate the assimilation balance between autotrophic and heterotrophic nutrition of corals that grow autotrophically (i.e., symbiont-derived photosynthate is a solo resource for corals) and heterotrophically (i.e., both symbiont-derived photosynthate and commercial coral foods are resources for corals) in laboratory aquariums. The results reveal that there is a large inter- and intra-specific diversity in the nitrogen isotope ratio for the corals in the autotrophic and heterotrophic experiments. For example, in the case of the star polyp coral Pachyclavularia violacea in autotrophic condition, the isotope ratios of amino acids are well explained by (1) assimilation of symbiont-derived photosynthate and (2) isotopic fractionation during metabolism (i.e., deamination) of amino acids. The isotope ratios of amino acids in a star polyp (Centre light) are identical to those of the algal symbionts, which suggests a little effect of the isotopic fractionation during metabolism. In contrast, the isotope ratios of amino acids in another star polyp (Fluorescent green) are largely elevated from those of the algal symbionts, which suggests the large effect of the isotopic fractionation during metabolism. Moreover, in the case of heterotrophic condition, a large diversity is also found in the nitrogen isotope ratio of amino acids, which is well explained by a large diversity in the assimilation balance between autotrophic and mixotrophic nutrition. The isotope ratios of amino acids in a star polyp (Metallic green) are almost identical to those of the algal symbionts because of little effect of heterotrophy (i.e., no uptake of the commercial coral foods), whereas the ratios of amino acids in another star polyp (Long polyp) are considerably elevated because of large effect of heterotrophy in their growth. Based on these results, we conclude that inter- and intra-specific diversity in the assimilation balance between autotrophic and heterotrophic nutrition is a critical factor controlling the nitrogen isotope ratios of corals, and suggest that the isotope ratios can be useful for identifying and quantifying the nutrition pathways of corals in natural ecosystems.