Unraveling the nitrogen isotopic signature of symbiotic corals

Coral reefs thrive in shallow, tropical, low nutrient waters. Nutrient inputs to a reef environment are often interpreted by measuring the nitrogen isotopic composition of reef organisms. The δ¹⁵N signature of scleractinian corals has been historically measured to assess the presence of anthropogenic influences such as sewage and fertilizer runoff. The majority of reef building corals form a symbiotic partnership with the dinoflagellate algae, Symbiodinium microadriaticum. The δ¹⁵N signature of symbiotic corals is complex as it is not only dependent on nitrogen acquisition by the coral, but also by the algal symbionts that reside within the gastrodermal tissue layer. The relationship between the δ¹⁵N of dissolved inorganic nitrogen (DIN) and the δ¹⁵N of coral tissue has not been established. The aim of this study is to identify considerations necessary when interpreting nitrogen sources based on δ¹⁵N of coral tissue. Incubations were carried out in order to measure isotopic fractionation associated with nitrate and ammonium incorporation by the Pacific branching coral, Pocillopora damicornis. We investigated the dependence of nitrogen isotope fractionation on species of DIN (nitrate or ammonium), concentration of DIN (range: 1-50 μM N), genetic diversity of algal symbionts (clade C or clade D) and light levels.