Studying the Effect of Elevated pCO2 on the Nitrogen Cycle Within the Coral Holobiont Using Stable Isotopes

Abstract

The efficient recycling of nitrogen plays an integral role in the health of the coral holobiont, it is therefore essential to understand how elevated pCO2 may affect this biogeochemical cycle under ocean acidification scenarios. We investigated how elevated pCO2 (~950 ppm pCO2) affects the nitrogen cycle in corals by making use of a natural CO2 seep on Shikine-jima, Japan. Colonies of two species of corals (Acropora solitaryensis and Porites heronensis) were sampled from a nearby reference site (~300 ppm pCO2), and fragmented into smaller pieces. After recovery, some fragments (n = 8) were individually incubated in chambers for three hours (incubation A) in order to determine changes in nitrate, nitrite, ammonium, total and organic dissolved nitrogen concentration and their respective nitrogen and oxygen isotopic composition. Following the initial incubation, half of the fragments were transplanted from the reference pCO2 site into the elevated pCO2 site for two weeks, and then a second incubation (n = 8, from each site) was carried out on the acclimatised fragments (incubation B). During incubation A, dissolved inorganic nitrogen decreased after three hours, as well as dissolved organic nitrogen (DON) for both species. Moreover, the nitrogen isotopic composition of total dissolved nitrogen (𝛿15NTDN) decreased over this period for both species. At the two-week point, there was a decrease in nitrate and nitrite concentrations for both species and treatments, with a concurrent increase in ammonium. For both species in the elevated pCO2 treatment there was a slight decrease in DON after three hours, however, there was an increase in DON after three hours in the reference pCO2 treatment. In the elevated pCO2 treatment there was an increase in 𝛿15NTDN after three hours for both species, conversely, there was a decrease after three hours for the reference pCO2 treatment. These preliminary results suggest elevated pCO2 may have an effect on the nitrogen cycle within the coral holobiont, specifically on the (re)cycling of DON.