Tropical Stony Corals Host Diverse Microbial Nitrogen Dynamics

Tyler Tamasi, S.M., 2019
Andrew Babbin, Advisor
Coral health hinges on an intricate relationship between the coral animal, photosynthetic algae of the family Symbiodiniaceae, and a microbial community of associates collectively termed the coral holobiont. This holobiont maintains the nutrient balance of their symbiosis amid reefs’ otherwise oligotrophic environment, including by cycling physiologically important nitrogen compounds. Nitrogen (N) fixation has been well quantified in corals in accounting for the source of their fixed N, but no complementary loss term had been directly detected. This pathway is important for understanding sources and sinks of nitrogen on reefs and how they may impact coral success.
Here we use 15N-tracer experiments to produce the first measurements of nitrate reduction, nitrite oxidation, and nitrous oxide (N2O) production in five species of reef-building corals in the Gardens of the Queen, Cuba. Nitrate reduction and nitrite oxidation are present in most species sampled, but ammonium oxidation is low potentially due to photoinhibition and competition with uptake. The brain coral Diploria labyrinthiformis exhibits the strongest potential for denitrification based on elevated rates of nitrate reduction and N2O production. The rates observed were consistent with previous measurements of N2O flux out of reef waters and directly confirm the potential of corals to carry out N loss.