Hiding in plain sight: The secret contribution of the solitary ascidian Herdmania grandis to temperate reef nitrous oxide production

Abstract

Large solitary ascidians, like Herdmania grandis (Heller), can dominate the benthic substrates of subtropical and temperate reefs; however, their influence on nitrogen cycling, particularly nitrous oxide (N2O) production, is unknown. Here, we incubated individual H. grandis and compared fluxes of dissolved inorganic and gaseous nitrogen species to fluxes from reef sediments. Nitrous oxide production rates per individual ascidian (21 ± 8 nmol ind h−1) are the highest reported for any marine invertebrate. An individual ascidian produced more N2O than 1 m2 of inter-reef sediment (1.7 ± 1.7 nmol m−2 h−1). Ascidian mediated N2O production was found to occur under nutrient depleted conditions. The addition of 15N labeled organic material showed that the microbiota associated with H. grandis is capable of both nitrification and denitrification, but the contribution of these pathways to N2O production could not be ascertained. As the ecology of temperate reefs change, any range expansion of H. grandis will increase coastal N2O production.