The Role of Nitrogen Metabolism in the Stability and Dysfunction of the Cnidarian-Dinoflagellate Symbiosis

作者：Oscar Crehan · DOI：10.26686/0v19-vy2x

Coral reefs are able to thrive in oligotrophic waters due to their symbiosis with dinoflagellate algae (family Symbiodiniaceae), which enables efficient nitrogen recycling and conservation within the holobiont. However, increased seawater temperatures can destabilise this partnership, causing bleaching, which can result in coral mortality. Thermally-tolerant symbionts can withstand higher temperatures, but they may provide the host with less carbon while assimilating more of the host’s nitrogen. The extent of bleaching can also be mitigated or exacerbated depending on the nitrogen form and concentration in the seawater. To elucidate nitrogen’s role in coral bleaching, this thesis aimed to determine how nitrogen assimilation of the host and symbiont varies with temperature and whether this response is influenced by symbiont identity. In Chapter 2, the tropical scleractinian coral Stylophora pistillata was grown at different temperatures (26°C, 30°C and 34°C), before being placed in nutrient-replete or -depleted seawater for 24 h. The corals were then incubated with 13C-labelled sodium bicarbonate and different 15N-labelled nitrogen forms (ammonium, urea, and dissolved free amino acids) to determine their assimilation rates. I found that nutrient depletion inhibited the assimilation of all nitrogen sources studied and that heat stress reduced the assimilation of ammonium and dissolved free amino acids. However, the host assimilated over 3-fold more urea at 30°C relative to 26°...