Coral holobiont functioning under global environmental change

The tropical scleractinian coral holobiont is comprised by the coral animal, dinoflagellate algae of the genus Symbiodinium, and a multitude of other microbes, specifically bacteria, archaea, viruses, and protists. These holobionts are the unit of ecological selection, and remarkably adapted to thrive under oligotrophic (nutrient-poor) conditions. The foundation for this adaptation is provided by the coral-algae symbiosis, a mutualistic nutrient exchange relationship between the coral and Symbiodinium, allowing for the high primary productivity and growth rates of tropical coral holobionts. The coral-algae symbiosis is maintained via nitrogen limitation by the host, and new nitrogen from heterotrophic feeding, dissolved nutrient uptake, or coral-associated nitrogen fixation activity is retained and taken up within the holobiont. This thesis highlights the importance of functional dependencies on nitrogen cycling, particularly the nitrogen fixation pathway, in coral holobiont functioning, and the importance of employing a large set of response parameters covering critical functions of the main holobiont members.