UC San Diego

Understanding the natural processes that drive resource distribution and the associated response of organisms is a principal goal in ecology. Mixotrophic organisms are particularly interesting in this regard because their fitness is linked to the resources essential for both primary producers (e.g., light and nutrients) and consumers (e.g., food). Reef-building corals are among the most widely distributed mixotrophs and form the foundation of one of the most productive and diverse marine ecosystems, coral reefs. To date, a disproportionate amount of research has focused on the role of endosymbiotic microalgae in defining coral nutrition and we have a limited understanding of how corals respond to variation in food availability through time and space. This dissertation examines how mixotrophic corals can modify their nutritional modes in accordance with resource availability at multiple spatial scales. The Southern Line Islands of Kiribati in the central Pacific Ocean span a known upwelling gradient and have distinct differences in nearshore primary production. Combining remotely sensed estimates of surface chl-a as a proxy for food availability and stable isotope analysis of a common coral species, I found that corals are more heterotrophic at more productive islands. I then extended this relationship globally by synthesizing published isotopic data on corals and found that large-scale patterns of chl-a can predict how heterotrophic corals are likely to be. I then developed a more precise method to studying coral trophic ecology at finer spatial scales. Using δ13C analysis of amino acids, I found extreme trophic plasticity (0-100% contribution of heterotrophic nutrition) among conspecific corals at the scale of meters to kilometers around Palmyra Atoll. Finally, I conducted a nutrient enrichment experiment to examine the physiological responses of corals to changes in autotrophic nutrition in the absence of heterotrophic nutrition. I found that elevated nutrient concentrations have species-specific effects on coral calcification likely due to modifications in resource sharing between corals and their endosymbionts. Collectively, the results of my dissertation address a critical knowledge gap in coral biology and provide a framework to resolve the importance of heterotrophic nutrition in the persistence of coral reef ecosystems in an era of global change.