Clash of dinoflagellates : nitrogen competition among coral-hosted symbiodiniaceae

Abstract

Symbiodiniaceae form essential symbioses with reef-building corals in oligotrophic waters. The symbiotic algae provide their host with essential resources through photosynthesis and cycling of nitrogenous wastes. I first reviewed the specificity of corals and their symbionts, in which advancement of molecular technology facilitated the discovery of Symbiodiniaceae diversity, with symbiont populations changing through time, seasons, and after disturbances. In addition, association with certain symbiont species is better under different environmental conditions such as high light, heat stress, and cold extremes. I then listed possible mechanisms that drive dominance and co-existence of Symbiodiniaceae and suggested competition theory from plant ecology as a testable mechanical framework for this research. In Chapter 2, I measured and demonstrated that five species of Symbiodiniaceae exhibited variation in capacity for maximum uptake (Vmax), or competitiveness (Ks) for limiting nitrate. I then linked these nitrate acquisition traits to their life-history traits (μ, cell size, and cell nitrogen). The ability to acquire and utilize limiting nitrate was species-specific and can be characterized into velocity-adapted (high Vmax and μ) and affinity-adapted (low Ks) strategies. This strategic assignment is the first step in understanding competition and co-existence in symbiont communities. In Chapter 3, I aimed to determine the growth characteristics of two Symbiodiniaceae, Cladocopium goreaui and Durusdinium trenchii. With the use of cell cycle progression, elemental stoichiometry (C and N), and isotopic composition (𝛿13C and 𝛿15N), I revealed variation in acclimation abilities to limiting nitrate between the two species. In Chapter 4, I combined three technologies to identify (fluorescent in situ hybridization, FISH) and isolate (flow cytometry, Flow) symbionts of interest, and simultaneously measure their metabolic function (stable isotope analyses, SIA) of specific genotypes (C. goreaui and D. trenchii) within a co-culture to represent a mixed microbe community. I demonstrated the impact of this technique by revealing species-specific changes in both carbon and nitrogen assimilation of the two symbionts when in competition for nutrient substrates relative to growth in isolation, which would likely have been misinterpreted by bulk analyses. Interactive effects of competition and temperature were also detected for limited nitrate, with C. goreaui a predicted winner in all scenarios over D. trenchii. In the last chapter, I developed a method for using a novel hollow-fiber membrane bioreactor (HFBR) for continuous culture of Symbiodiniaceae. Much of the research on

Symbiodinaceae has been conducted with batch culture, which is time-intensive and has limited control over growth conditions (e.g., resources are exhausted, toxins accumulate, and cells senesce). HFBR supported higher cell density and yielded viable cells for at least 306 days. The application of HFBR may be a powerful tool to improve research on the nutritional ecology of Symbiodiniaceae. In summary, multiple approaches were adopted to define differences in physiological, biochemical, and cellular responses among Symbiodiniaceae in the context of shifting resource landscapes. These methods are symbiont-centric and aimed to be transferable between free-living and in hospite conditions, which may allow a better understanding on the symbiont structuring mechanism under changing climate.