Quantifying and characterising the activity and relevance of microbial associates in tropical coral nitrogen cycling

Thomas David Glaze

doi:10.25918/thesis.306

The nitrogen (N) cycling community in tropical coral microbiomes is purported to provide a range of alternative N uptake, release and recycling mechanisms, potentially having both beneficial and deleterious impacts on holobiont fitness and function. While the molecular evidence strongly indicates a dynamic N cycling community is present, direct quantification of fluxes is limited. This thesis aimed to address some of these knowledge gaps, utilising a range of isotopic tracers, nutrient perturbation analyses, mass balance models, and captured metagenomics analyses to quantify coral-associated N2 fixation, nitrification, denitrification, and DNRA, evaluate their respective relevance to coral N budgets, and determine the drivers of flux variability. N2 fixation was active across experiments (25 – 500 µmol N m-2 d-1), contributing significantly to coral N demand (1 – 5 %), and Symbiodiniaceae was demonstrated as the primary site of uptake. Activity in the central Great Barrier Reef was shown to be irradiance-dependent, correlating positively with cross-continental irradiance increases, and exhibiting day-time upregulation, though southern GBR corals did not express the same diel pattern. Denitrification was confirmed active, albeit sporadically, though flux rates (0 – 5 µmol N m-2 d-1) were not sufficient to meaningfully impact the coral N cycle. Nitrification was confirmed in some incubations, but not others, with activity primarily occurring under night-time conditions, suggesting coral nitrifiers are photo-inhibited. While measured rates were relatively low, with limited impact on the holobiont N cycle, consumption of nitrified nitrate was not traced, and gross fluxes may be more relevant. DNRA was found to be a significant DIN recycling mechanism using a nitrate mass balance model, though the ammonium preference of coral substrates, on which the model assumes, was invalidated in later chapters. Under the observed experimental conditions, DNRA was unlikely a relevant contributor to the holobiont. With the exception of diazotrophs, the N cycling community in coral holobionts was not found to not significantly impact the N budget, at least in oligotrophic corals under a nutrient pulse. Experimental incubations under both ambient and chronically elevated nutrient concentrations are required to confirm these findings hold under ambient conditions and chronically eutrophic environments. In addition to the microbial influences, novel findings in host and Symbiodiniaceae DIN cycling were also observed. The absence of ammonium preference, coupled with the lack of nitrate uptake inhibition recorded in P. cylindrica imply nitrate is a significant N input to GBR corals. Cross-shelf patterns in the magnitude and partitioning of DIN correlated with site water temperatures, indicating behavioural shifts of Symbiodiniaceae towards parasitism, further reinforcing the synergism of nutrient and thermal stressors.

Quantifying and characterising the activity and relevance of microbial associates in tropical coral nitrogen cycling

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Quantifying and characterising the activity and relevance of microbial associates in tropical coral nitrogen cycling

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