Investigating nitrogen cycling within the coral holobiont using stable isotope and metagenomic analysis

Abstract

Corals and their symbionts, collectively known as the coral holobiont, have evolved to live in low nutrient (oligotrophic) conditions. However, many reefs now face elevated levels of nutrients (e.g. nitrogen) from agricultural run-off and poor sewage management. While many experiments have shown increased concentrations of nitrogen (N) can harm corals very little is known about the N cycle within the coral holobiont. To date, most studies investigating N within corals have focused on the relationship between corals and a type of symbiotic algae (Symbiodinium) that lives inside the corals and their response to different N concentrations. Furthermore, these studies focused on a form of N known as dissolved inorganic N (DIN). To fully understand the N cycle within corals it is necessary to investigate the rates of uptake and assimilation of DIN as well as another form of N: dissolved organic nitrogen (DON e.g. urea or free and dissolved amino acids). Additionally, the identification of microbial symbionts and their contribution to the N cycle is vital for a better understanding of N cycling within the coral holobiont as a whole. To address these questions, we will incubate coral fragments taken from the waters around Hong Kong in sealed containers with isotope enriched seawater. Containers will be respectively spiked with δ15 N-enriched nitrate and δ18 O-enriched water, and with δ15 N and δ13C-enriched urea and δ18 O-enriched water. Samples will be collected at the start of the experiment (before spiking) and then at regular intervals to monitor progress. Control incubations will be carried out and sampled simultaneously. Coral soft tissue will also be sampled after the incubation period is over. After subsequent analysis this will allow the uptake of nitrate, ammonium and DON to be quantified, as well as the rate of N-assimilation by coral soft tissue. In addition to the above experimentation identification of microbial communities within corals and quantification of the role they play in the N cycle will be undertaken. Due to the difficulties of culturing the vast majority of microbes, non-culture methods with high throughput detection are more applicable for characterizing microbial communities within the coral holobiont, therefore, we will use metagenomics to identify microbial communities. In combination with NanoSIMS technology this will help to identify the route different forms of N take within the coral holobiont and which microbes are involved.