Changes in Bacterial Communities, Carbon and Nitrogen Dynamics on Coral Surfaces Following Mortality: Potential Implications for Reef Systems

Prokaryotic energy and nutrient cycling is critical to coral reef ecosystems. Living coral surfaces can harbour diverse but markedly different bacterial communities than other marine surfaces and, these communities may alter considerably following a disturbance. The consequences of these changes may include; alteration of fluxes and routes of organic and inorganic matter cycling, accelerated eutrophication and proliferation of algal blooms, all of which may ultimately lead to changes in trophic interactions. Coral reefs worldwide are experiencing a decline in health of a magnitude not seen in recorded history. In view of current trends and events over the last 30 years, it appears inevitable that we will lose a large percentage of living coral in the coming decades. It is therefore necessary to gain a greater understanding of the consequences associated with a substantial increase in the area of dead coral substratum. In particular the changes in the bacterial associates of coral surfaces following mortality may lead to dramatic changes in carbon and nutrient cycling, which may have implications throughout the ecosystem. This study investigated the primary colonization and early succession of bacteria on dead coral surfaces following mortality. This included an examination of the associated changes in production, carbon and nitrogen dynamics of the developing biofilms, and the consequent contributions to reef carbon and nutrient budgets. Inferences were made to the ecosystem changes that may be associated with a decrease in living coral and concomitant increase in bacterial mediated carbon and nutrient processes. Coral mortality via thermal stress resulted in the development of markedly different bacterial communities to those of live corals. Analysis of bacterial primary recruitment and early succession using Fluorescence in situ hybridisation (FISH) allowed direct visualization of bacteria and their spatial heterogeneity within and across samples. Members of the Cytophaga-Flavobacterium-Bacteriodes (CFB) and the gamma Proteobacteria dominated samples from dead corals, while members of the alpha Proteobacteria were apparent over time after coral mortality. Profiling of bacterial populations using Denaturing Gradient Gel Electrophoresis (DGGE) and Terminal- Restriction Fragment Length Polymorphism (T-RFLP) showed significant differences in surface bacterial populations between live and dead coral and that the bacteria on dead corals were dynamic over time post-mortem. In addition, T-RFLP allowed the identification of bacterial species that have previously been linked to nitrogen processes. The results of molecular analysis suggest that significant changes in surface biogeochemistry may occur concurrently with the changes in coral-surface bacteria following coral mortality. Surface associated oxygen metabolism was investigated using a combination of oxygen microelectrode profiles and Pulse Amplitude Modulated (PAM) Fluorometry. Results demonstrated a significant increase in oxygenic photosynthesis from dead coral biofilms. The associated P/R ratios implied a net increase in autotrophic carbon fixation, which was also demonstrated by increases in photosynthetic pigments and algal biomass. The developing biofilms also had greater photosynthetic efficiency, particularly at lower light levels, suggesting a greater capacity for carbon fixation. Changes in nitrogen processes were investigated by nitrogen fixation assays as well as fluxes of NH3/NH4[superscript]+ and NO[subscript]x. A significant increase in nitrogen fixation was observed from dead coral surfaces. These nitrogen fixation rates represented a significant input of new nitrogen to coral reefs, particularly in the initial two weeks after coral mortality. Analyses of nitrification and denitrification processes indicated these processes were absent in the initial development period of the dead coral algal-bacterial community. Assessment of the coral surface algal-bacterial biofilm showed an increase in carbon and nitrogen within the biofilms, which suggested that biochemical activity within the biofilm was primarily responsible for the increase in algal biomass observed. Overall, the results demonstrate a change in coral surface biogeochemical processes, which in the context of coral bleaching, may have ecosystem wide consequences. Coral bleaching has already been responsible for the death of large areas of coral. Death of coral over large spatial scales and the consequent increase in bacterial-mediated nutrient cycling may provide a significant input of new nitrogen to the system. This thesis explores the potential significance of these changes for the long term alteration of coral reefs subject to thermal bleaching