Abstract
BackgroundCoral-associated bacteria are increasingly considered to be important in coral health, and altered bacterial community structures have been linked to both coral disease and bleaching. Despite this, assessments of bacterial communities on corals rarely apply sufficient replication to adequately describe the natural variability. Replicated data such as these are crucial in determining potential roles of bacteria on coral.Methodology/Principal FindingsDenaturing Gradient Gel Electrophoresis (DGGE) of the V3 region of the 16S ribosomal DNA was used in a highly replicated approach to analyse bacterial communities on both healthy and diseased corals. Although site-specific variations in the bacterial communities of healthy corals were present, host species-specific bacterial associates within a distinct cluster of gamma-proteobacteria could be identified, which are potentially linked to coral health. Corals affected by “White Syndrome” (WS) underwent pronounced changes in their bacterial communities in comparison to healthy colonies. However, the community structure and bacterial ribotypes identified in diseased corals did not support the previously suggested theory of a bacterial pathogen as the causative agent of the syndrome.Conclusions/SignificanceThis is the first study to employ large numbers of replicated samples to assess the bacterial communities of healthy and diseased corals, and the first culture-independent assessment of bacterial communities on WS affected Acroporid corals on the GBR. Results indicate that a minimum of 6 replicate samples are required in order to draw inferences on species, spatial or health-related changes in community composition, as a set of clearly distinct bacterial community profiles exist in healthy corals. Coral bacterial communities may be both site and species specific. Furthermore, a cluster of gamma-proteobacterial ribotypes may represent a group of specific common coral and marine invertebrate associates. Finally, the results did not support the contention that a single bacterial pathogen may be the causative agent of WS Acroporids on the GBR.
Highlights
The successful establishment and survival of coral reefs in tropical nutrient-limited waters is made possible by the symbiosis between two phylogenetically distinct eukaryotes, cnidarians and dinoflagellates of the genus Symbiodinium
Water associated bacterial communities Principal component analyses (PCA) showed that the bacterial communities associated with Acropora hyacinthus, Stylophora pistillata, and the surrounding reef water were significantly different from each other (R = 0.975, p = 0.001; with each subsequent pairwise comparison having a p = 0.001)
Certain water-associated bacteria were found in coral samples of A. hyacinthus and S. pistillata, these were not abundant within or across samples and are likely to represent contaminants from the water column
Summary
The successful establishment and survival of coral reefs in tropical nutrient-limited waters is made possible by the symbiosis between two phylogenetically distinct eukaryotes, cnidarians and dinoflagellates of the genus Symbiodinium. Unlike Symbiodinium which are strictly intracellular, bacteria are found in a range of coral associated micro-niches, including the mucus layer covering coral host tissues [11,12,13], on the coral tissue surface [9], and intracellularly [14,15,16]. Within these bacterial communities, there are several specific groups of bacteria that, to date, are associated with corals [2,3,17], and evidence suggests that this specificity extends further to distinct coral hosts [7,17]. Replicated data such as these are crucial in determining potential roles of bacteria on coral
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