Metagenomic analysis of stressed coral holobionts

Abstract

The coral holobiont is the community of metazoans, protists and microbes associated with scleractinian corals. Disruptions in these associations have been correlated with coral disease, but little is known about the series of events involved in the shift from mutualism to pathogenesis. To evaluate structural and functional changes in coral microbial communities, Porites compressa was exposed to four stressors: increased temperature, elevated nutrients, dissolved organic carbon loading and reduced pH. Microbial metagenomic samples were collected and pyrosequenced. Functional gene analysis demonstrated that stressors increased the abundance of microbial genes involved in virulence, stress resistance, sulfur and nitrogen metabolism, motility and chemotaxis, fatty acid and lipid utilization, and secondary metabolism. Relative changes in taxonomy also demonstrated that coral-associated microbiota (Archaea, Bacteria, protists) shifted from a healthy-associated coral community (e.g. Cyanobacteria, Proteobacteria and the zooxanthellae Symbiodinium) to a community (e.g. Bacteriodetes, Fusobacteria and Fungi) of microbes often found on diseased corals. Additionally, low-abundance Vibrio spp. were found to significantly alter microbiome metabolism, suggesting that the contribution of a just a few members of a community can profoundly shift the health status of the coral holobiont.

Dominance of Endozoicomonas bacteria throughout coral bleaching and mortality suggests structural inflexibility of the Pocillopora verrucosa microbiome

“…Importantly, we did not observe an intrusion and propagation of putative opportunistic or pathogenic bacteria from the surrounding seawater, which harbored bacterial communities that were highly distinct from bacteria associated with P. verrucosa (Figure 4). While the absence of bacterial community changes in coral holobionts counters previous work (Vega Thurber et al., 2009, 2012; Ziegler et al., 2016), recent work from the Red Sea reports on similarly stable bacterial communities in P. verrucosa across sites subject to differential anthropogenic impact (sewage, municipal waste water, and sediment input). Notably, this bacterial community “stability” in P. verrucosa as reported by Ziegler et al.…”

Investigating the Driving Mechanisms Behind Differences in Bleaching and Disease Susceptibility Between Two Scleractinian Corals, Pseudodiploria Strigosa and Diploria Labyrinthiformis

“…Other acidification studies used lower pH values of 7.3 (Meron et al 2011) and6.7 (Vega Thurber et al 2009). In these studies, an increase in Alphaproteobacteria and decrease in Deltaproteobacteria and Bacteroidetes was detected within the SML at pH 7.3 (Meron et al 2011), and a decrease in Alphaproteobacteria and increase in Beta-, Delta-and Epsilonproteobacteria was detected at pH 6.7 (Vega Thurber et al 2009). Although the current acidification study did not involve microbial analysis, and did not detect changes in the SML thickness, the changes in microbial populations detected in other studies suggest that the SML is affected by acidification in other ways.…”

“…Therefore, another potential consequence of SML thinning may be the reduction in the availability of DOC and nutrients in coral mucus. This may explain the population shift in the coral associated microbial community that was detected in Porites compressa fragments subjected to temperature stress (Vega Thurber et al 2009). A reduction in the available nutrients for heterotrophic bacteria in the SML would limit the number and diversity of microbes within the SML, potentially affecting the symbiotic relationship between corals and their microbial associates.…”

“…In addition to the protective physical properties, the SML also contains a microbial community that is crucial to the health of the coral (Bythell and Wild 2011). In the past ten years many studies have focused upon the composition and role of this microbial community Kellogg 2004;Bourne 2005;Guppy and Bythell et al 2006;Littman et al 2009;Shnit-Orland and Kushmaro 2009;Vega Thurber et al 2009;Meron et al 2011;Sweet et al 2011). This body of research has led to the development of the coral probiotic hypothesis, stating that coral-associated microbes may serve to protect their host based on their ability to rapidly evolve protection mechanisms .…”

“…For example, increased temperature and acidification caused the SML-associated microbial functional gene community to change (Vega Thurber et al 2009), and microbial communities shifted significantly at a lowered pH of 7.3 (Meron et al 2011). It was not determined in these studies whether the observed microbial functional gene and community shifts occurred in response to the changes in environmental conditions, or due to altered properties of the SML under the influence of these factors, which secondarily influenced the microbial communities.…”