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ItemVariation in Coral Microbiome Composition and Transcriptional Activity of Three Corals over Diel Cycles(Georgia Institute of Technology, 2020-05) Caughman, Alicia MarieUnderstanding the dynamics of coral microbiome composition and function is important because microbiomes play important roles in coral health and metabolism. While numerous long-term studies have investigated changes in the microbiome due to various physical or biotic stressors, little is known about the stability of the coral microbiome over diel cycle. For certain parameters (e.g., temperature, pH), the magnitude of diel fluctuation can exceed that observed in mean values over seasons, especially on shallow reefs. Such short-term environmental heterogeneity can affect longer term trends, for example by influencing the extent to which corals acclimate to stress and increase resilience. This study examined diel dynamics of microbiomes in three coral species (Porites lutea, Porites cylindrica, and Pocillopora damicornis) from a shallow, backreef lagoon in Mo’orea (French Polynesia). Porites is relatively resistant to stress, being one of the last coral genera to succumb to bleaching or several diseases, and one of the more abundant genera of corals remaining on degraded reefs. In contrast, Pocillopora is less resistant to many of these stresses and bleaches easily but is one of the genera that commonly recruits and rebounds rapidly after disturbances or strong stresses. We assessed microbiome taxonomic composition and relative transcriptional activity by analyzing 16S rRNA gene and transcript sequences from six time points over 48 hours for each of these coral species. Results showed that composition in P. damicornis varied significantly over the diel period, while composition in both Porites species remained more stable. However, the taxonomic composition of the transcript pool did not vary significantly over time across all corals sampled. This shows that diel stability of these coral microbiomes is dependent on host species, which could contribute to differences in host responses to environmental stressors.
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ItemSignificant changes in microbial community composition in the Gulf of Mexico "Dead Zone" over a diel cycle(Georgia Institute of Technology, 2014-05-02) Cartee, John C.The structure and diversity of microbial communities associated with the oxygen minimum zone located on the Louisiana Shelf in the northern Gulf of Mexico deadzone was studied through amplicon analysis of the 16S rRNA gene. The oxygen minimum zone located on the Louisiana Shelf is a region of reduced oxygen concentrations, containing dynamic and diverse microbial communities that thrive under microaerophilic and anaerobic conditions. The Gulf of Mexico contains one of the largest zones of coastal hypoxia (region of reduced dissolved oxygen concentrations) which is dominated by complex microbial communities that contribute to marine biogeochemical cycling on a global scale. Here we used next-generation sequencing technology to track the microbial community at a single site over a day-night (diel) cycle. Two varying depths were used to collect seawater samples which were used for amplicon sequencing of the 16S rRNA gene (rDNA). By comparing our genetic data to coupled measurements of oxygen and nutrients, we determined how microbial community composition changes in response to day-night gradients and to environmental variation in oxygen and substrate availability.
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ItemThe Effects of Variation in Electron Donor Concentration and Type on Deep-Sea Endosymbiont Community Composition and Gene Expression(Georgia Institute of Technology, 2014-05-02) Shockey, Abigail C.Chemosynthetic symbioses are among the most prevalent microbial symbioses found in marine systems. These associations often dominate reducing environments such as hydrothermal vents, where they play critical roles in biogeochemical cycling. Among the diverse number of organisms that participate in chemosynthetic symbioses is Ifremeria nautilei, a gastropod found surrounding the deep-sea hydrothermal vents of the South Pacific Ocean. Little is known about how chemosynthetic symbiont community composition and gene expression change in response to gradients of electron donors in the vent environment. Understanding these changes offers significant insight into the environmental conditions and physiological mechanisms necessary to sustain the relationship present between host and symbiont. To address this question, individual Ifremeria were collected from the Lau Basin hydrothermal vent system and placed in pressurized, sterile aquaria under the following conditions: i) no electron donor, ii) 100 µM hydrogen sulfide, iii) 300 µM hydrogen sulfide, iv) 300 µM thiosulfate. Stable carbon isotope (13C) incorporation rates were determined for each condition, with 300 µM thiosulfate yielding the highest average rate of carbon incorporation. Amplicon (16S rRNA gene) and metatranscriptomic sequencing were used to compare the phylogenetic diversity and differential gene expression of the symbiotic communities in gill tissue excised from Ifremeriain each treatment. Amplicon analyses revealed two major symbiont lineages within the phylum γ-proteobacteria: putative sulfur-oxidizing symbionts of the Chromatiales and methane-oxidizing symbionts of the Methylococcales. Of these, Chromatiales symbionts dominated, consisting of a single operational taxonomic unit (OTU) representing 81.2-99.6% of the symbiont population. Methylococcales symbionts were represented by two distinct OTUs (0.003-17.5% of sequences) and were present in all host individuals, excluding those exposed to 300uM hydrogen. Preliminary results of the metatranscriptome analysis confirm the expression of genes from both symbiont pools, including genes mediating sulfur oxidation and methane oxidation, despite an assumed lack of methane in the treatments. Genes for sulfur oxidation were ten-fold higher in abundance than those for methane oxidation. These results confirm that Ifremeria engages in a "dual" symbiont strategy using thiotrophic and methanotrophic partners and that this community may be sensitive to changes in electron donor availability, due potentially to symbiont competition within the host, host sanctions of symbiont "cheaters," or direct effects of substrate (sulfide) toxicity.