The Effects of Variation in Electron Donor Concentration and Type on Deep-Sea Endosymbiont Community Composition and Gene Expression
Shockey, Abigail C.
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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.