The independent partnership of Thyasira cf. gouldi and its sulfur oxidizing symbiont

McCuaig, Bonita (2018) The independent partnership of Thyasira cf. gouldi and its sulfur oxidizing symbiont. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Symbiotic relationships between invertebrates and chemoautotrophic bacteria can be found in many marine environments. Here we focus on describing the genomic content of the extracellular symbionts of Thyasira cf. gouldi. Previous work described three symbiont phylotypes designated A, B, and C. These bacteria are all closely related sulfur oxidising gammaproteobacteria. Investigation of the ribulose-1,5-bisphosphate carboxylase (RuBisCo) gene showed a high level of diversity between bacterial populations hosted by Thyasira cf. gouldi. The RuBisCo had a higher diversity than the 16S rRNA gene and showed possible horizontal gene transfer events within phylotype A. A small number of host individuals also appeared to contain mixed populations of RuBisCo phylotypes not identified in 16S rRNA sequences. Draft genomes were created from sequencing of host gill tissue. These samples were selected to include bacteria of each phylotype. The draft genomes created were highly fragmented, but many genes were identified. The metabolic capabilities were very similar and encode for carbon fixation through the Calvin-Benson-Basham cycle. Energy for this process is obtained through oxidation of sulfur compounds through both a reverse dissimilar sulfate reduction cycle and a bacterial sulfur oxidation cycle. The bacteria may also be able to utilize nitrogen or hydrogen oxidation for respiration. A functional tricarboxylic acid cycle was also identified in all three genomes suggesting the bacteria may be capable of heterotrophy. All metagenomes contained chemotaxis and flagellar genes. An extracellular chromosome encoding a type IV secretion system was identified in the draft genome of phylotype B. If this plasmid is truly from the symbiont it could be used to export compounds to the host. Both phylotypes A and B contained genes involved in biofilm formation. A biofilm would benefit extracellular bacteria by creating a barrier from the host immune system and creating a controlled environment where the symbiont can exclude other environmental bacteria. Some genes associated with host interactions were also identified, although these were less conserved than the metabolic functions. Our results support the evidence that Thyasira cf. gouldi obtains its symbionts from the environment. The draft genomes contained evidence of a flexible metabolism that can adapt to changing sediment conditions.

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