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  • (2022) Nguyen, Viet Hung
    Thesis
    Sponges can harbour diverse communities of microbial symbionts, collectively referred to as a holobiont. Sponge symbionts play important ecological and mutualistic roles, including cycling of carbon, nitrogen, and sulfur; provisioning the host with essential compounds; and producing bioactive metabolites that confer fitness advantages to the holobiont. Recent metagenomics and bioinformatics advances facilitate genomic reconstruction and metabolic characterisation of uncultured prokaryotic species. Leveraging these tools, I reconstructed 75 metagenome-assembled genomes (MAGs) that represent 21 novel sponge-associated species: ten Gammaproteobacteria, six Acidimicrobiia, and five Acidobacteriota. The gammaproteobacterial species were metabolically diverse, likely representing adaptations to diverse habitats associated with different sponge species. Two species from the Candidatus genus Azotimanducus comprised almost identical patterns of organoheterotrophy that likely enabled them to colonize the same sponge host, but differed in other features that likely allowed for niche partitioning and cohabitation. Unlike the sponge-associated Gammaproteobacteria, the Acidimicrobiia shared very similar genomic features. Of particular interest, sponge-associated Acidimicrobiia were predicted to produce bioactive compounds that may modulate host signalling pathways, suggesting a potential role in host health. The sponge-associated Acidobacteriota likely predominantly formed symbiosis with their hosts prior to the phylogenetic split between sponges and corals. All five acidobacteriotal species shared similar patterns of organoheterotrophy, likely allowing for scavenging organic substrates from the host environment. Another feature that was specifically enriched in the novel sponge-associated Acidobacteriota was their capacity to produce diverse B-vitamins, with Candidatus Versatilivorator vitaminiformans comprising the genetic capacity to produce all of them. All 21 novel species also comprised unique respiratory, degradation, biosynthetic, and defensive features that likely mediate their interactions with the corresponding hosts. Features shared by the majority of the species were also identified. Altogether, the comprehensive genomic characterisation of sponge symbionts in this thesis has uncovered unique and shared features, highlighting the importance of extensive surveys into uncultured sponge symbiont diversity and function.

  • (2020) Khan, Mahjabeen
    Thesis
    Pseudomonas aeruginosa causes both contact lens and non-contact lens-related keratitis (corneal infection). This opportunistic bacterium naturally has the ability to resist the mechanism of action of many antibiotics which are used for treatment. P. aeruginosa resistance patterns and the mechanism of resistance in isolates from keratitis are not well understood. This thesis described the phenotypic and genotypic patterns of antimicrobial resistance and compared these between ocular isolates of P. aeruginosa from Australia (contact lens) and India (non-contact lens). Changes in the antimicrobial susceptibility between isolates over time were also analysed. Susceptibility to antibiotics, multipurpose disinfecting solutions and disinfectants was analysed for twenty-seven Australian isolates from contact lens-related keratitis and forty non-contact lens-related isolated from India. The whole genomes of fourteen Australian (historical and recent) and twelve Indian isolates were sequenced using Illumina® MiSeq®. Computational analysis of the genomes was performed to analyse their core and pan genomes and these were examined for the presence of acquired resistance genes, virulence genes, gene mutations, and these compared to their phenotypic resistance to antibiotics. Indian isolates possessed large pan genomes with more acquired resistance (30) genes and larger numbers of genetic variations. The Indian isolates contained clones of three sequence types ST308, ST316 and ST491, whereas Australian isolates contained only one sequence type ST233. Isolates with larger gene variations had mutations in the DNA mismatch repair system. Most multi-drug resistant Indian (non-contact lens) isolates were exoU +. Indian isolates had large accessory genes compared to Australian isolates and this increased the pan genome size of the Indian isolates. The number of core genome mutations were larger in the Indian isolates a median of 50006 (IQR=26967-50600) compared to Australian isolates a median of 26317 (IQR=25681-33780). There were differences between isolates from Australia and India with respect to their antibiotic resistance and associated genes. Indian strains had more genetic diversity and were multi-drug resistant. However, there was no evidence of substantial genetic or phenotypic changes within isolates from their respective countries.

  • (2021) Webb, David
    Thesis
    Planetary waves play a role in a large variety of oceanic and climate dynamics. In particular, Kelvin waves can provide rapid teleconnections from large-scale climate and weather events to remote regions of the globe. Kelvin waves may be partially responsible for linking climatic changes in Southern Ocean winds to increases in subsurface warming around Antarctica that can lead to glacial ice-melt and increases in global sea level rise. Kelvin waves may also link changes in Southern Ocean winds to increases in North Atlantic Deep Water (NADW) formation and an enhancement of the Atlantic Meridional Overturning Circulation (AMOC), which is responsible for circulating a vast amount of the ocean’s heat and nutrient content. However, the exact role of Kelvin waves in these processes is unclear. This thesis aims to further clarify the role that Kelvin waves play in these high-latitude climate processes. First, we use a suite of idealized models in order to better understand the dynamics of barotropic Kelvin waves around Antarctica. We find that super-inertial (high frequency) barotropic Kelvin waves are nearly completely scattered away from the Antarctic coastline due to a combination of coastal geometry and bathymetry. Sub-inertial (low frequency) barotropic Kelvin waves are mostly scattered away from the Antarctic coastline due to bathymetry, however a significant amount of barotropic Kelvin wave energy remains at the Antarctic coastline after one circumnavigation of the continent, enabling a gradual build-up of energy along the coast and the ability to sustain a barotropic Kelvin wave signal around Antarctica over time. Secondly, we perform a diagnostic study using theory and a range of varying resolution model simulations to quantify the amount of subsurface warming along the West Antarctic Peninsula caused by barotropic Kelvin waves via an induced bottom Ekman flow that advects warm Circumpolar Deep Water onto the Antarctic continental shelf. We find that barotropic Kelvin waves can account for a substantial amount of warming within one year, depending on the background temperature gradients and thickness of the bottom Ekman layer. Lastly, we explore the role of Kelvin waves in linking Southern Ocean wind-stress to NADW formation and the AMOC by analysing ensemble simulations from a fully-coupled ocean-sea-ice model at 1/4 degree horizontal resolution (50 vertical levels). We find first mode baroclinic Kelvin waves to propagate along a global coastal and equatorial waveguide from the Southern Ocean forcing region to the North Atlantic, where downwelling waves initiate an enhancement of the AMOC by making surface waters denser.