AbstractsBiology & Animal Science

Macroalgae, the ecosystem engineers of temperate marine habitats, are particularly vulnerable to climate change. Environmental stress is linked to a decline in macroalgal health and can render the host susceptible to attack by opportunistic pathogens. Bacteria form an important part of the macroalgal holobiont; however, bacterial pathogens are also major contributors to disease in macroalgae. Despite the importance of bacterial interactions with macroalgal hosts, there remains a paucity of knowledge regarding the molecular mechanisms that facilitate bacterial colonisation in this context. The aim of work conducted in this thesis was to investigate the molecular mechanisms that facilitate bacterial colonisation and persistence on macroalgae using two model systems- the red alga Delisea pulchra and the green alga Ulva autsralis. Increased sea surface temperatures reduce the host defence capacity of D. pulchra, and increase the susceptibility of the macroalga to infection by opportunistic pathogens, including the Roseobacter species, Nautella sp. R11. This research utilised molecular techniques, including allelic exchange mutagenesis and quantitative proteomics, in combination with attachment assays and colonisation experiments to identify molecular factors that contribute to the virulence of Nautella sp. R11. This is the first study to elucidate a role for the stress-resistance enzyme, glutathione peroxidase, and two cell density dependant transcriptional regulators (VarR and RaiR) in mediating virulence in a macroalgal pathogen. The second model system focused on the mechanisms utilised by the bacterium Pseudoalteromonas tunicata to competitively colonise the surface of U. australis. The study identified a role for the P. tunicata lipoprotein, PtlL32, in mediating the interaction between the bacterial epibiont and its host macroalga. This work provides the first evidence of a role for LipL32 outside of the human pathogen, Leptospira interrogans, where the protein facilitates attachment to mammalian extracellular matrix factors. The work presented in this thesis builds upon the previously limited understanding of the molecular factors that facilitate bacterial colonisation on macroalgae and provides insight into the dual-function characteristics that confer virulence in environmental bacteria.