Bacteria are ubiquitous to marine living surfaces, taking on a broad spectrum of roles from mutualistic to pathogenic. Despite their universality, much remains unknown about their basic ecology and interactions with higher organisms. To address this gap, this thesis firstly examines the bacterial communities associated with three co-occurring marine eukaryote hosts from temperate Australia: the demosponge Cymbastela concentrica, the subtidal red macroalga Delisea pulchra and the intertidal green macroalga Ulva australis. Molecular characterisation of the bacterial communities was undertaken using 16S rRNA gene library analysis to compare within-host (alpha) and between-host (beta) diversity for the three microbial communities. This study highlights the potentially substantial contribution host-associated microorganisms could have on marine microbial diversity. The remaining focus for this thesis was on the bacterial community associated with D. pulchra. This alga produces a suite of biologically active secondary metabolites (furanones) that non-toxically inhibit acyl homoserine lactone (AHL)-driven quorum sensing in bacteria, affecting a range of phenotypes including colonisation and virulence traits. The ecology of D. pulchra???s epiphytic bacteria was investigated using a mechanistic approach to explain bacterial colonisation patterns. In particular, concepts and models of ecological succession founded in eukaryote ecology were investigated. The thesis concludes with a study investigating the effect of furanones and elevated temperature on bacteria-induced disease and thallus bleaching of D. pulchra. In the presence of furanones colonisation and infection of two Roseobacter isolates from D. pulchra???s epiphytic bacterial community were inhibited. Ruegeria strain R11 was demonstrated to have temperature regulated virulence, which caused thallus bleaching in furanone-free algae. The implications of elevated sea temperatures resulting from global warming for algal health are discussed.

Bacteria are ubiquitous to marine living surfaces, taking on a broad spectrum of roles from mutualistic to pathogenic. Despite their universality, much remains unknown about their basic ecology and interactions with higher organisms. To address this gap, this thesis firstly examines the bacterial communities associated with three co-occurring marine eukaryote hosts from temperate Australia: the demosponge Cymbastela concentrica, the subtidal red macroalga Delisea pulchra and the intertidal green macroalga Ulva australis. Molecular characterisation of the bacterial communities was undertaken using 16S rRNA gene library analysis to compare within-host (alpha) and between-host (beta) diversity for the three microbial communities. This study highlights the potentially substantial contribution host-associated microorganisms could have on marine microbial diversity. The remaining focus for this thesis was on the bacterial community associated with D. pulchra. This alga produces a suite of biologically active secondary metabolites (furanones) that non-toxically inhibit acyl homoserine lactone (AHL)-driven quorum sensing in bacteria, affecting a range of phenotypes including colonisation and virulence traits. The ecology of D. pulchra???s epiphytic bacteria was investigated using a mechanistic approach to explain bacterial colonisation patterns. In particular, concepts and models of ecological succession founded in eukaryote ecology were investigated. The thesis concludes with a study investigating the effect of furanones and elevated temperature on bacteria-induced disease and thallus bleaching of D. pulchra. In the presence of furanones colonisation and infection of two Roseobacter isolates from D. pulchra???s epiphytic bacterial community were inhibited. Ruegeria strain R11 was demonstrated to have temperature regulated virulence, which caused thallus bleaching in furanone-free algae. The implications of elevated sea temperatures resulting from global warming for algal health are discussed.