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.

Delisea pulchra is a red macroalga that produces furanones, a class of secondary metabolites that inhibit the growth and colonization of a range of micro- and macroorganisms. In bacteria, furanones specifically inhibit acyl homoserine lactone (AHL)- driven quorum sensing, which is known to regulate a variety of colonization and virulence traits. This thesis aims to unveil multiple aspects of the chemically mediated interactions between an alga and its bacterial flora. It was demonstrated that the quorum sensing genetic machinery of bacteria is laterally transferred, making traditional 16S rRNA gene based-diversity techniques poorly suited to identify quorum sensing species. Previous studies had shown that AHL-producing bacteria belonging to the roseobacter clade can be readily isolated from D. pulchra. Because of this, it was decided to use a roseobacter epiphytic isolate from this alga, Ruegeria strain R11, to conduct a series of colonization experiments on furanone free and furanone producing D. pulchra. Furanones were shown to inhibit Ruegeria strain R11's colonization and infection of D. pulchra. In addition, it was demonstrated that Ruegeria strain R11 has temperature-regulated virulence, similar to what is seen for the coral pathogen Vibrio shiloi. Rising ocean temperatures may explain bleached D. pulchra specimens recently observed at Bare Island, Australia. To assess whether quorum sensing is common within the roseobacter clade, cultured isolates from the Roseobacter, Ruegeria and Roseovarius genera were screened for AHL production. Half of the bacteria screened produced the quorum sensing signal molecules, AHLs. These AHLs were identified using an overlay of an AHL reporter strain in conjunction with thin layer chromatography (TLC). The prevalence of quorum sensing within the roseobacter clade, suggests that these species may occupy marine niches where cellular density is high (such as surface associated communities on substratum and marine eukaryotes). Diversity studies in marine microbial communities require appropriate molecular markers. The 16S rRNA gene is the most commonly used marker for molecular microbial ecology studies. However, it has several limitations and shortcomings, to which attention has been drawn here. The rpoB gene is an alternate ???housekeeping??? gene used in molecular microbial ecology. Therefore, the phylogenetic properties of these two genes were compared. At most taxonomic levels the 16S rRNA and rpoB genes offer similar phylogenetic resolution. However, the 16S rRNA gene is unable to resolve relationships between strains at the subspecies level. This lack of resolving power is shown here to be a consequence of intragenomic heterogeneity.

Delisea pulchra is a red macroalga that produces furanones, a class of secondary metabolites that inhibit the growth and colonization of a range of micro- and macroorganisms. In bacteria, furanones specifically inhibit acyl homoserine lactone (AHL)- driven quorum sensing, which is known to regulate a variety of colonization and virulence traits. This thesis aims to unveil multiple aspects of the chemically mediated interactions between an alga and its bacterial flora. It was demonstrated that the quorum sensing genetic machinery of bacteria is laterally transferred, making traditional 16S rRNA gene based-diversity techniques poorly suited to identify quorum sensing species. Previous studies had shown that AHL-producing bacteria belonging to the roseobacter clade can be readily isolated from D. pulchra. Because of this, it was decided to use a roseobacter epiphytic isolate from this alga, Ruegeria strain R11, to conduct a series of colonization experiments on furanone free and furanone producing D. pulchra. Furanones were shown to inhibit Ruegeria strain R11's colonization and infection of D. pulchra. In addition, it was demonstrated that Ruegeria strain R11 has temperature-regulated virulence, similar to what is seen for the coral pathogen Vibrio shiloi. Rising ocean temperatures may explain bleached D. pulchra specimens recently observed at Bare Island, Australia. To assess whether quorum sensing is common within the roseobacter clade, cultured isolates from the Roseobacter, Ruegeria and Roseovarius genera were screened for AHL production. Half of the bacteria screened produced the quorum sensing signal molecules, AHLs. These AHLs were identified using an overlay of an AHL reporter strain in conjunction with thin layer chromatography (TLC). The prevalence of quorum sensing within the roseobacter clade, suggests that these species may occupy marine niches where cellular density is high (such as surface associated communities on substratum and marine eukaryotes). Diversity studies in marine microbial communities require appropriate molecular markers. The 16S rRNA gene is the most commonly used marker for molecular microbial ecology studies. However, it has several limitations and shortcomings, to which attention has been drawn here. The rpoB gene is an alternate ???housekeeping??? gene used in molecular microbial ecology. Therefore, the phylogenetic properties of these two genes were compared. At most taxonomic levels the 16S rRNA and rpoB genes offer similar phylogenetic resolution. However, the 16S rRNA gene is unable to resolve relationships between strains at the subspecies level. This lack of resolving power is shown here to be a consequence of intragenomic heterogeneity.