Host-pathogen interactions have been widely studied in humans and terrestrial plants, but are much less well explored in marine systems. Here we show that a marine macroalga, Delisea pulchra, utilizes a chemical defence - furanones - to inhibit colonization and infection by a novel bacterial pathogen, Ruegeria sp. R11, and that infection by R11 is temperature dependent. Ruegeria sp. R11 formed biofilms, invaded and bleached furanone-free, but not furanone-producing D. pulchra thalli, at high (24°C) but not low (19°C) temperatures. Bleaching is commonly observed in natural populations of D. pulchra near Sydney, Australia, during the austral summer when ocean temperatures are at their peak and the chemical defences of the alga are reduced. Furanones, produced by D. pulchra as a chemical defence, inhibit quorum sensing (QS) in bacteria, and this may play a role in furanone inhibition of R11 infection of furanone-free thalli as R11 produces QS signals. This interplay between temperature, an algal chemical defence mechanism and bacterial virulence demonstrates the complex impact environmental change can have on an ecosystem.

Host-associated bacterial communities are potentially critical components of marine microbial diversity, yet our understanding of bacterial distribution on living surfaces lags behind that for planktonic communities. We used 16S rRNA gene library analysis to compare within-host (alpha) and between-host (beta) diversity among bacterial communities on 3 co-occurring marine eukaryotes from temperate Australia: the demosponge Cymbastela concentrica, the red macroalga Delisea pulchra and the green intertidal alga Ulva australis. The bacterial community on C. concentrica had high phylum-level diversity (7 phyla including 3 proteobacterial classes) but relatively low 'species' richness (estimated at 24 species). Among the algae, D. pulchra contained 7 phyla including an estimated 79 species, while the U. australis library yielded only 4 phyla with an estimated 36 species. Alpha-, Delta-and Gammaproteobacteria were well represented in all libraries, while Planctomycetes and Bacteroidetes were relatively common on the 2 algae, but absent or rarely encountered in the sponge. At the phylum level, the community of C. concentrica largely mirrored that found in other marine sponges (e.g. Proteobacteria, Actinobacteria, Nitrospira), although large numbers of diatoms and the presence of Verrucomicrobia were atypical. Overall, within-host (alpha) diversity was relatively high, at least for C. concentrica and D. pulchra, while between-host (beta) diversity depended heavily on the phylogenetic level examined. Generally, there was a remarkable lack of overlap at the species level. No species showed universal distribution across hosts, indicating high beta diversity at the species level. At the level of phyla, however, both universal (e.g. Proteobacteria) and distinct (e.g. Nitrospira) groups existed. This study is among the first to compare patterns of alpha and beta diversity for microbial communities associated with co-occurring marine eukaryotes. KEY WORDS: Bacterial communities · Diversity · Marine · Host organisms · Sponge · MacroalgaResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 48: 217-229, 2007 1999, Rappé et al. 2000, Venter et al. 2004, Rusch et al. 2007, with the alphaproteobacterial SAR 11 clade accounting for up to 50% of all microbial cells in ocean surface waters (Morris et al. 2002). Cyanobacteria of the genera Synechococcus and Prochlorococcus are also extremely prevalent (Partensky et al. 1999, Scanlan & West 2002, while other frequently encountered marine taxa include the Bacteroidetes, Actinobacteria, Planctomycetes and Chloroflexi (Giovannoni et al. 1996, Simon et al. 1999, Cottrell & Kirchman 2000, Venter et al. 2004, Giovannoni & Stingl 2005, Rusch et al. 2007). This apparent global distribution of various bacterial phyla (and even particular species, e.g. Mullins et al. 1995 suggests a reasonable degree of uniformity to marine bacterioplankton communities. Despite this overall uniformity, however, compositional differences have also been observe...

Nautella sp. R11, a member of the marine Roseobacter clade, causes a bleaching disease in the temperate-marine red macroalga, Delisea pulchra. To begin to elucidate the molecular mechanisms underpinning the ability of Nautella sp. R11 to colonize, invade and induce bleaching of D. pulchra, we sequenced and analyzed its genome. The genome encodes several factors such as adhesion mechanisms, systems for the transport of algal metabolites, enzymes that confer resistance to oxidative stress, cytolysins, and global regulatory mechanisms that may allow for the switch of Nautella sp. R11 to a pathogenic lifestyle. Many virulence effectors common in phytopathogenic bacteria are also found in the R11 genome, such as the plant hormone indole acetic acid, cellulose fibrils, succinoglycan and nodulation protein L. Comparative genomics with non-pathogenic Roseobacter strains and a newly identified pathogen, Phaeobacter sp. LSS9, revealed a patchy distribution of putative virulence factors in all genomes, but also led to the identification of a quorum sensing (QS) dependent transcriptional regulator that was unique to pathogenic Roseobacter strains. This observation supports the model that a combination of virulence factors and QS-dependent regulatory mechanisms enables indigenous members of the host alga's epiphytic microbial community to switch to a pathogenic lifestyle, especially under environmental conditions when innate host defence mechanisms are compromised.