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...

Marine Ulvacean algae are colonized by dense microbial communities predicted to have an important role in the development, defense and metabolic activities of the plant. Here we assess the diversity and seasonal dynamics of the bacterial community of the model alga Ulva australis to identify key groups within this epiphytic community. A total of 48 algal samples of U. australis that were collected as 12 individuals at 3 monthly intervals, were processed by applying denaturing gradient gel electrophoresis (DGGE), and three samples from each season were subjected to catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). CARD-FISH revealed that the epiphytic microbial community was comprised mainly of bacterial cells (90%) and was dominated by the groups Alphaproteobacteria (70%) and Bacteroidetes (13%). A large portion (47%) of sequences from the Alphaproteobacteria fall within the Roseobacter clade throughout the different seasons, and an average relative proportion of 19% was observed using CARD-FISH. DGGE based spatial (between tidal pools) and temporal (between season) comparisons of bacterial community composition demonstrated that variation occurs. Between individuals from both the same and different tidal pools, the variation was highest during winter (30%) and between seasons a 40% variation was observed. The community also includes a sub-population of bacteria that is consistently present. Sequences from excised DGGE bands indicate that members of the Alphaproteobacteria and the Bacteroidetes are part of this stable sub-population, and are likely to have an important role in the function of this marine epiphytic microbial community.

The nature of unusual aquatic microbial formations in flooded passages of cave systems in the Nullarbor region of Australia was investigated using electron microscopy and DNA analysis. The caves are located in a semiarid region but intersect the watertable at depths of approximately 100 m below the surface. Throughout submerged portions of the caves divers have noted the presence of unusual microbial formations. These 'microbial mantles' comprise sheets or tongues of mucoid material in which small crystals are embedded. Examination of the biomass revealed it to be primarily composed of densely packed, unbranched filaments, together with spherical-, rod- and spiral-shaped cells, and microcrystals of calcite in a mucoid matrix. Molecular phylogenetic analysis of the community structure revealed approximately 12% of clones showed high similarity to autotrophic nitrite-oxidizing bacteria (Nitrospira moscoviensis). The remainder of the clones exhibited a high proportion of phylogenetically novel sequence types. Chemical analysis of water samples revealed high levels of sulphate and nitrate together with significant nitrite. The community structure, the presence of nitrite in the water, and the apparent absence of aquatic macrofauna, suggest these microbial structures may represent biochemically novel, chemoautotrophic communities dependent on nitrite oxidation.

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