Temperature induced bacterial virulence and bleaching disease in a chemically defended marine macroalga.

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.

Ability of Pseudoalteromonas tunicata to colonize natural biofilms and its effect on microbial community structure.

We investigated the effectiveness of surface colonization by the epiphytic marine bacterium Pseudoalteromonas tunicata firstly on a complex biofilm community on glass slides, and secondly, on the epiphytic community of Ulva australis. The effectiveness of P. tunicata was compared with the performance of Phaeobacter sp. 2.10, also a marine epiphytic isolate in the U. australis colonization experiments. Pseudoalteromonas tunicata cells were able to colonize the glass slide community at densities found naturally in the water column (9.7 x 10(4) cells mL(-1)). However, P. tunicata was a poor invader of the epiphytic community on U. australis at densities of 10(6) cells mL(-1). At densities of 10(8) cells mL(-1), P. tunicata again exerted little impact on the epiphytic community. Phaeobacter sp. 2.10 was also a poor invader at lower densities, but was able to invade and become dominant at densities of 10(8) cells mL(-1). Differences in the ability of P. tunicata and Phaeobacter sp. 2.10 to invade natural communities may be due to differences in the antibacterial compounds produced by the two species. These experiments suggest that epiphytic communities may have protective effects compared with inanimate surfaces.

Variability and abundance of the epiphytic bacterial community associated with a green marine Ulvacean alga.

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.

Catalyzed reporter deposition-fluorescence in situ hybridization allows for enrichment-independent detection of microcolony-forming soil bacteria.

Advances in the growth of hitherto unculturable soil bacteria have emphasized the requirement for rapid bacterial identification methods. Due to the slow-growing strategy of microcolony-forming soil bacteria, successful fluorescence in situ hybridization (FISH) requires an rRNA enrichment step for visualization. In this study, catalyzed reporter deposition (CARD)-FISH was employed as an alternative method to rRNA enhancement and was found to be superior to conventional FISH for the detection of microcolonies that are cultivated by using the soil substrate membrane system. CARD-FISH enabled real-time identification of oligophilic microcolony-forming soil bacteria without the requirement for enrichment on complex media and the associated shifts in community composition.

A CARD-FISH protocol for the identification and enumeration of epiphytic bacteria on marine algae.

A CARD-FISH protocol was developed and applied to analyse surface-associated bacteria on the marine algae Ulva lactuca, Delisea pulchra, Corallina officinalis, Amphiroa anceps, Porphyra sp. and Sargassum linearifolium. The combination of Alexa(546)-labelled tyramide as the reporter molecule with SYBR Green II counterstain allowed for superior detection of the hybridised probe fluorescence against plant tissue from which pigment autofluorescence has been reduced.