Bacteria play a crucial role in the health and survival of the world’s coral populations. The attachment of bacteria and development of a biofilm is a major factor in the bacteria’s influence on the coral communities and whether the coral will, through symbiotic relationships, be sustained or recede through disease.
The authors created artificial resin nubbins of the scleractinian coral Acropora muricata possessing similar contours to natural coral. After dip-coating in different agar types (plain agar and agar containing a range of healthy and stressed coral exudates) the nubbins were placed on the reef and samples taken at times up to 96 hours to determine bacterial biofilm diversity at different stages. Microscope slides were also dipped in plain agar and placed on the reef at the same time to determine the difference of biofilm formation between a smooth surface and a textured surface (artificial nubbin). The nubbins showed greater 16S rRNA (hypervariable between bacterial species) diversity after 4 hours indicating that textured surfaces with features such as crevices allow motile bacteria to form a biofilm whereas exposure to hydrodynamic forces inhibited growth on the slides. There was no significant difference in the 16S rRNA diversity between each different agar type and, therefore, further experiments used artifical nubbins coated in plain agar. This indicates that coral morphology, more so than chemical factors, is crucial for biofilm formation and, therefore, those early colonisers are key to later development of coral microbial communities.
Significant differences in biofilm composition between seasons were also observed with up to 22% difference between each season. Although no exclusive ribotypes existed, shifts in dominance of different bacterial species could be seen between summer and winter seasons with γ-proteobacteria spp. dominant in winter and Flavobacteriaceae spp. and Pseudoalteromonas spp. dominant in summer. Different seasons also showed significant differences in the bacterial communities between the early colonising bacteria and the bacteria found in the mature biofilm. Further seasonal analysis showed that the ribotype diversity of biofilms was similar to the water column after 8 hours in winter whereas summer showed no overall pattern indicating less stable biofilm formation. Also, between both seasons dominant ribotypes of pioneer species between 2-4 hours were absent in mature biofilms with Vibrio spp. being present in some samples at 2 hours but not present after 6 hours.
Samples from the water column and swabs of SML were collected alongside biofilm samples for comparison and showed that, after 96 hours, there were significant differences in the diversity of the water column and the SML. The water column had a stable community including α-proteobacteria spp. and Flavobacteriaceae spp. whereas the SML had a similar diversity as the 96 hour biofilm containing γ-proteobacteria spp. Where the authors hypothesised that early biofilm diversity would be similar to diversity of the water column the results indicate that developing biofilm bacteria consist mainly of less abundant bacterial species from the water column. The differences in developing bacterial communities on corals are also influenced by antimicrobials in coral exudates to prevent microbial fouling and also by other commensal bacteria producing antimicrobials. This can be seen where Pseudoalteromonas spp. actively inhibit other species such as Vibrio spp. where, at 2 hours, Vibrios are present in the biofilm but, after 6 hours, they have been outcompeted by the Pseudoalteromonas spp. This relationship is also seen where the presence of Vibrio inhibiting bacteria are reduced on stressed coral allowing Vibrio spp. to cause disease and subsequent coral bleaching.
Spatial variability was also assessed around the reef for differences in both the water column and differences in developing biofilms. Each site was chosen for differences in benthic structure and differences in the predicted water movements. 16S rRNA showed high similarities at each site after 24 hours for dominant biofilm bacteria. The overall bacterial communities and bacterial communities of the water column at each site were also significantly difference except at two different sites which could indicate rapid benthic-pelagic coupling in the communities. Therefore it is impossible to determine whether late developing communities are controlled by initial colonisers or by continual settlement from the water column.
This study has sown that developing bacterial communities found on biofilms remains distinct from that of the supply (water column). Also, surface structure significantly affects the initial bacterial assemblages and, therefore, is the key factor driving change.
Review of : Sweet,M.J; Croquer, A; Bythell,J.C; Development of Bacterial Biofilms on Artificial Corals in Comparison to Surface-Associated Microbes of Hard Corals; 2011; PLoS One.2011; 6(6): e21195
2 comments:
Hi Theo
Interesting blog.
This is proving to be a very complex subject in marine symbiosis. I spent the summer working with a PhD student looking at antagonism between the different species of bacteria found in the coral mucus. Originally, I thought that for any one coral there would be just a hand full of bacterial species. However, for one species of Acropora coral there were over 40species of bacteria and the way they affected eachother was so complex. Some species produced antibiotics and so hindered other species, while the presence of some species caused others to swarm. The data we got was mind boggerling.
I was fortunate enough to examine Michael Sweet for his PhD. He used some really imaginative approaches to reproduce coral surfaces for this experimental approach and told some great stories of the problems of planting them on reefs and following them up. It's a rare example of a real attempt to conduct experiments in natural coral ecosystems.
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