A review of: Dalit Meron, Elinor Atias, Lilach Iasur Kruh, Hila Elifantz, Dror Minz, Maoz Fine and Ehud Bani (2011) The impact of reduced pH on the microbial community of the coral Acropora eurystom
It is well known that corals are particularly fragile. Tourism, overfishing and environmental change have all contributed to a decrease in the numbers of these living structures. Their symbiotic relationship with zooxanthallae and other bacteria or archaea may be flexible, but rapid environmental change could lead to further destruction. Ocean acidification is an emerging source of stress to corals and is the result of increased CO2 in our atmosphere. This affects the relationships between corals and their symbiotic bacteria and also profound physiological changes have been recorded. CO2 decreases the saturation state of the sea with respect to two coral building minerals, calcite and aragonite. These allow for the building of calcium carbonate structures and their decrease could lead to physiological changes, not only in corals but in any calcium carbonate depositing organisms, like shellfish. This journal however investigates the coral Acropora eurystom with particular focus on the changes that take place to its microbial community in relation to the increased acidity of the sea.
DGGE (Denaturing Gradient Gel Electrophresis) was used to determine whether pH affected the microbial community in different fractions of A. eurystom. In each of the corals’ mucus, tissue and skeleton fractions, the analysis revealed two completely different symbiotic communities when kept at two different pH’s, 8.2 (ambient seawater) and 7.3. To quantify the species diversity of the microbial communities the Shannon – Wiener index was calculated. With indexes of 4.34 and 3.94 it is clear to see there is an increased microbial diversity found in the corals maintained at pH7.3 compared to those kept at pH 8.2, respectively. The authors here suggest that this may be due to intermediate disturbance, the disturbance being, in this case, a lowering of the pH which favours a greater diversity of bacteria. However, 16s rRNA gene libraries, used to interpret the diverse bacterial colonies, show that some of the DNA sequences found in corals at this lowered pH are closely related to groups found in diseased or bleached corals. Bacteria such as Alphaproteobacteria, Vibrionaceae and Alteromonadaceae were either isolated to corals maintained at a lower pH or their species richness increased once pH was lowered; Alphaproteobacteria increased from 11 – 20% once the pH was lowered from 8.2 to 7.3. Opportunistic bacteria that favour more acidic conditions therefore cause a shift in the microbial community of A. eurystom towards that of an unhealthy one. It is important to note however that there was no disease found during this experiment.
The antibacterial ability of 165 isolated microbes found in the coral host was tested as it is an important marker of host colonisation. Five indicators were used including E. coli and the coral pathogen Vibrio coralliilyticus, known for causing bleaching. An increase of antibacterial activity from 17 – 42% was recorded once pH was lowered and conditions became more acidic. The increased antibacterial activity supports other papers which have associated secondary metabolites such as antibiotics with increased stress and therefore change of microbial community within the coral.
This paper provides evidence that bacterial shifts occur within coral hosts when its surroundings change. This is due to opportunistic or specific bacteria, which flourish in acidic conditions, outcompeting those which are strongest in ambient seawater. The bacteria found at a lowered pH, however, are only related to known pathogens and no exact matches were detected. Therefore, longer term experiments need to take place in order to provide sound evidence that these new bacteria will be severely detrimental to the holobionts health, otherwise couldn’t they just become harmless new tenants?
8 comments:
Hi Sara
I have seen many papers that looked at the affect of rising sea temp on coral bacterial communities. Good to see some research has been carried out focusing on a different factor.
You mentioned that a lowerring in pH resulted in a bacterial shift but were only related to known pathogens. I was just wandering if that ment no Vibrio coralliilyticus isolates were found?
Hello Lee. Yes, I have also seen effect of temperature on coral health being described in many papers, this, I think, is the first I've seen investigating pH. Answering your question, the paper mentions an increase of Vibrionacea, which includes some species of Vibrio (vague but this is all the authors have written). The paper uses V. coralliilyticus as a tester of the antibacterial activity of isolates identified, but this is the only place where it is mentioned specifically. I think that if it was identified in the corals at a lower pH it would have been highlighted as it is, of course, a well known coral pathogen. The paper does state that no identical sequences of known corals were detected only close homologues. SO you're correct, no V. coralliilyticus isolates were detected and this is why I think longer experiments should take place to see how damaging these similar isolates could be.
Sara - these OA experiments (similar to the many conducted in Plymouth) are of course subject to the proviso that in the real world, such extreme shifts in pH are unlikely and there will be a gradual decrease in pH over many years, so some adaptation may occur. The exception would be in the case of sudden CO2 release from proposed undersea CO2 storage systems. In this case, it looks like they acidified the water and plonked half of the corals straight in, so it is morel like the CO2 storage release accident scenario. After 10 weeks the community had shifted. What I find a bit surprising is that they didn't look at all at the health or physiological status of the corals (e.g. the activity of zooxanthellae).
This is really interesting and seems to have a similar idea to the paper ive read for the seminar next week (Mao-jones et al 2010), which is about the mechanism behind temperature causing microbial shifts. This also concludes the affects of opportunistic pathogenic bacteria, that can survive higher temperatures than the beneficial bacteria. It seems interesting that the pathogenic bacteria always seem to be more resistant than the beneficial and I wonder whether there are any examples where this isnt the case!
Colin - The pH of the water was decreased gradually over the ten weeks but this is not long enough for adaptation to occur I imagine. It is apparently the first investigation that does gradually decrease the pH and look at how this effects holobionts over a long (ish) period of time, compared to others which place some coral in one pH and another at a lower pH. However intermediate readings were not taken and as you mentioned neither was the activity of zooxanthellae. Would a longer experiment be viable in the lab?
Il try to find a paper which looks at zooxanthellae health as pH decreases over time. or do you have any suggestions?
natasha I'l let you know if I find anything on increasing temperature benefiting beneficial bacteria.
Sara - there is a recent paper that looks at pH effects on the zoox. Showing that responses vary according to the phylotype. thi might be worth a look. There is an abstract here
http://oceanacidification.wordpress.com/2011/05/16/differential-effects-of-ocean-acidification-on-growth-and-photosynthesis-among-phylotypes-of-symbiodinium-dinophyceae/
Thank you Colin
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