Ocean acidification: the end for corals?

Coral reefs are massive structures, covering around 620,000km², and have given rise to incredibly diverse community assemblages. Included in this diversity are the photosynthetic symbionts associated mutualistically with the corals themselves and the other microorganisms which inhabit the coral tissues and structures. The interactions between these organisms are important for coral health and despite many reports focussing negatively on the relationship between microorganisms and corals; it is likely that most of the interactions are in fact positive and very influential as the microorganisms may provide nutrients and antimicrobial agents for protection. This has lead to the development of the coral probiotic hypothesis by Reshef et al. (2006) which suggests that by changing the coral-resident community, the coral holobiont can adapt more quickly and effectively to the changes in environmental conditions that we impose on them (increased temperatures, CO² concentration etc.).

This study focuses on the importance of reduced pH through ocean acidification and the effects it has on the scleractinian coral Acropora eurystoma and its microbial community. This was achieved by taking coral samples from the Red Sea which were split into two groups and subjected to two different pH’s: 8.2 (ambient seawater) and 7.3 (acidified). This was done for 10 weeks, after which the samples were fractionated and the bacteria isolated from each fractionation: skeleton, tissue and mucus. The DNA were extracted and PCR amplification of the 16S rRNA was carried out, the products of which were put through DGGE analysis in order to construct clone libraries to enable identification of the OTU’s present in each coral fractionation for each pH treatment. Finally, antibacterial screening was carried out on five common indicator bacterial strains to determine whether the coral bacteria had any beneficial antibacterial properties.

The DGGE analysis showed that in all fractionations, community composition was affected by pH. This was supported by the clone libraries which found a total of 103 OTU’s for the reduced pH of 7.3, compared to 74 for the ambient pH of 8.2. This increase was seen across several bacterial groups including the two dominant ones: Gammaproteobacteria and Cyanobacteria. Differences were also found in the antibacterial screening tests as the corals maintained at pH 7.3 showed a marked increase in bacteria producing antibacterial activity than those maintained at 8.2. The most noteworthy of these were the Vibrionaceae and Rhodobacteraceae which comprised 50 and 29% of the activity, respectively.

The increased diversity of bacteria in the reduced pH treatment could initially suggest that ocean acidification may not be as seriously damaging to some corals as has been previously thought. In fact, where the coral probiotic hypothesis is concerned, a greater diversity would seemingly allow corals more flexibility to adapt to changing environmental factors, as there would be more options of potentially beneficial bacteria to inhabit their tissues. However, as the authors note that the reduced pH used was not representative of the predicted levels for the next 100 years, it may be that such diversity would not be available in reality and so corals could still have trouble adapting to changing conditions and still be caused damage. Furthermore, it appears that several of the OTU’s found at the lower pH were genetically similar to strains that have been associated with coral disease previously; therefore this greater diversity may well be harmful to corals. Although, it must be noted that the abundance of some harmful bacteria such as Desulfobacter, a sulphate-reducing group partially responsible for black band disease, diminished in the reduced pH. This shows the need for more detailed and realistic research as in order for it to be useful, we must be able to apply it properly to real world scenarios by using real world parameter levels.

While several groups of the OTU’s found in the pH 7.3 fractionations are usually considered to be harmful to corals, none of the coral specimens in either treatment showed any sign of bleaching or disease. However, other authors have reported that just a few slight changes to the coral bacterial community can dramatically change the health of the coral and so here, in the reduced pH treatment, the shift to a more opportunistic set of OTU’s known to be associated with stressed or diseased corals seems to be an important biomarker for the risk that reduced pH may impose on corals and perhaps, over a longer time period signs of disease may have begun to show.

Consequently, this study appears to be of great importance as it highlights the need to monitor coral microbial communities more closely and to understand the relationship within the holobiont. With more detailed research of the area we may be able to protect corals and the mass of life they support better in the future, even though we cannot turn back time and undo the damage we have already inflicted upon them.

A review of: Dalit Meron, Elinor Atias, Lilach Iasur Kruh, Hila Elifantz, Dror Minz, Maoz Fine and Ehud Banin (2010) The impact of reduced pH on the microbial community of the coral Acropora eurystoma. The ISME Journal; 5, 51-60.