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Coral Microbial Communities Along a Natural pH Gradient
Volume 16, Number 6: 6 February 2013

In a study published in the ISME Journal, Meron et al. (2012) write that ocean acidification "has the potential to cause significant perturbations to the physiology of ocean organisms, particularly those such as corals that build their skeletons/shells from calcium carbonate," and they say that this phenomenon "could also have an impact on the coral microbial community, and thus may affect coral physiology and health." However, they note that most studies of declining pH effects on corals and/or their associated microbiota have typically been done under "controlled laboratory conditions," which approach clearly ignores any impacts declining pH values may have on the coral holobiont, some of which may be negative and some positive, which in the latter case is referred to as the probiotic hypothesis, as per Reshef et al. (2006).

Departing from this common protocol, the six scientists took advantage of a natural pH gradient off the coast of Ischia (Gulf of Naples, Italy), which is created by an underwater CO2 flux from volcanic vents (Hall-Spencer et al., 2008). This they did for the purpose of examining potential impacts of a range of pH conditions (7.3 to 8.1) on coral microbial communities living under natural real-world conditions, focusing on two Mediterranean coral species: Balanophyllia europaea and Cladocora caespitosa. And what did they find?

"We found," as the research team writes in the abstract of their paper, that (1) "pH did not have a significant impact on the composition of associated microbial communities in both coral species," that (2) "corals present at the lower pH sites exhibited only minor physiological changes," and that (3) "no microbial pathogens were detected." Thus, they conclude that "at least for these two coral species, reduced pH does not seem to significantly reduce coral health," which further suggests that some of the contrary results obtained in laboratory studies could be due to the fact that "laboratory environments cannot mimic the dynamism and microbial diversity present in nature," as well as the possibility that "aquarium conditions themselves contribute to stress or disturbance in the microbial community," which view is supported in part by the finding of Kooperman et al. (2007) that "the same coral species has different associated microbial communities in the laboratory compared with field conditions."

Clearly, we need more studies of the potential effects - or non-effects - of declining seawater pH to be conducted in the real world of nature.

Sherwood, Keith and Craig Idso

Hall-Spencer, J.M., Rodolfo-Metalpa, R., Martin, S., Ransome, E., Fine, M., Turner, S.M., Rowley, S.J., Tedesco, D. and Buia, M.-C. 2008. Volcanic carbon dioxide vents show ecosystem effects of ocean acidification. Nature 454: 96-99.

Kooperman, N., Ben-Dov, E., Kramarsky-Winter, E., Barak, Z. and Kushmaro, A. 2007. Coral mucus-associated bacterial communities from natural and aquarium environments. FEMS Microbiology Letters 276: 106-113.

Meron, D., Rodolfo-Metalpa, R., Cunning, R., Baker, A.C., Fine, M. and Banin, E. 2012. Changes in coral microbial communities in response to a natural pH gradient. ISME Journal 6: 1775-1785.

Reshef, L, Koren, O., Loya, Y., Zilber-Rosenberg, I. and Rosenberg, E. 2006. The coral probiotic hypothesis. Applied Environmental Microbiology 8: 2067-2073.