How low can pH go? The effects of ocean acidification on biofilm microbial composition

Biofilms are made up of the early colonisers of rocky intertidal ecosystems; they act as the basis for succession and are vital to the grazing of organisms at higher trophic levels. Microbes are a large component of these biofilms and so it is important to study the diversity of such epilithic communities in order to monitor the effects of changing environmental conditions.

As such, in a study by Taylor et al, molecular techniques were used to examine microbial community composition at three sites along a pH gradient caused by a large CO₂ seep beneath Levante Bay in Italy. The seep itself produces around 3.6 tonnes of CO₂ per day which causes localised seawater acidification around it, leading to a pH of 5.6 that increases to normal Mediterranean sea pH as you move away from the seep. This can therefore act as an environmental model for the effects of global ocean acidification on intertidal epilithic microbial diversity.

Comparison of OTUs found at each site showed that community composition differed significantly between all three sites. And, while primary producers such as Proteobacteria, Bacteroidetes and Cyanobacteria were found to be dominant in all the sites, the diversity and abundance of such groups individually varied significantly between sites. The ecological processes driving these differences are currently unknown however it is thought that decreasing pH may incur improved carbon turnover and therefore create competition, niche modification and cross-feeding that will result in altered community composition.  In other words, changes in community composition the sites may indicate the identity of the potential ‘winners’ and ‘losers’ of the future changing environmental parameters caused by ocean acidification.

The importance of this is the use of a seep site; by using in situ sampling as opposed to laboratory manipulations it is possible to examine communities that have experienced ‘real’ environmental selection. This therefore lends a predictive advantage to such a study. However, there is a drawback to this technique in that it cannot possibly take into account all conceivable variables that may have led to this change in community composition. It is therefore my opinion that it is only the amalgamation of in situ environmental sampling around CO₂ seeps and the use of highly selective mesocosm-based laboratory experiments that will provide the most accurate model for the predicted future impacts of ocean acidification on microbial diversity and in turn ecosystem functioning.

Taylor, J. D, Ellis, R, Milazzo, M, Hall-Spencer, J. M. and Cunliffe, M.. (2014). Intertidal epilithic bacteria diversity changes along a naturally occurring carbon dioxide and pH gradient. FEMS Microbial Ecology. 89, 670-678.