16SrRNA gene microbial community profiling about a naturally occuring pH gradient.

Intertidal epilithic bacteria communities are important components of coastal ecosystems, yet few studies have assessed their diversity and how it may be affected by changing environmental parameters. Submarine volcanic CO2 vents produce localised areas of CO2-enriched seawater with reduced pH levels. We utilised the well-characterised seawater pH/CO2 gradient at Levante Bay (Italy) to determine changes in bacteria diversity using high-throughput sequencing of 16S rRNA genes. Intertidal epilithic biofilms were sampled from three sites exposed to seawater with varying pH levels; site 1 pH 8.2 (normal), site 2 pH 7.6 and site 3 pH 7.2. Alphadiversity of total bacteria communities and Cyanobacteria communities was significantly different between sites, being lowest at site 3. Comparison of communities between sites (betadiversity) showed that both total bacteria communities and Cyanobacteria communities were significantly different. All communities were dominated by the phyla Proteobacteria, Bacteroidetes and Cyanobacteria, however there were significant differences between sites in the relative abundance of specific orders. This study provides the first detailed assessment of intertidal epilithic bacteria community diversity, and shows that diversity is significantly different along a naturally occurring seawater pH/CO2 gradient. This information supports the evaluation of the impacts of future ocean acidification on coastal marine ecosystems.