To study the effect of reduced pH on bacterial biofilms on tropical coral reefs, settlement tiles were deployed along pH gradients in December 2011 at two reefs in Papua New Guinea which hosted hydrothermal CO2 seeps. Biofilm samples were collected 5 and 13 months after deployment. This pangaea entry contains the table with the bacterial community composition based on Automated Ribosomal Intergenic Spacer Analysis (ARISA; arisa_OTU.txt), a table with the bacterial community composition of a subset of samples based on amplicon sequence counts (ENA project accession: PRJEB14127; amplicon_OTU.txt). A description of the columns in these data tables is provided in Readme.txt. Additional information about the sequenced samples (including ENA sample accession numbers) is provided in the environmental data set. Further data on this project is associated with a separate publication (DOI: 10.1038/srep09537).