With increasing plastic pollution, the biofilm community on marine plastic debris is becoming increasingly important, but its responses to global changes such as ocean acidification are unknown. We compared the whole metagenome of biofilms on plastic drinking bottles kept in subtidal waters naturally enriched in CO₂, and for comparison normal ambient CO₂ levels, off the southern coast of Japan. In an earlier study using 16s amplicon data, we found significant changes in biofilm bacterial community. Here, metagenomics allowed the full range of taxa and functional genes to be compared. Analysis showed a biofilm community incorporating a broad range of the diversity of life, including eukaryotes and archaea, and that ocean acidification affected all groups. There was a large increase in the proportion of eukaryotes with increasing acidification. There was also a major increase in diatom relative abundance, but a decrease in their diversity, while cyanobacteria stayed at similar relative abundance. Decreased prevalence of genes associated with cell-cell interactions suggested weaker biofilm intergration and resilience. There was an increase in stress genes with acidification, suggesting decreased resilience of the biofilm community, but an increase in genes associated with detoxification and aromatic compound breakdown suggesting increased ability to degrade xenobiotic pollutants. However, antibiotic resistance genes decreased overall. There were few major nitrogen and cycling gene changes, apart from decreased abundance of ammonification genes, suggesting that the role in nutrient processes of plastics in an acidified ocean may not change much. However, the abundance in iron acquisition genes decreased with acidification, suggesting decreased sequestering of this nutrient.  The snapshot provided by this small scale experiment suggests that as the world’s oceans acidify, the plastisphere will undergo significant changes with changes in taxonomic composition towards a greater role for diatoms, and changes in nutrient cycling.
 

biofilms, carbon dioxide, functional genes, ocean acidification, plastisphere