Species turnover underpins the effect of elevated CO2 on biofilm communities through early succession

Biofilms harbour a wealth of microbial diversity and fulfil key functions in coastal marine ecosystems. Elevated carbon dioxide (CO₂) conditions affect the structure and function of biofilm communities, yet the ecological patterns that underpin these effects remain unknown. We used high-throughput sequencing of the 16S and 18S rRNA genes to investigate the effect of elevated CO₂ on the early successional stages of prokaryotic and eukaryotic biofilms at a CO₂ seep system off Shikine Island, Japan. Elevated CO₂ profoundly affected biofilm community composition throughout the early stages of succession, leading to greater compositional homogeneity between replicates and the proliferation of the potentially harmful algae Prymnesium sp. and Biddulphia biddulphiana. Species turnover was the main driver of differences between communities in reference and high CO₂ conditions, rather than differences in richness or evenness. Our study indicates that species turnover is the primary ecological pattern that underpins the effect of elevated CO₂ on both prokaryotic and eukaryotic components of biofilm communities, indicating that elevated CO₂ conditions represent a distinct niche selecting for a distinct cohort of organisms without the loss of species richness.