Responses of Intertidal Bacterial Biofilm Communities to Increasing pCO₂

The effects of ocean acidification on ecosystems remain poorly understood, because it is difficult to simulate the effects of elevated CO₂ on entire marine communities. Natural systems enriched in CO₂ are being used to help understand the long-term effects of ocean acidification in situ. Here, we compared biofilm bacterial communities on intertidal cobbles/boulders and bedrock along a seawater CO₂ gradient off Japan. Samples sequenced for 16S rRNA showed differences in bacterial communities with different pCO₂ and between habitat types. In both habitats, bacterial diversity increased in the acidified conditions. Differences in pCO₂ were associated with differences in the relative abundance of the dominant phyla. However, despite the differences in community composition, there was no indication that these changes would be significant for nutrient cycling and ecosystem function. As well as direct effects of seawater chemistry on the biofilm, increased microalgal growth and decreased grazing may contribute to the shift in bacterial composition at high CO₂, as documented by other studies. Thus, the effects of changes in bacterial community composition due to globally increasing pCO₂ levels require further investigation to assess the implications for marine ecosystem function. However, the apparent lack of functional shifts in biofilms along the pCO₂ gradient is a reassuring indicator of stability of their ecosystem functions in shallow ocean margins.