Ocean acidification alters microeukaryotic and bacterial food web interactions in a eutrophic subtropical mesocosm.

Ocean acidification (OA) is known to influence biological and ecological processes, mainly focusing on its impacts on single species, but little has been documented on how OA may alter plankton community interactions. Here, we conducted a mesocosm experiment with ambient (∼410 ppmv) and high (1000 ppmv) CO 2 concentrations in a subtropical eutrophic region of the East China Sea and examined the community dynamics of microeukaryotes, bacterioplankton and microeukaryote-attached bacteria in the enclosed coastal seawater. The OA treatment with elevated CO 2 affected taxa as the phytoplankton bloom stages progressed, with a 72.89% decrease in relative abundance of the protist Cercozoa on day 10 and a 322% increase in relative abundance of Stramenopile dominated by diatoms, accompanied by a 29.54% decrease in relative abundance of attached Alphaproteobacteria on day 28. Our study revealed that protozoans with different prey preferences had differing sensitivity to high CO 2 , and attached bacteria were more significantly affected by high CO 2 compared to bacterioplankton. Our findings indicate that high CO 2 changed the co-occurrence network complexity and stability of microeukaryotes more than those of bacteria. Furthermore, high CO 2 was found to alter the proportions of potential interactions between phytoplankton and their predators, as well as microeukaryotes and their attached bacteria in the networks. The changes in the relative abundances and interactions of microeukaryotes between their predators in response to high CO 2 revealed in our study suggest that high CO 2 may have profound impacts on marine food webs. • The communities were strongly affected by phytoplankton bloom stages. • Ocean acidification (OA) affected some taxa as the phytoplankton bloom stages progressed. • OA changed the co-occurrence network complexity and stability of microeukaryotes. • OA altered the proportions of potential interactions between phytoplankton and their predators. [ABSTRACT FROM AUTHOR]