Ocean acidification (OA) is one of the most widespread anthropogenic influences on
Earth. Many studies have assessed its impacts on single species, but there is scant
knowledge about how OA alters planktonic organism interactions. Here, we
conducted a mesocosm experiment in a eutrophic region of the East China Sea, to
examine community composition dynamics and network structure of
microeukaryotes, bacterioplankton and eukaryote-attached bacteria in coastal
seawater treated with 1000 ppmv atmospheric CO2 (HC, compared with 410 ppmv
CO2), by sequencing the18S rDNA gene V9 region and 16S rDNA gene V4-V5
region. The diversity of organisms found in each treatment was not significantly
different, but succession and community structure were significantly altered by HC.
Metazoa had higher relative abundance under HC throughout the experiment whereas
Cercozoa had reduced relative abundance at the mid-stage of a phytoplankton bloom.
Stramenopiles dominated by Bacillariophyta, as well as eukaryote-attached
Proteobacteria, were more abundant under HC conditions, at the late stage of a
phytoplankton bloom. This is likely a combined response to eutrophication and HC as
such profound changes have not been recorded in mesocosm experiments that used
lower nutrient levels. High CO2 increased the prevalence of predatory/exoparasitic
attached bacteria whilst the microeukaryotic community formed a more stable and
more complex network in response to HC. Thus OA is expected to profoundly change
planktonic food webs in eutrophic coastal systems.