Microzooplankton grazing responds to simulated ocean acidification indirectly through changes in prey cellular characteristics

ABSTRACT: Microzooplankton (MZP) grazing is a factor that regulates oceanic primary production and is a controlling mechanism for marine biogeochemical cycling. Despite the prominent ecological role of MZP, few studies have explored their responses to ocean acidification (OA). Studies to date generally indicate that MZP are affected indirectly by OA through changes in phytoplankton prey composition and biomass concentration. Here, we conducted a series of experiments testing whether OA-induced changes in cellular characteristics of individual prey species can cause changes in MZP grazing. Two tintinnid ciliates (Eutintinnus sp. and Schmidingerella sp.) and a heterotrophic dinoflagellate (Oxyrrhis marina) were offered phytoplankton prey (Emiliania huxleyi) cultured under 3 pCO2 concentrations. Using linear mixed effects models, we found that Eutintinnus sp. and O. marina exhibited a step-wise increase in ingestion rates on E. huxleyi cells cultured under elevated pCO2. Schmidingerella sp. ingestion showed a non-linear response, whereby cells cultured under high pCO2 were ingested at higher rates than cells from moderate pCO2. The percentages of all 3 MZP populations observed feeding were higher on E. huxleyi cells cultured under elevated pCO2, with Eutintinnus sp. showing a step-wise increase. We postulate that this response is caused by the observed increased coccosphere volume in E. huxleyi cells cultured under elevated pCO2. If changes in phytoplankton cell volume are widespread under OA, this could be an important mechanism by which MZP grazing behavior shifts and planktonic food web dynamics are altered in the future ocean.