Relative importance of bacterivorous mixotrophs in an estuary-coast environment

Mixotrophic eukaryotes are important bacterivores in oligotrophic open oceans, but their significance as grazers in more nutrient-rich waters is less clear. Here, we investigated the bacterivory partition between mixotrophs and heterotrophs in a productive, estuary-influenced coastal region in the East China Sea. We found ubiquitous, actively feeding phytoplankton populations and taxa with mixotrophic potential by identifying ingestion of fluorescent prey surrogate and analyzing community 18S rRNA gene amplicons. Potential and active mixotrophs accounted for 10–63% of the total eukaryotic community and 17–69% of bacterivores observed, respectively, contributing 6–48% of estimated in situ bacterivory. The much higher mixotroph fitness outside of the turbid plume was potentially driven by increased light and decreased nutrient availability. Our results suggest that, although heterotrophs dominated overall in situ bacterivory, mixotrophs were abundant and important bacterivores in this low-latitude mesotrophic coastal region.