In the marine planktonic community, nanoflagellates (NF), including heterotrophic nanoflagellates (HNF) and mixotrophic nanoflagellates (MNF), are considered the main consumers of prokaryotes, channeling organic matter and energy to higher trophic levels. NF grazing on prokaryotes in the sunlit ocean has been extensively studied. However, studies to date mostly considered NF as a black box and focused mainly on the euphotic zone communities, leaving their taxonomic identities and impacts on prokaryotic communities, especially in deep waters, largely ignored. In the present study, samples were collected from the South China Sea from the euphotic zone to mesopelagic waters. We employed the tracer ingestion method to uncover the abundance of NF and their grazing on prokaryotes. FISH-TSA was used to target the potential key MNF group, and high throughput sequencing on V4 hypervariable regions of the 18S rRNA was used to reveal potential active NF grazers. Meanwhile, multiple environmental variables were measured to explore the factors that influence the distribution and grazing of NF functional groups across the water column.

protistan grazing, mesopelagic zone, South China Sea, FISH-TSA, high throughput sequencing