Distribution and abundance of uncultured heterotrophic flagellates in the world oceans

TLDR

Heterotrophic flagellates are key picoplankton grazers, yet their taxonomic composition remains largely unknown, with many 18S rDNA sequences not matching cultured organisms. The study aims to determine the abundance of several marine stramenopile lineages in surface waters. Abundance was quantified using molecular probes and fluorescent in situ hybridization in surface marine waters. These protists are free‑living bacterivorous flagellates, widely distributed across all five oceans, comprising up to 35 % of heterotrophic flagellates, with MAST‑4 alone representing 9 % except in polar waters, and likely contributing substantially to picoplankton grazing and nutrient remineralization.

Abstract

Summary Heterotrophic flagellates play fundamental roles in marine ecosystems as picoplankton grazers. This recognized importance contrasts with our ignorance of the taxonomic composition of this functional group, which remains mostly unidentified by microscopical and culturing approaches. Recent molecular marine surveys based on 18S rDNA genes have retrieved many sequences unrelated to cultured organisms and marine stramenopiles were among the first reported uncultured eukaryotes. However, little is known about the organisms corresponding to these sequences. Here we determine the abundance of several marine stramenopile lineages in surface marine waters using molecular probes and fluorescent in situ hybridization. We show that these protists are free‐living bacterivorous heterotrophic flagellates. They were widely distributed, occurring in the five world oceans, and accounted for a significant fraction (up to 35%) of heterotrophic flagellates in diverse geographic regions. A single group, MAST‐4, represented 9% of cells within this functional assemblage, with the intriguing exception of polar waters where it was absent. MAST‐4 cells likely contribute substantially to picoplankton grazing and nutrient re‐mineralization in vast areas of the oceans and represent a key eukaryotic group in marine food webs.