Eukaryotic plankton diversity in the sunlit ocean

TLDR

Marine plankton sustain global biological and geochemical cycles, yet biodiversity surveys have been geographically limited and have not captured the full size spectrum. We sampled 334 size‑fractionated photic‑zone plankton communities worldwide during the Tara Oceans expedition and analyzed their 18S rDNA across the intermediate size range (0.8 µm to a few mm). The analysis revealed ~150,000 eukaryotic OTUs, one‑third of which are unclassified, with most diversity in heterotrophic protists that are parasites or symbionts, and with lineages spanning all taxonomic levels.

Abstract

Marine plankton support global biological and geochemical processes. Surveys of their biodiversity have hitherto been geographically restricted and have not accounted for the full range of plankton size. We assessed eukaryotic diversity from 334 size-fractionated photic-zone plankton communities collected across tropical and temperate oceans during the circumglobal Tara Oceans expedition. We analyzed 18S ribosomal DNA sequences across the intermediate plankton-size spectrum from the smallest unicellular eukaryotes (protists, >0.8 micrometers) to small animals of a few millimeters. Eukaryotic ribosomal diversity saturated at ~150,000 operational taxonomic units, about one-third of which could not be assigned to known eukaryotic groups. Diversity emerged at all taxonomic levels, both within the groups comprising the ~11,200 cataloged morphospecies of eukaryotic plankton and among twice as many other deep-branching lineages of unappreciated importance in plankton ecology studies. Most eukaryotic plankton biodiversity belonged to heterotrophic protistan groups, particularly those known to be parasites or symbiotic hosts.