Marked changes in diversity and relative activity of picoeukaryotes with depth in the world ocean

Caterina R Giner,Ramiro Logares,Massimo C Pernice,Carlos M Duarte,Josep M Gasol,Vanessa Balagué,Ramon Massana

doi:10.1038/s41396-019-0506-9

Abstract

Microbial eukaryotes are key components of the ocean plankton. Yet, our understanding of their community composition and activity in different water layers of the ocean is limited, particularly for picoeukaryotes (0.2–3 µm cell size). Here, we examined the picoeukaryotic communities inhabiting different vertical zones of the tropical and subtropical global ocean: surface, deep chlorophyll maximum, mesopelagic (including the deep scattering layer and oxygen minimum zones), and bathypelagic. Communities were analysed by high-tthroughput sequencing of the 18S rRNA gene (V4 region) as represented by DNA (community structure) and RNA (metabolism), followed by delineation of Operational Taxonomic Units (OTUs) at 99% similarity. We found a stratification of the picoeukaryotic communities along the water column, with assemblages corresponding to the sunlit and dark ocean. Specific taxonomic groups either increased (e.g., Chrysophyceae or Bicosoecida) or decreased (e.g., Dinoflagellata or MAST-3) in abundance with depth. We used the rRNA:rDNA ratio of each OTU as a proxy of metabolic activity. The highest relative activity was found in the mesopelagic layer for most taxonomic groups, and the lowest in the bathypelagic. Altogether, we characterize the change in community structure and metabolic activity of picoeukaryotes with depth in the global ocean, suggesting a hotspot of activity in the mesopelagic.

Highlights

Supplementary information The online version of this article contains supplementary material, which is available to authorized users.Protists are key components of marine microbial communities, playing a central role in marine food webs [1], in carbon cycling [2]
Bray–Curtis dissimilarities were used as an estimator of beta diversity, which were clustered using non-metric multidimensional scaling (NMDS)
To assess the relative activity of taxonomic groups, rRNA:rDNA ratios were calculated for each Operational Taxonomic Units (OTUs) in each sample by dividing the relative abundance of the OTU in the RNA between their relative abundance in the DNA

Summary

Introduction

Protists are key components of marine microbial communities, playing a central role in marine food webs [1], in carbon cycling [2]. Despite their importance, the distribution and activity of protists in the global ocean is still poorly understood. Previous studies on the distribution of protists along the water column are limited to a few specific oceanic regions and indicate a clear differentiation between epipelagic and deep ocean communities [7,8,9,10,11,12,13,14]

Methods

Results

Conclusion