Small pigmented eukaryote assemblages of the western tropical North Atlantic around the Amazon River plume during spring discharge

Sophie Charvet,Eunsoo Kim,Joseph Montoya,Solange Duhamel,Ajit Subramaniam

doi:10.1038/s41598-021-95676-2

Abstract

Small pigmented eukaryotes (⩽ 5 µm) are an important, but overlooked component of global marine phytoplankton. The Amazon River plume delivers nutrients into the oligotrophic western tropical North Atlantic, shades the deeper waters, and drives the structure of microphytoplankton (> 20 µm) communities. For small pigmented eukaryotes, however, diversity and distribution in the region remain unknown, despite their significant contribution to open ocean primary production and other biogeochemical processes. To investigate how habitats created by the Amazon river plume shape small pigmented eukaryote communities, we used high-throughput sequencing of the 18S ribosomal RNA genes from up to five distinct small pigmented eukaryote cell populations, identified and sorted by flow cytometry. Small pigmented eukaryotes dominated small phytoplankton biomass across all habitat types, but the population abundances varied among stations resulting in a random distribution. Small pigmented eukaryote communities were consistently dominated by Chloropicophyceae (0.8–2 µm) and Bacillariophyceae (0.8–3.5 µm), accompanied by MOCH-5 at the surface or by Dinophyceae at the chlorophyll maximum. Taxonomic composition only displayed differences in the old plume core and at one of the plume margin stations. Such results reflect the dynamic interactions of the plume and offshore oceanic waters and suggest that the resident small pigmented eukaryote diversity was not strongly affected by habitat types at this time of the year.

Highlights

Small pigmented eukaryotes (⩽ 5 μm) are an important, but overlooked component of global marine phytoplankton
The sampled stations were classified into 5 habitat types (Fig. 1a,b) as described by Weber et al.[35]: young plume core (YPC), old plume core (OPC), west plume margin (WPM), east plume margin (EPM) and oceanic seawater (OSW)
Each habitat was characterized by a unique combination of sea surface salinity, sea surface temperature, nitrate availability index, mixed layer depth and chlorophyll maximum depth[35]

Summary

Introduction

Small pigmented eukaryotes (⩽ 5 μm) are an important, but overlooked component of global marine phytoplankton. During peak discharge in the spring, the ARP is pushed northwards by the North Brazil current along the eastern coast of South America, is carried eastwards later in the year by the North Equatorial Countercurrent[2] During this northward progression, the plume mixes gradually into the coastal and offshore waters on either side, establishing a diverse range of ecological niches impacting phytoplankton community structure far from the mouth of the river[1,3,4]. The small-sized pigmented eukaryotes (≤ 5 μm) are ubiquitous throughout the global oceans, reaching peak abundance at the deep chlorophyll maximum[7,8,9] or at the base of the surface mixed layer[10]. Taxonomic variability is observed along latitudinal gradients with a marked increase in eukaryote contribution to small-sized phytoplankton communities, mostly by Chlorophyta, from the North Pacific to the A rctic[25,26,27]

Objectives

Methods

Results