# Diversity and distribution of the eukaryotic picoplankton in the oxygen minimum zone of the tropical Mexican Pacific

## TL;DR

This study explores the diversity and distribution of tiny eukaryotic plankton in low-oxygen zones of the tropical Mexican Pacific, revealing how oxygen levels influence their communities.

## Contribution

The study provides new insights into picoeukaryotic diversity and distribution patterns in oxygen minimum zones using metabarcoding and flow cytometry.

## Key findings

- Picoeukaryotic diversity was highest at the lower oxycline (10 μM O2) compared to surface and subsurface layers.
- Different picoeukaryotic communities were found at each depth, with Mamiellophyceae dominating the surface and Syndiniales, Radiolaria, and Ochrophyta prevalent at the oxycline.
- Oxygen concentration was identified as a key driver of microbial distribution and niche specialization in the water column.

## Abstract

The ecology of eukaryotic picoplankton in oxygen minimum zones (OMZs) is crucial to understand global primary production, trophic dynamics and plankton diversity. This study analyses picoeukaryotic diversity and distribution patterns along the water column at two locations (slope and oceanic) in the tropical Mexican Pacific OMZ using metabarcoding and flow cytometry. Well-known groups of Chlorophytes (Mamiellophyceae) and Ochrophytes (Chrysophyceae, Dictyochophyceae, Pelagophyceae) occurred in high relative abundances, whereas less-known groups such as Chloropicophyceae and Prasinodermophyta were found in lower abundances. Picoeukaryotic diversity was higher at the lower end of the oxycline (10 μM O2) than at the surface and subsurface layers. Differential distributions of picoeukaryotes were also detected along the water column, with almost exclusive communities at each depth. Mamiellophyceae dominated the surface and subsurface layers, whereas Syndiniales (parasitic dinoflagellates), Radiolaria, Ochrophyta, and Sagenista (MArine STramenopiles -MAST groups-) were prevalent at the oxycline. Post-upwelling oceanographic conditions possibly contributed to shape the differences in community composition and distribution. These findings highlight that oxygen concentration is a key factor driving microbial distribution and that oxyclines provide specialized niches that promote high picoplankton diversity and multiple trophic strategies including autotrophy, mixotrophy, heterotrophy and parasitism.