Imprint of Trace Dissolved Oxygen on Prokaryoplankton Community Structure in an Oxygen Minimum Zone

Luis Medina Faull,Paraskevi Mara,Virginia P Edgcomb,Gordon T Taylor

doi:10.3389/fmars.2020.00360

Luis Medina Faull, Paraskevi Mara + Show 2 more

Open Access

https://doi.org/10.3389/fmars.2020.00360

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Abstract

The Eastern Tropical North Pacific (ETNP) is a large, persistent, and intensifying oxygen minimum zone (OMZ) that accounts for almost half of the total area of global OMZs. Within the OMZ core (~350-700 m depth), dissolved oxygen is typically near or below the analytical detection limit. Steep oxygen gradients above and below the OMZ core lead to vertical structuring of microbial communities that also vary between particle-associated (PA) and free-living (FL) size fractions. Here, we use 16S amplicon sequencing (iTags) coupled with multivariate statistics to analyze the diversity and distribution of prokaryotic populations between FL and PA size fractions and among the range of existing redox conditions. Nonmetric Dimensional Scaling (nMDS) revealed that bacterial and archaeal phylogenetic representations were significantly different between size fractions. Based on analysis of similarities (ANOSIM) and iTag profiles, composition of PA assemblages was less influenced by the prevalent depth-dependent biogeochemical regime than the FL fraction. Trace oxygen concentrations (~0.33 M) were present throughout the OMZ core at our sampling location. Consequently, nitrite accumulations typically reported for OMZ cores were absent. Furthermore, sequences for anammox bacteria (Brocadiales genus Candidatus Scalindua), commonly found at oxic-anoxic boundaries in other systems, were not recovered here. However, ammonia-oxidizing bacteria (AOB) and archaea (AOA) distributions and maximal autotrophic carbon assimilation rates (1.4 µM C d-1) coincided with a pronounced ammonium concentration maximum near the top of the OMZ core. In addition, members of the genus Nitrospina, a dominant nitrite-oxidizing bacteria (NOB) were present suggesting that both ammonia and nitrite oxidation occur at almost undetectable oxygen concentrations. We conclude that the presence of trace oxygen in the OMZ core of this region of the ETNP OMZ is sufficient to support active nitrification.

Highlights

In response to ongoing climate change and localized human activities, concentrations of dissolved oxygen have been declining in the open ocean and in coastal marine systems (Breitburg et al, 2018)
The hydrographic conditions at our study area were distinct from those previously reported in the Eastern Tropical North Pacific (ETNP) (Podlaska et al, 2012) and other oxygen minimum zone (OMZ), such as the ETSP (Bristow et al, 2016)
Oxygen profiles measured in situ from all 16 stations occupied during the cruise, consistently indicated trace oxygen was present in the OMZ core of this region

Summary

Introduction

In response to ongoing climate change and localized human activities, concentrations of dissolved oxygen have been declining in the open ocean and in coastal marine systems (Breitburg et al, 2018). Some of the highest rates of nitrogen loss have been recorded in OMZs of the Eastern Tropical North Pacific (ETNP) and South Pacific (ETSP) (Callbeck et al, 2017; Penn et al, 2019), the permanently stratified Cariaco Basin (Montes et al, 2013), the Arabian Sea (Ward et al, 2009), and the OMZ of the Benguela upwelling system (Kuypers et al, 2005) In these systems, the microbial processes of canonical denitrification (heterotrophic reduction of nitrate to nitrogen intermediates and often to dinitrogen gas) and anammox (anaerobic ammonium oxidation) lead to nitrogen losses that can potentially limit primary production (Ward et al, 2007). Oceanic nitrous oxide emissions (a potent greenhouse gas) from microbial denitrification occurring in OMZs is estimated to account for at least one third of global natural nitrous oxide emissions (Naqvi et al, 2010)

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