Aquatic Respiration Rate Measurements at Low Oxygen Concentrations

Moritz Holtappels,Tim Kalvelage,Marcel M M Kuypers,Gaute Lavik,Niels Peter Revsbech,Laura Tiano

doi:10.1371/journal.pone.0089369

Moritz Holtappels, Tim Kalvelage + Show 4 more

Open Access

https://doi.org/10.1371/journal.pone.0089369

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Abstract

Despite its huge ecological importance, microbial oxygen respiration in pelagic waters is little studied, primarily due to methodological difficulties. Respiration measurements are challenging because of the required high resolution of oxygen concentration measurements. Recent improvements in oxygen sensing techniques bear great potential to overcome these limitations. Here we compare 3 different methods to measure oxygen consumption rates at low oxygen concentrations, utilizing amperometric Clark type sensors (STOX), optical sensors (optodes), and mass spectrometry in combination with 18-18O2 labeling. Oxygen concentrations and consumption rates agreed well between the different methods when applied in the same experimental setting. Oxygen consumption rates between 30 and 400 nmol L−1 h−1 were measured with high precision and relative standard errors of less than 3%. Rate detection limits in the range of 1 nmol L−1 h−1 were suitable for rate determinations in open ocean water and were lowest at the lowest applied O2 concentration.

Highlights

The oxygen concentration in the ocean interior depends on a balance between oxygen consumption and oxygen transport from surface waters, where oxygen is produced and exchanged with the atmosphere
Reduced ventilation is the main cause for anoxia in semi enclosed seas such as the Baltic Sea [1] and the Black Sea [2], whereas a combination of sluggish ventilation of upwelling source waters and high organic matter input causes the development of oceanic oxygen minimum zones (OMZs) at the western continental margins [3] and the Arabian Sea [4]
Until hour 14, the offset between STOX sensor and optodes was less than 3% of the O2 concentration

Summary

Introduction

The oxygen concentration in the ocean interior depends on a balance between oxygen consumption and oxygen transport from surface waters, where oxygen is produced and exchanged with the atmosphere. Basin scale estimates can be derived from particulate carbon flux modeling [9], or time integrated oxygen consumption is back-calculated from the assumed residence time of water masses and the oxygen deficit, i.e. the apparent oxygen utilization (AOU) [3]. Validation of these estimates with experimentally measured oxygen consumption rates is scarce. Even more challenging is the investigation of oxygen dynamics at the low O2 concentrations found in OMZ waters It requires a low detection limit of less than 1–2 mmol L21, which is the current limit of common methods used in oceanographic surveys such as Winkler titration, electrochemical and optical sensors [12]. A low detection limit is necessary to study (i) the kinetics of the various oxygen consuming processes such as heterotrophic respiration and ammonium and nitrite oxidation; (ii) the inhibitory thresholds of anoxic processes such as anammox and denitrification; (iii) and the interaction of these oxic and anoxic processes

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