Determination of Respiration Rates in Water with Sub-Micromolar Oxygen Concentrations

Abstract

It is crucial for our study and understanding of element transformations in low-oxygen waters that we are able to reproduce the in situ conditions during laboratory incubations to an extent that does not result in unacceptable artifacts. In this study, we have explored how experimental conditions affect measured rates of O 2 consumption in low-O 2 waters from the anoxic basin of Golfo Dulce (Costa Rica) and oceanic waters off Chile-Peru. High-sensitivity optode dots placed within all-glass incubation containers allowed for high resolution O 2 concentration measurements in the nanomolar and low µM range and thus also for the determination of rates of oxygen consumption by microbial communities. Consumption rates increased dramatically (from 3 and up to 60 times) by prolonged incubations, and started to increase after 4-5 h in surface waters and after 10-15 h in water from below the upper mixed layer. Estimated maximum growth rates during the incubations suggest the growth of opportunistic microorganism with doubling times as low as 2.8 and 4.6 h for the coastal waters of Golfo Dulce (Costa Rica) and oceanic waters off Chile and Peru, respectively. Deoxygenation by inert gas bubbling led to increases in subsequently determined rates, possibly by liberation of organics from lysis of sensitive organisms, particle or aggregate alterations or other processes mediated by the strong turbulence. Stirring of the water during the incubation led to an about 50% increase in samples previously deoxygenated by bubbling, but had no effect in untreated samples. Our data indicate that data for microbial activity obtained by short incubations of minimally manipulated water are most reliable, but deoxygenation is a prerequisite for many laboratory experiments, such as determination of denitrification rates, as O 2 contamination by sampling is practically impossible to avoid.

Anaerobic methane oxidation is an important sink for methane in the ocean's largest oxygen minimum zone

“…a,b). A similar acceleration during batch incubation has been observed for other microbial processes in OMZs and is generally considered as a “bottle effect” with the initial rates assumed to reflect in situ activity more accurately than rates during the acceleration period (e.g., Thamdrup et al ; Garcia‐Robledo et al ). Accordingly, all rates and rate constants reported here are based on 24 h incubations.…”

Microbial Respiration, the Engine of Ocean Deoxygenation

“…Good agreement was found between measurements of oxygen concentration and oxygen consumption made by STOX sensors, optode spots and membrane inlet mass spectrometric analysis of 18−18 O 2 (Holtappels et al, 2014). Using these new nanomolar techniques, in situ oxygen and respiration measurements derived from consumption of oxygen, showed that aerobic respiration occurs efficiently at extremely low oxygen concentrations (Tiano et al, 2014;Kalvelage et al, 2015;Garcia-Robledo et al, 2016). As well as the difficulty in measuring very low oxygen concentrations and rates of oxygen consumption, TABLE 1 | Recent midwater (100-1000 m) respiration estimates derived from dissolved oxygen consumption during a bottle incubation ( O 2 ), activity of the electron transport system (ETS), the reduction of the tetrazolium salt (INT), or the time resolved estimate of the amount of dissolved oxygen consumed since a water parcel was last in contact with the atmosphere, the oxygen utilization rate (OUR).…”

Oxygen Profiles Across the Sea-Surface Microlayer—Effects of Diffusion and Biological Activity

“…Such measurements would only be valid under the assumption that the process rates are not affected by the low concentration levels. However, this assumption seems reasonable, as values for the apparent half-saturation constant of the Michaelis-Menten equation (K m ) for microbial oceanic O 2 consumption have been shown to be in the nanomolar range (Tiano et al, 2014;Garcia-Robledo et al, 2016). Also, for compounds with a natural low ambient concentration and where a suitable microsensor exists, e.g., for nitrous oxide (N 2 O), SML microprofiling can likely be used to determine reaction rates in situ.…”