the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Apr 2026
An intermediary scale setup to measure O2 fractionation factors of aquatic biosphere
Abstract. Earth atmospheric O2 is mainly produced by biosphere photosynthesis, and biosphere respiration is also one of the main consumers of this gas. The evolution of the elemental and isotopic composition of atmospheric O2 is thus linked to global biosphere productivity. Quantitative interpretation of the isotopic composition of O2 in the past as archived in polar ice cores relies on robust estimates of oxygen fractionation factors associated with the relevant biological processes: photosynthesis and respiration. In the past decades, some determinations of these biological fractionation factors were performed in uncontrolled large-scale environments or at the scale of the micro-organisms in conditions very different from the natural environment. This leads to uncertainties in the applicability of these determinations for the interpretation of isotopic composition of atmospheric O2. In order to come up with coherent estimates of oxygen biological fractionation factors applicable to the scale of plants or ecosystems, we developed a set-up of 4 closed biological chambers as a scaled-down replica of aquatic biosphere, with controlled environment parameters, and measured the evolution of O2 concentration and of its isotopic composition. We present here 3 measurement series using this set-up run with the freshwater species Chlorella vulgaris and lasting between 2 and 9 months. These measurement series enabled us to validate and optimise our newly developed system for aquatic closed biological chambers. We also determined the isotopic discrimination associated with 18O/16O of O2 during respiration as -30 ‰ which is in the upper part of the distribution of the previously published values.
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Status: open (until 08 Jul 2026)
- RC1: 'Comment on egusphere-2026-1369', Anonymous Referee #1, 01 May 2026 reply
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- 1
The reviewed manuscript presents measurements of an important parameter: The discrimination against 18O of O2 during respiration. The authors here choose to use a mesocosm approach. The average values reported here are at the top of the range of previously reported values.
While these results are interesting and important, I feel a lot is lacking. A large part of the previous work they compare to is based on experimental work in the ocean, which should reflect discrimination in natural ecosystems. As the authors note, the (dark respiration) discrimination in their mesocosm experiment is at least partly a function of the temperature, nutrient status, and experiment temperature. Thus, it is unclear how the results obtained in the experiments here could be extrapolated to Earth’s oceans.
The authors acknowledge this limitation and suggest that further experiments should test for the effects of the temperature, species, etc. To me, the advantage of a controlled lab experiment is that one can set up these parameters and understand how they control the isotopic discrimination. Moreover, in such lab experiments, the question is whether this strong discrimination is cyanide-resistant respiration-related, which could be answered by the use of inhibitors. But as a minimum, it seems to me that the temperature effect on the results presented here should be tested before the manuscript is resubmitted.
In addition, some of the reported discrimination values reported here are extreme, about -40 permil. This is well beyond what is known for respiration processes. How is this explained?
As for discrimination during photosynthesis, the authors acknowledge that their results are too uncertain to shed any new light on this question. So I wonder if including these results and discussing them adds much to the paper.
In general, the manuscript is well written and easy to follow.
Minor comments:
Title: Instead of “intermediary scale setup”, maybe a “mesocosm” will be a shorter and more often used term?
Line 70: In this paragraph, the following papers should be cited and added to the discussion :
Severinghaus, J. P., Beaudette, R., Headly, M. A., Taylor, K., & Brook, E. J. (2009). Oxygen-18 of O2 records the impact of abrupt climate change on the terrestrial biosphere. Science, 324(5933), 1431-1434.
Tables and figures: Please report epsilon in permil (i.e., -29 instead of -0.029), as in the text.