This manuscript by Paul et al. is a methods development paper primarily. It describes the development of a multiplexing system and the application of two different analytical techniques – Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) and Isotope Ratio Mass Spectrometry (IRMS) – to determine the elemental and isotopic composition of O2 produced by biological processes occurring in a closed chamber. Using OF-CEAS for O2 isotope measurement is a relatively novel idea and Paul et al. effectively demonstrate its capability in this work.

The methods section is very well written in this work and would enable readers to recreate this experimental design.

A few minor comments on the work are highlighted below:

1. Line 91 -92: A sentence explaining how d18O is used to reconstruct oceanic vs terrestrial productivity would be helpful.

1. Line 105: Replace “despite our system...” with “although our system...”

1. Line 108-121: Adding a sentence explaining what overall factors are considered when choosing type of species for experimental setup would be helpful

1. Line 236: ‘reference’ is highlighted, citation missing

1. Line 292: “Experimental problem:. Please describe what the experimental problem was. Since this is a new method and would potentially be adopted by other labs, it would be useful to know what the problem was and how it was addressed.

1. Line 341: Explain why maize was chosen as the preferred option

1. Figures: Adding shaded error bars/ or some sort of way to indicate uncertainty in regression slopes would be visually helpful.

The article is well structured and very well written. The topic is of interest to the readers of this Journal. However, the following minor revisions should be made:

1- a scientific article on an innovative but non-exclusive topic deserves a more extensive literature search. This would also make the introduction of the paper more robust

2- figure 2 has too small, unreadable lettering. I suggest replacing it

3- In general, I always find it confusing to merge discussions with conclusions. In any case, I suggest adding in the conclusions your group's future perspectives on this very interesting research.