|Apologies for the tardiness of this review -- the original one was deleted as I was trying to submit, so it had to wait until I found some more time. |
This revision by Paul et al. addresses many of my prior concerns, but I do still have some after a second reading.
First, the formulations of Equation 8 and 14 is still confusing and counter-intuitive. I think it would be better to put a minus sign in front of dn(O2) and dn_total_respi, respectively:
n(O2)_t+dt = n(O2)_t - dn(O2)
n(O2)_t+dt = n(O2)_t - dn_total_respi + dn_photosynthesis
It preserves the intuitive sense that respiration removes O2, even though the value of those quantities is positive. The equations derived from these would then need to be modified accordingly. Equation 19 and aN in L567, for example, would need a negative sign in front of F_total_respi to preserve the same sense.
Second, the estimation of fractionation factors in the light seems to underestimate systematic uncertainties. The fractions and fractionation factors for Mehler and photorespiration are assumed based on prior literature but it is not clear whether (or how) these assumptions were included in Monte Carlo uncertainty estimates. While some effort has been made to do some sensitivity tests in 3.2.2 and in the supplement, they could use with some elaboration and justification. How sensitive are the estimates to the two different values of 18e_Mehler?
Finally, is there a working rationale for why terrestrial plant photosynthesis would enrich O2 in 18O if chloroplasts do not? The result is unexplained (similar to that of Eisenstadt) and therefore a bit unsatisfying. Can one rule out remnants of some oxygen-consuming process like light-dependent oxygen consumption at a terminal oxidase (Mehler/PTOX etc.)?
Overall, I suspect these concerns and questions require major revisions to address -- although I believe they can be fixed.
L48-50: Dole's precise measurement of the 18O/16O ratio dates back to 1935 (10.1021/ja01315a511) or 1936 (10.1063/1.1749834) when he reported the relative atomic weight of oxygen in air and Lake Michigan water. His result translates somewhere in the ballpark of +21 per mil, not the 23.88 value implied in this sentence. Incidentally, the 23.88 per mil value from Barkan and Luz is perhaps overly precise given the more recent measurements from Pack (24.15; 10.1038/ncomms15702) and Wostbrock (24.05; already cited). I suggest rephrasing the latter part of this sentence to be more clear and representative of the literature and using "~24 per mil" instead.
L61: The isotopic discrimination found by Guy et al. varied a bit depending on origin (cyano/diatom/chloroplast). This sentence should at least acknowledge that what is being reported is an average of the three results.
L82/83, 663/664: Nannocloreopsis --> Nannochloropsis & oreinhardtii --> reinhardtii
L85: better be --> be better
L88: delete "the" before Southeast Asia
L122: delete "the" before dark respiration
L584-599: Please include more information about the sensitivity tests in the main text. I don't know from here what the numerical range of each variable was, how it was chosen, nor how multivariate problems were addressed (e.g., photorespiration and Mehler). The supplement only shows a couple tests -- I imagine the authors could do a Monte Carlo estimate quite easily and derive a more accurate uncertainty range from that.
L607: DARK respiration
L608-610: Theta, the slope of ln(1+d17O) vs. ln(1+d18O), and gamma are not comparable, as they are defined differently. Generally speaking, if theta = 0.516, then the observed triple-isotope slope and gamma in experiments would be larger. This section seems to suggest that they are equivalent.
L614-624: I think it would be more appropriate to say that the increase in D17O in these experiments is due to light-dependent processes rather than photosynthesis specifically, given that it was not possible to separate them.
L647-650: It is worth noting that Guy et al. (1989) (10.1007/BF00392616) found similar fractionation factors in P. triconutum and land plants.