In the manuscript " Excess labile carbon promotes diazotroph abundance in heat-stressed octocorals, " Xiang et al. tested the effects of DOC concentration and temperature increments (and the interactions of these two variables) on octocorals-dwelling diazotrophs populations. The authors have previously used an experimental setting to test other hypotheses related to these two variables. They found that the temperature, in combination with DOC additions, has significant effects in both octocorals species, increasing the abundance of diazotrophs. The results presented in the present manuscript are relevant, and the coral reef scientific community would benefit from the generated knowledge. The manuscript is well-written, and the results support the conclusions. The statistical approach is well documented, and the data is available, making the manuscript reproducible. I would like to see this manuscript published after some improvements. I recommend that authors change some parts of the manuscript, making it reach a broader audience than the microbiology-related field. Please see the specific comments below.

Abstract:
Lines 31-32: “stimulated diazotroph population and activity”. I understand that “stimulated” means the increase in population by the number of cells. I suggest making this information clear in the abstract/title. Maybe using the term growth would be a solution.

Line 31-31: “resulting in excess nitrogen availability”. Is this a good or a bad thing for corals? I suggest including a brief sentence making it clear here in the abstract.

Lines 40-43: “Our data suggest that labile DOC enrichment and concomitant heat stress could disrupt the nitrogen limitation in octocorals by stimulating diazotroph proliferation.”. I suggest clarifying whether these findings are good or bad for corals' survival and conservation. From my point of view, this is a significant finding, and a broader audience (coral reef scientific community) would benefit from this information. Please make sure to revise the manuscript considering my suggestion.

Introduction
Line 92: I suggest including a concluding remark or summarization of the article's findings.

Material and Methods
Lines 98-100: I suggest including the information on the sampling location of the corals.

Lines 118-121: Would authors describe why they chose these glucose concentration levels? If authors could relate to what is observed in reefs would be a piece of great information in the manuscript.

Results
The authors have not reported results regarding the octocorals' physiology or health status. Would authors have this type of information? All the data/results presented in this manuscript are relevant, but having the impacts of the treatments measured at the visible eye level would improve the quality and reach a broader audience. I understood that this is a result presented in ref 29. However, would it be possible to mention this in the Results section once this is the same experiment?

Subsection “Labile DOC enrichment does not affect relative diazotroph abundance in octocorals at ambient temperature”
Please explain the results for all DOC addition levels as did in the subsequent subsection.

Line 207: Add “of the experiment” after “at the beginning”. Replace “21” with “Twenty-one”.

Discussion

Authors have discussed the possible specificity of diazotroph abundance patterns in octocorals. Considering authors have monitored the health status and observed bleaching at a certain point of the experiment, would it be possible to propose a diazotroph threshold for bleaching? Previous studies demonstrated that coral reefs' microbial communities (seawater) would be the first sign of fragile reef health (e.g., DOI 10.3389/fmicb.2018.02203). The information generated in the present study would provide tools for future molecular/microbiological approaches to coral reef monitoring.

Lines 228-229: I suggest including this information in the abstract.

Line 241: “Diazotroph abundances in octocorals are host species-specific”. I suggest tuning down this affirmation. Despite the clear patterns presented in the results concerning the differences between the octocorals, the authors compared only two species.

Line 268: I suggest changing from “Labile DOC stimulates diazotroph abundances in heat-stressed octocorals” to something like “Labile DOC stimulates diazotroph abundance increase in heat-stressed octocorals”