Dear Dr. Küsel

Thank you for submitting your manuscript to BG.

The reviewers made constructive comments, which have been adequately addressed.

However, before publication I would like to have your attention for some minors concerns. I side with reviewers that the oxygenated condition is over emphasized. Pls carefully reason the relevant sentences, particularly in the abstract.

(1) L7-8. abbreviation. CLSM was used referring to confocal laser scanning microscopy. Pls provide the abbreviations for Scanning electron microscopy and Fourier transform infrared spectra; state the results that are directly obtained from these technique in abstract if flexible.
(2) L17. Delete either “based on DNA and RNA” or “RNA and DNA” in Line 18
(3) L20. Any evidence for “fully oxygen-saturated conditions” in this study, the phrase might be deleted.
(4) L23. Any experimental evidence for “lower photosynthetic activities” in this study? If not, pls rephrase it.
(5) L30-31. I am not sure whether this statement can be rephrased in a better manner. It seems that the co-existence of these stuffs are clearly demonstrated in this study. But it might not be very safe to suggest the Tribonema sp. provide microenvironment for neutrophilic FeOB, why not the other way around; In addition, if there were no solid evidence of oxygen concentration dynamics as the reviewer specifically pointed out, the author may put himself/herself in a dangerous position to challenge the current dogma. I guess these statements can be tuned down to some degree. For example. Collectively, our results suggest that in heavy metal-rich environment the coexistence of oxygen-generating alage Tribonema-like and strictly microaerophilic neutrophilic FeOB-like microbes (can be better rephrased)
(6) P7708. I side with reviewers that one cannot rule out the possibility of chemolithotrophic lifestyle of organisms in this study. It is certainly true that Emerson et al., 2013 showed the absence of genes for organic carbon assimilation; and no heterotrophic growth was observed for their isolate. However, this may not be the case in this study because the absence of evidence is not the evidence of absence.

In addition, I am wondering what is the point mentioning "metal-polluted" in the title. I had a quick look and found that the effect of heavy metal appears to be descriptive and rarely justified. Maybe it can be briefly discussed in the materials and methods, if heavy metal here is not a key concern relating iron encrustations on algae-Gallionella aggregated communities.

Best Wishes

Zhongjun Jia