This perspective was an interesting and enjoyable to read contribution that identifies and more closely examines some of the most salient challenges that microbes in the atmosphere encounter. A deeper examination of these challenges reveals that some of them may be as or more challenging that previously believed, while some may be less relevant, due to the unique circumstances of the atmosphere, and in cloud and aerosol phases. The conclusions presented are mainly based on modelling, but based on realistic approximations and our currently knowledge. As yet, little empirical data exists to validate many of these suppositions. However, the authors seem to be cognizant of these limitations (in referencing literature where unexpected findings were made e.g. Liu et al. 2012). Nevertheless, the perspective builds a more detailed and nuanced examination of these factors than I have seen elsewhere.

Specific comments:

While microbial interactions can have beneficial outcomes, there can also be negative outcomes/ antagonistic interactions. The physical separation of cells that is more prevalent in the atmosphere than in other environments will not only reduce/eliminate beneficial interactions, but will reduce/ eliminate negative interactions such as competition and direct antagonism. The net outcome on this may be beneficial or detrimental, which likely depends on the impacted organism and the context.

I appreciated the discussion on water activity, and the calculations estimating the potential hydration shell that might exist under different conditions. One aspect that was missing for me in this perspective is the impact of pH in aerosols and clouds on microbial cells – most microbes are not well adapted to living at pH 5, so this presents another stressor for microbes in the atmosphere.

Several studies are beginning to build convincing links between environmental factors (e.g. UV, temperature) on microbial community structure and diversity in the aerosol phase (e.g. Archer et al. 2019 Nature Microbiology, Gusareva et al. 2019 PNAS). Developing a controlled experimental system for forming these links is difficult to conceptualize, but environmental sampling is challenged by the numerous confounding variables. With sufficiently well-resolved and controlled environmental sampling, disentangling the impacts of specific variables should be feasible, for at least some environmental factors.

The calculation on settling velocity of microbial cells does not take into account air currents. In addition, it is not impossible that some microbes or microbial spores have evolved for long-range dispersal by air – this has long been known for plant seeds, and there is mounting evidence of this for fungi (e.g. Borgmann-Winter et al. 2023, Ecology 104), therefore it is not unlikely that this is relevant for prokaryotes.  Previous modelling work has suggested that long-range microbial dispersal is likely (Wilkinson et al. 2012 J. Biogeography 39).

General comments:

Ervens et al. presents a thoughtful, well-written piece summarizing previous, and motivating future, research on microorganisms in the atmosphere. The detailed figures were especially informative and effectively conveyed the concepts discussed throughout the article. While some considerations are not wholly original, they are clearly and concisely encapsulated here.

Specific comments:

Page 3, Lines 57-59: The sentence discussing settling velocity is a bit unclear. Consider replacing “particle size” with “particle diameter”. Are you assuming that doubling the number of cells would double the particle diameter? What about in the case of high RH or a cloud droplet where a second cell may just displace water (cf. Figure 3)?

Page 3, Lines 63-64. Please provide a reference for these statements. It may be appropriate to cite Fankhauser et al. (2019) who were among the first to suggest that microbes were physically isolated from one another in the atmosphere.

Page 6, Section 2.3: This section assumes that microorganisms are metabolically active in the atmosphere. The article would benefit from a brief discussion of dormancy, in relation to this and other stressors.

pH response (Author Response to Referee #1): The inclusion of a new subsection on effect of pH is appreciated. It is suggested to add additional commentary in light of work by  Liu et al. (2023, ACP) whose laboratory experiments reported on the effects of pH (in combination with light exposure) on bacterial survival.

Technical corrections:

Page 2, Line 29: The word “role” is written twice.

Page 4, Line 79: Missing period after closed parenthesis and “Novel”.

Page 4, Line 88: Extraneous closed parenthesis before comma.

Page 5, Line 108: Extraneous period between times and during.