EGU General Assembly 2020
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Photochemistry versus biological activity towards organics in cloud water

Amina Khaled, Minghui Zhang, Pierre Amato, Anne-Marie Delort, and Barbara Ervens
Amina Khaled et al.
  • Institut de chimie de Clermont Ferrand, Chemistry, Aubière, France (

The aqueous phase of clouds is a complex atmospheric medium containing a multitude of organic and inorganic species with different reactivities. The main oxidant towards organics in the aqueous phase is the OH radical. Many studies have identified biological material as a major fraction of ambient aerosol loading with bacteria being a small fraction. Laboratory experiments in our and other research groups have shown that microbial degradation of small organics (e.g., formic and acetic acids) can efficiently occur in artificial and real cloud water in competition to chemical radical reactions. However, in current models, it is usually assumed that bacteria are not metabolically active in the atmosphere. The aim of our study is to identify conditions, under which biological activity is significant in the multiphase system for specific organic compounds. Using a cloud multiphase process model, we compare the predicted fractions of organics consumed by radicals in the gas and aqueous phases to that by microbial processes of bacteria in the aqueous phase over large ranges of microphysical (e.g., cloud liquid water content, drop number), biological (cell concentration and activity) and chemical parameters (reaction rate constants and Henry’s law constants). We identify the organic properties and cloud parameters under which metabolic processes represent major atmospheric sinks for organics. In our cloud model, we consider the fact that only a small number fraction of droplets contain active bacteria cells. As many other models might not be able to describe such microphysical details, we also suggest simplified model approaches to represent microbial activity in clouds.

How to cite: Khaled, A., Zhang, M., Amato, P., Delort, A.-M., and Ervens, B.: Photochemistry versus biological activity towards organics in cloud water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1517,, 2019.

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  • CC1: Comment on EGU2020-1517, Kumar Sarang, 04 May 2020

    Your study seems to be really interesting. Is there any possibility of carrying out a collaborative study for my compounds? This seems to be an interesting pathway for degradation.

    • AC1: Reply to CC1, Amina Khaled, 06 May 2020

      Our group has mostly focused on the biodegradation of small organics (formic, acetic acid, formaldehyde, C2-C4 dicarboxylic acid, etc) by bacteria that we identified in cloud water.  
      Plenty of alcohols and aldehydes take part in the metabolic processes, so there are chances that also GLVs can indeed be utilized. There are, for example, some enzymes that convert hexenol into hexenal and vice versa. However, it seems that these processes do not efficiently occur in bacteria that are abundant in clouds (such as Pseudomonas syringae). Also for pentenol, there are some metabolic pathways but they are very specific to some bacteria strains. Thus, we expect that GLVs are inefficiently utilized and thus their biodegradation rates may be slow. 
      As you have shown that the chemical reactivity of GLVs in the aqueous phase is fairly high, one may expect that the chemical loss will dominate for these compounds over biodegradation. Biodegradation experiments are very laborious and time-consuming so that we currently focus more on organics that have been commonly identified in cloud water and where we have evidence of efficient utilization and thus relatively high biodegradation rates. 
      Please let us know if you would like to have more information on our experiments. We will be happy to share more details. 

  • CC2: Comment on EGU2020-1517, Ulrich Krieger, 04 May 2020

    I wrote in the chat:

    "I am suprised that they can stay dormant in the aerosol phase for a couple of days. I would have thought that osmotic pressure kills them."

    Are there any mechanistic studies around? If the aerosol is a sulfate dominated liquid one I cannot see how a bacterium could survive. But may be on the surface of a dust particle? Is there anything known about the type of aerosol when they survive for several days?

    • AC2: Reply to CC2, Amina Khaled, 06 May 2020

      Bacteria have been shown in several studies to survive in aerosols for up to several days, e.g. (Griffin, 2007) and even non-spore forming cells can endure desiccation for several months in soils, e.g (Chen and Alexander, 1973). It is probable though that some cells will not maintain and survive atmospheric transport. A recent review article nicely summarizes various aspects of bacteria transport and survival emitted during sand storms (Behzad et al., 2018). It is discussed there that there is some selection to bacteria that endure the stressful conditions. 
      Also recent studies from our group have shown that microorganisms (Arthrobacter sp., Dioszegia hungarica and P. syringae) can indeed resist and survive in harsh conditions encountered in clouds: exposure to H2O2 and ultraviolet light (oxidative stress), evaporation condensation cycle (osmotic shock), and freeze-thaw cycles (multiple stresses: cold shock, oxidative stress and osmotic stress) (Joly et al., 2015). Microorganisms have developed various means of enduring atmospheric stress: (1) the presence of pigments: roughly 50% of strains isolated from clouds are pigmented protecting microorganisms from extreme and cold temperature, (2) exopolymeric substances and bio surfactant synthesis allows for the formation of aggregates and biofilms and (3) some microorganisms form spores (bacillus, yeast and fungi). We have shown that microorganisms isolated from clouds can grow in cloud water in a laboratory (Amato et al., 2007). This metabolic activity was confirmed by assaying the ATP content found in cloud water samples.
      We will be happy to share more details and to keep the discussion going. 

      Amato, P., Parazols, M., Sancelme, M., Laj, P., Mailhot, G. and Delort, A.-M.: Microorganisms isolated from the water phase of tropospheric clouds at the Puy de Dôme: major groups and growth abilities at low temperatures, FEMS Microbiology Ecology, 59(2), 242–254, doi:10.1111/j.1574-6941.2006.00199.x, 2007.
      Behzad, H., Mineta, K. and Gojobori, T.: Global Ramifications of Dust and Sandstorm Microbiota, Genome Biology and Evolution, 10(8), 1970–1987, doi:10.1093/gbe/evy134, 2018.
      Chen, M. and Alexander, M.: Survival of soil bacteria during prolonged desiccation, Soil Biology and Biochemistry, 5(2), 213–221, doi:, 1973.
      Griffin, D. W.: Atmospheric Movement of Microorganisms in Clouds of Desert Dust and Implications for Human Health, Clin. Microbiol. Rev., 20(3), 459, doi:10.1128/CMR.00039-06, 2007.
      Joly, M., Amato, P., Sancelme, M., Vinatier, V., Abrantes, M., Deguillaume, L. and Delort, A.-M.: Survival of microbial isolates from clouds toward simulated atmospheric stress factors, Atmospheric Environment, 117, 92–98, doi:10.1016/j.atmosenv.2015.07.009, 2015.

      • CC3: Reply to AC2, Ulrich Krieger, 08 May 2020

        Thanks a lot, I will take a closer look,

        Cheers, Uli