Photoheterotrophy provides increased fitness in airborne bacteria: Aerosol simulation chamber studies

The atmosphere harbors a great diversity of microorganisms. Among them, some taxa of bacteria, such as Methylobacterium species are abundant and recurring members of the viable fraction (Amato et al., 2017; Woo and Yamamoto, 2020). These include non-obligate light-users, and we postulate that this function could be linked with their prevalence and survival capacity in the atmosphere. The alternative use of light to generate biochemical energy (ATP) through photoheterotrophy and anoxygenic photosynthesis is known to enhance survival under nutrient-deficient conditions (Soora and Cypionka, 2013). The use of light could therefore be beneficial and favor survival by supporting the maintenance of metabolic activity in the atmospheric environment,  with dispersed droplets or particles where access to substrates is limited.

To test the hypothesis that photoheterotrophy is beneficial to the survival of airborne bacteria, two distinct phenotypes of the same strain (with or without the photosynthetic pigment bacteriochlorophyll; [BChl+] or [BChl-], respectively, which can be controlled by growing cells under dark or light conditions, respectively) of a facultative photoheterotrophic strain of Methylobacterium sp. (R17b-9), isolated from clouds, were injected into the atmospheric simulation chamber (ASC) "ChAMBRe" (Vernocchi et al., 2023). Their survival was monitored for 2 hours while being exposed to different light intensities. During experimentation in the ASC, cell viability, cultivability, ATP concentration and residence time were measured.

Bacteria containing bacteriochlorophyll retained greater viability and cultivability than those lacking this photosynthetic pigment.  Light exposure on [BChl-] phenotype had a negative impact on cultivability, but not on viability. The mean half-lives (measuring by cultures) of bacteria [BChl-] was ~100-700 min depending on light intensity whereas there was no loss of cultivability over time for bacteria with pigment independently from light exposure. The ATP/cell ratio was 3 times greater for bacteria with bacteriochlorophyll than without. In addition, bacteria with bacteriochlorophyll sedimented 1.71 times faster than their counterparts without the pigment. This study supports the idea that not all bacteria are equal to atmospheric transport, and that specific phenotypic traits can be involved. It is possible that the widespread distribution, at low level, of photoheterotrophy in bacteria in the global environment could be promoted by their increased ability to disperse aerially.

 

Reference

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