Metabolic modulations of Pseudomonas graminis in response to H2O2 in cloud water.

In cloud water, microorganisms are exposed to very strong stresses especially related to the presence of reactive oxygen species including H2O2 and radicals, which are the driving force of cloud chemistry. In order to understand how the bacterium Pseudomonas graminis isolated from cloud water respond to this oxidative stress, it was incubated in microcosms containing a synthetic solution of cloud water in the presence or in the absence of H2O2. P. graminis metabolome was examined by LC-MS and NMR after 50 min and after 24 hours of incubation. After 50 min, the cells were metabolizing H2O2 while this compound was still present in the medium, and it was completely biodegraded after 24 hours. Cells exposed to H2O2 had a distinct metabolome as compared to unexposed cells, revealing modulations of certain metabolic pathways in response to oxidative stress. These data indicated that the regulations observed mainly involved carbohydrate, glutathione, energy, lipid, peptides and amino-acids metabolisms. When cells had detoxified H2O2 from the medium, their metabolome was not distinguishable anymore from unexposed cells, highlighting the capacity of resilience of this bacterium. This work illustrates the interactions existing between the cloud microbial metabolome and cloud chemistry.

Siderophores in Cloud Waters and Potential Impact on Atmospheric Chemistry: Production by Microorganisms Isolated at the Puy de Dôme Station.

A total of 450 bacteria and yeast strains isolated from cloud waters sampled at the puy de Dôme station in France (1465 m) were screened for their ability to produce siderophores. To achieve this, a high-throughput method in 96-well plates was adapted from the CAS (chrome azurol S) method. Notably, 42% of the isolates were siderophore producers. This production was examined according to the phyla of the tested strains and the type of chelating functional groups (i.e., hydroxamate, catechol, and mixed type). The most active bacteria in the clouds belong to the γ-Proteobacteria class, among which the Pseudomonas genus is the most frequently encountered. γ-Proteobacteria are produced in the majority of mixed function siderophores, such as pyoverdines, which bear a photoactive group. Finally, siderophore production was shown to vary with the origin of the air masses. The organic speciation of iron remains largely unknown in warm clouds. Our results suggest that siderophores could partly chelate Fe(III) in cloud waters and thus potentially impact the chemistry of the atmospheric aqueous phase.