H₂O₂ modulates the energetic metabolism of the cloud microbiome

Abstract. Chemical reactions in clouds lead to oxidation processes driven by radicals (mainly HO^⚫, NO₃^⚫, or HO₂^⚫) or strong oxidants such as H₂O₂, O₃, nitrate, and nitrite. Among those species, hydrogen peroxide plays a central role in the cloud chemistry by driving its oxidant capacity. In cloud droplets, H₂O₂ is transformed by microorganisms which are metabolically active. Biological activity can therefore impact the cloud oxidant capacity. The present article aims at highlighting the interactions between H₂O₂ and microorganisms within the cloud system.

First, experiments were performed with selected strains studied as a reference isolated from clouds in microcosms designed to mimic the cloud chemical composition, including the presence of light and iron. Biotic and abiotic degradation rates of H₂O₂ were measured and results showed that biodegradation was the most efficient process together with the photo-Fenton process. H₂O₂ strongly impacted the microbial energetic state as shown by adenosine triphosphate (ATP) measurements in the presence and absence of H₂O₂. This ATP depletion was not due to the loss of cell viability. Secondly, correlation studies were performed based on real cloud measurements from 37 cloud samples collected at the PUY station (1465 m a.s.l., France). The results support a strong correlation between ATP and H₂O₂ concentrations and confirm that H₂O₂ modulates the energetic metabolism of the cloud microbiome. The modulation of microbial metabolism by H₂O₂ concentration could thus impact cloud chemistry, in particular the biotransformation rates of carbon compounds, and consequently can perturb the way the cloud system is modifying the global atmospheric chemistry.