Comparative gene expression during cloudy and clear conditions: (A) RNA-to-DNA concentration ratio for clouds and clear atmospheric conditions; dots indicate individual samples, and boxplots display medians, 25th and 75th percentiles, and whiskers 1.5 interquartile ranges; ** indicates highly significant difference, highlighting the relatively higher total RNA concentration in clouds (Mann-Whitney test, p-value=0.004); (B) Non-Metric Multidimensional Scaling (NMDS) polygons based on the 21,046 functional gene entries in MGs and MTs, illustrating the distinction between MGs and MTs, and the differences between MTs during cloudy and clear conditions; (C) volcano plot representing differential gene expression between atmospheric conditions, with genes significantly overexpressed in and outside clouds in blue and red, respectively; grey dots represent genes whose expression level is not affected; dashed lines represent significance thresholds; DEA coefficient: Differential Expression Analysis (DEA) coefficient, from the MTXmodel R package (78); this illustrates that overexpression of distinct genes depends on the atmospheric condition; (D) numbers of overrepresented Gene Ontology terms (GOs) derived from gene annotations during cloudy and clear conditions, in blue and red, respectively, referring to their classification as Biological Processes (BP), Cellular Components (CC) and Molecular Functions (MF). A number of biological functions are overexpressed in clouds compared with clear conditions. — biorxiv.org
Editor’s note: with recent speculation about the potential habitability of the clouds of Venus and the long-hypothesized notion of airborne life forms on Saturn and Jupiter these studies from Earth microbes living aloft might serve as a template to further consider how that might actually happen.
Bacteria cells and fungal spores can aerosolize and remain suspended in the atmosphere for several days, exposed to water limitation, oxidation, and lack of nutrients. Using comparative metagenomics/metatranscriptomics, we show that clouds are associated with the activation of numerous metabolic functions in airborne microorganisms, including fungal spore germination.
The whole phenomenon mirrors the rapid recovery of microbial activity in soils after rewetting by rain, known as the “Birch effect”. Insufficient nutrient resources in cloud droplets cause a famine that recycling cellular structures could alleviate.
The recovery of metabolic activity by microorganisms in clouds could favor surface invasion upon deposition, but it may also compromise further survival upon cloud evaporation. In any case, clouds appear as floating biologically active aquatic systems.
Raphaëlle Péguilhan, Florent Rossi, Muriel Joly, Engy Nasr, Bérénice Batut, François Enault, Barbara Ervens, Pierre Amato
Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻
