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ABSTRACT Aerobiology research focusing on bioaerosol particle dynamics has catalogued the identity, distribution, and abundance of airborne microbes in a broad variety of indoor environments and, more recently, indoor disinfection methods for medically relevant microbes. Given their importance in environmental health and our constant exposure to airborne microbes in our daily lives, surprisingly little is known about the activity of live bioaerosols and their metabolic responses to aerosolization and suspension stress. In this context, microbial messenger RNA (mRNA) is a powerful information source of near-real-time organismal responses that cannot be attained through genomic, proteomic, or metabolomic studies. This review discusses current knowledge from transcriptomic studies describing airborne bacterial cellular activity in response to a myriad of environmental stresses imparted rapidly upon aerosolization and continued suspension as a microscopic bioaerosol. In the context of transcriptome profiling, potential artifacts associated with aerosolization/collection of bioaerosols are discussed from the perspective of preserving mRNA and maintaining its fidelity as it exists in airborne microbes. Recommendations for advancing live bioaerosol metabolic profiling through gene expression studies are presented to mitigate inherent artifacts and challenges with modern bioaerosol experiments. These recommendations include the use of larger experimental chambers, temperature control during aerosolization processes, and liquid capture bioaerosol sampling into a nucleic acid preservative to improve the fidelity of collected RNA and better capture the transcriptional activity of airborne microorganisms. Eventually, improvements in profiling bioaerosol activity can contribute toward answering fundamental questions on the aerobiome such as: is the atmosphere a temporary highway or a habitat for microorganisms? 

The generation of hydrogen and reduced carbon compounds during serpentinization provides sustained energy for microorganisms on Earth, and possibly on other extraterrestrial bodies (e.g., Mars, icy satellites). However, the geochemical conditions that arise from water-rock reaction also challenge the known limits of microbial physiology, such as hyperalkaline pH, limited electron acceptors and inorganic carbon. Because cell membranes act as a primary barrier between a cell and its environment, lipids are a vital component in microbial acclimation to challenging physicochemical conditions. To probe the diversity of cell membrane lipids produced in serpentinizing settings and identify membrane adaptations to this environment, we conducted the first comprehensive intact polar lipid (IPL) biomarker survey of microbial communities inhabiting the subsurface at a terrestrial site of serpentinization. We used an expansive, custom environmental lipid database that expands the application of targeted and untargeted lipodomics in the study of microbial and biogeochemical processes. IPLs extracted from serpentinite-hosted fluid communities were comprised of >90% isoprenoidal and non-isoprenoidal diether glycolipids likely produced by archaeal methanogens and sulfate-reducing bacteria. Phospholipids only constituted ~1% of the intact polar lipidome. In addition to abundant diether glycolipids, betaine and trimethylated-ornithine aminolipids and glycosphingolipids were also detected, indicating pervasive membrane modifications in response to phosphate limitation. The carbon oxidation state of IPL backbones was positively correlated with the reduction potential of fluids, which may signify an energy conservation strategy for lipid synthesis. Together, these data suggest microorganisms inhabiting serpentinites possess a unique combination of membrane adaptations that allow for their survival in polyextreme environments. The persistence of IPLs in fluids beyond the presence of their source organisms, as indicated by 16S rRNA genes and transcripts, is promising for the detection of extinct life in serpentinizing settings through lipid biomarker signatures. These data contribute new insights into the complexity of lipid structures generated in actively serpentinizing environments and provide valuable context to aid in the reconstruction of past microbial activity from fossil lipid records of terrestrial serpentinites and the search for biosignatures elsewhere in our solar system. 

Airborne murine coronavirus was assessed for its sensitivity to the vapors of chemicals commonly used to disinfect indoor surfaces. As a model for the chemical sensitivity of airborne SARS-CoV-2, the infectious potential of airborne Mouse Hepatitis Virus (MHV) was tracked in the presence of the following pure chemical vapors, each of which was below its permissible exposure limit (PEL) as regulated by the US National Institute of Occupational Safety and Health (NIOSH): <50ppmv for glycol; <1ppmv for HOCl; and <1ppmv for H2O2. Along with its growth media, infectious MHV was aerosolized in a particle size distribution between 0.5 l/m and 3.2 l/m into a sealed, dark, 9m3 chamber maintained at 22 C and 60% RH, including levels of chemical vapors maintained below their respective PELs. As judged by the TCID50 of airborne MHV collected by condensation, this airborne virus was rapidly inactivated by HOCl vapor, incurring an average of 99% infectious potential loss after 16 ± 4 min exposure to <0.2 ppmv HOCl. Airborne MHV responded with a 99% loss of infectious potential in 38 ± 10 min of exposure to <0.9ppmv H2O2; and, a 99% loss of infectious potential in 33 ± 15 min when exposed to a gas-phase dipropylene glycol blend <20ppmv as TVOC. The juxtaposition of quantitative RT-PCR and TCID50 responses suggest that even low levels of gas-phase HOCl exposures can damage the genome of airborne coronavirus in relatively short time frames (c.a. < 5 mins). 

Abstract Metagenome assembled genomes (MAGs) and single amplified genomes (SAGs) affiliated with two distinct Methanobacterium lineages were recovered from subsurface fracture waters of the Samail Ophiolite, Sultanate of Oman. Lineage Type I was abundant in waters with circumneutral pH, whereas lineage Type II was abundant in hydrogen rich, hyperalkaline waters. Type I encoded proteins to couple hydrogen oxidation to CO2 reduction, typical of hydrogenotrophic methanogens. Surprisingly, Type II, which branched from the Type I lineage, lacked homologs of two key oxidative [NiFe]-hydrogenases. These functions were presumably replaced by formate dehydrogenases that oxidize formate to yield reductant and cytoplasmic CO2 via a pathway that was unique among characterized Methanobacteria, allowing cells to overcome CO2/oxidant limitation in high pH waters. This prediction was supported by microcosm-based radiotracer experiments that showed significant biological methane generation from formate, but not bicarbonate, in waters where the Type II lineage was detected in highest relative abundance. Phylogenetic analyses and variability in gene content suggested that recent and ongoing diversification of the Type II lineage was enabled by gene transfer, loss, and transposition. These data indicate that selection imposed by CO2/oxidant availability drove recent methanogen diversification into hyperalkaline waters that are heavily impacted by serpentinization.