| Summary: | Bacterial growth at low pressure is a new research area with implications for predicting microbial activity in clouds, the bulk atmosphere on Earth, and for modeling the forward contamination of planetary surfaces like Mars. Here we describe experiments on the recovery and identification of 23 species of bacterial hypobarophiles (def., growth under hypobaric conditions of approximately 1-2 kPa) in 11 genera capable of growth at 0.7 kPa. Hypobarophilic bacteria, but not archaea or fungi, were recovered from soil and non-soil ecosystems. The highest numbers of hypobarophiles were recovered from Arctic soil, Siberian permafrost, and human saliva. Isolates were identified through 16S rRNA sequencing to belong to the genera Carnobacterium, Exiguobacterium, Leuconostoc, Paenibacillus, and Trichococcus. The highest population of culturable hypobarophilic bacteria (5.1 x 104 cfu/g) was recovered from Colour Lake soils from Axel Heiberg Island in the Canadian arctic. In addition, we extend the number of hypobarophilic species in the genus Serratia to 6 type-strains that include S. ficaria, S. fonticola, S. grimesii, S. liquefaciens, S. plymuthica, and S. quinivorans. Microbial growth at 0.7 kPa suggests that pressure alone will not be growth-limiting on the martian surface, or in Earth's atmosphere up to an altitude of 34 km.
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