title: Glycerol enhances fungal germination at the water-activity limit for life.
abstract: For the most-extreme fungal xerophiles, metabolic activity and cell division typically halts between 0.700 and 0.640 water activity (approximately 70.0-64.0% relative humidity). Here, we investigate whether glycerol can enhance xerophile germination under acute water-activity regimes, using an experimental system which represents the biophysical limit of Earth's biosphere. Spores from a variety of species, including Aspergillus penicillioides, Eurotium halophilicum, Xerochrysium xerophilum (formerly Chrysosporium xerophilum) and Xeromyces bisporus, were produced by cultures growing on media supplemented with glycerol (and contained up to 189 mg glycerol g dry spores-1). The ability of these spores to germinate, and the kinetics of germination, were then determined on a range of media designed to recreate stresses experienced in microbial habitats or anthropogenic systems (with water-activities from 0.765 to 0.575). For A. penicillioides, Eurotium amstelodami, E. halophilicum, X. xerophilum and X. bisporus, germination occurred at lower water-activities than previously recorded (0.640, 0.685, 0.651, 0.664 and 0.637 respectively). In addition, the kinetics of germination at low water-activities were substantially faster than those reported previously. Extrapolations indicated theoretical water-activity minima below these values; as low as 0.570 for A. penicillioides and X. bisporus. Glycerol is present at high concentrations (up to molar levels) in many types of microbial habitat. We discuss the likely role of glycerol in expanding the water-activity limit for microbial cell function in relation to temporal constraints and location of the microbial cell or habitat. The findings reported here have also critical implications for understanding the extremes of Earth's biosphere; for understanding the potency of disease-causing microorganisms; and in biotechnologies that operate at the limits of microbial function.
authors.affiliation: Institute for Global Food Security, School of Biological Sciences, MBC, Queen's University Belfast, Belfast, BT9 7BL, Northern Ireland.
authors.forename: Andrew
authors.initials: A
authors.lastname: Stevenson
authors.affiliation: Institute for Global Food Security, School of Biological Sciences, MBC, Queen's University Belfast, Belfast, BT9 7BL, Northern Ireland.
authors.forename: Philip G
authors.initials: PG
authors.lastname: Hamill
authors.affiliation: Applied Mycology Group, Cranfield Soil and AgriFood Institute, Cranfield University, Cranfield, Bedford, MK43 OAL, UK.
authors.forename: Ángel
authors.initials: Á
authors.lastname: Medina
authors.affiliation: Independent Safety Office, European Space Agency, 2200 AG Noordwijk, The Netherlands.
authors.forename: Gerhard
authors.initials: G
authors.lastname: Kminek
authors.affiliation: SETI Institute, Mountain View, California, 94043, USA.
authors.forename: John D
authors.initials: JD
authors.lastname: Rummel
authors.affiliation: CBS-KNAW Fungal Biodiversity Centre, Uppsalalaan 8, Utrecht, CT, 3584, The Netherlands.
authors.forename: Jan
authors.initials: J
authors.lastname: Dijksterhuis
authors.affiliation: School of Pharmacy and Biomolecular Sciences, University of Brighton, Huxley Building, Lewes Road, Brighton, BN2 4GJ, UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Timson
authors.affiliation: Applied Mycology Group, Cranfield Soil and AgriFood Institute, Cranfield University, Cranfield, Bedford, MK43 OAL, UK.
authors.forename: Naresh
authors.initials: N
authors.lastname: Magan
authors.affiliation: Department of Microbiology, Swedish University of Agricultural Sciences, Box 7025, Uppsala, 75007, Sweden.
authors.forename: Su-Lin L
authors.initials: SL
authors.lastname: Leong
authors.affiliation: Institute for Global Food Security, School of Biological Sciences, MBC, Queen's University Belfast, Belfast, BT9 7BL, Northern Ireland.
authors.forename: John E
authors.initials: JE
authors.lastname: Hallsworth
mesh_terms: D001230:Aspergillus / Q000378:metabolism; D017753:Ecosystem; D032641:Eurotiales / Q000378:metabolism; D005658:Fungi / Q000378:metabolism / Q000502:physiology*; D005990:Glycerol / Q000378:metabolism*; D013172:Spores, Fungal / Q000378:metabolism / Q000502:physiology*; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water; D005990:Glycerol