Some extreme halophiles exhibit optimal rates of growth and metabolism at saturated NaCl (approx. 5 M; 0.755 water activity) (Stevenson et al., 2015; Lee et al., 2018). Some lower water-activity brines (acid brines, and those dominated by MgCl2 or CaCl2) are too stressful to permit metabolism (Hallsworth et al., 2007; Yakimov et al., 2015; La Cono et al., 2019; Benison et al., in preparation). Whereas it can be argued that microbial cells are perpetually stressed, and that stress enhances their vitality (Hallsworth, 2018), combinations of low water-activity+osmotic stress+chaotropicity, low pH, and/or high ionic strength can prevent all life processes and cause cell-system failure (Hallsworth et al., 2007; Alves et al., 2015; Fox-Powell et al., 2016). The question remains, what is the ultimate value at which the most-halotolerant microbe is inhibited by water activity per se. We aimed to investigate whether brines of <0.755 water activity can permit cellular function and found that kosmotropic substances, when incorporated into to chaotropic brines, enable proliferation of halophiles between 0.748 and 0.635 (Stevenson et al., 2015; Hallsworth et al., 2007; Yakimov et al., 2015; Stevenson et al., 2015). Compatible solutes, essential for salt tolerance, are either chaotropic (glycerol, fructose) or kosmotropic (K+, proline, ectoine, mannitol, trehalose) (Chin et al., 2010; Cray et al., 2013; Stevenson et al., 2017). Using the fungal xerophile/halophile Aspergillus penicillioides (JH06THJ; syn. FRR 6206 Williams & Hallsworth, 2009; Hallsworth, 2019), we assayed germination of conidia that had accumulated glycerol on an assay-medium supplemented with NaCl+glycerol. Differentiation, germination and cell division were observed at 0.585 water activity (Stevenson et al., 2017); equivalent to a theoretical concentration of 8 M NaCl (Lee et al., 2018). The finite solubility of this salt may have stabilised the genetic composition of halophile populations and limited the action of natural selection in driving their evolution towards greater xerophilicity (Lee et al., 2018). We conclude that saturated NaCl lies only part way along the thermodynamic scale for microbial life, and does not represent a limit for life.
|Title of host publication||Studia Universitatis Babes-Bolyai Biologia|
|Place of Publication||Biologia, Romania|
|Publication status||Published - Jun 2019|