Ice nuclei in soil compromise cold hardiness of hatchling painted turtles (Chrysemys picta)

Hatchling painted turtles (Chrysemys picta) commonly overwinter within thei r natal nests and survive exposure to temperatures as low as -12 degrees C by supercooling. We report that the supercooling capacity of hatchling C. p icta is reduced by direct contact with nest soil which, in samples from nor thwestern and north-central Nebraska, Indiana, and Ontario, contained poten t ice nuclei active in the range of -3.5 degrees to -5 degrees C. These nuc lei were sensitive to autoclaving and extractable in water. The supercoolin g capacity of C. picta hatched in native nest soil, or hatched in sterilize d vermiculite (which lacks water-extractable nuclei), and subsequently expo sed to nest soil, was reduced by similar to 10 degrees C relative to contro l turtles that were hatched and reared in sterilized vermiculite. The effec t of these nuclei was potentiated by the presence of environmental moisture , although even transient exposure to dry nest soil markedly reduced superc ooling capacity in similar to 50% of the turtles. Unlike turtle species tha t hibernate underwater (Sternotherus odoratus. Chelydra serpentina, Apalone spinifera), hatchlings of C. picta exhibited an extraordinary capacity for supercooling (temperature of crystallization, - 16 degrees C to - 20 degre es C) when cooled in isolation from external ice nuclei. However, hatchling s of these four species were equally susceptible to inoculation by suspensi ons of the ice-nucleating bacterium, Pseudomonas syringae. Indirect evidenc e suggests that the soil nuclei are associated with such microbes. Nucleati ng activity was higher in soil collected within nests than in soil collecte d at the same depth, adjacent to these nests. Differences in the activities of ice nuclei in nesting soils may account for geographic and local variat ion in winter survival of hatchling C. picta. Our finding that similar agen ts occur in various other terrestrial habitats in central North America sug gests that such nuclei may pose a formidable challenge to the overwintering survival of ectothermic animals that rely on supercooling to withstand fro st exposure.