ICE NUCLEI IN SOIL COMPROMISE COLD HARDINESS OF HATCHLING PAINTED TURTLES (CHRYSEMYS PICTA)

Abstract

Hatchling painted turtles (Chrysemys picta) commonly overwinter within their natal nests and survive exposure to temperatures as low as −12°C by supercooling. We report that the supercooling capacity of hatchling C. picta is reduced by direct contact with nest soil which, in samples from northwestern and north-central Nebraska, Indiana, and Ontario, contained potent ice nuclei active in the range of −3.5° to −5°C. These nuclei were sensitive to autoclaving and extractable in water. The supercooling capacity of C. picta hatched in native nest soil, or hatched in sterilized vermiculite (which lacks water-extractable nuclei), and subsequently exposed to nest soil, was reduced by ∼10°C relative to control turtles that were hatched and reared in sterilized vermiculite. The effect of these nuclei was potentiated by the presence of environmental moisture, although even transient exposure to dry nest soil markedly reduced supercooling capacity in ∼50% of the turtles. Unlike turtle species that hibernate underwater (Sternotherus odoratus, Chelydra serpentina, Apalone spinifera), hatchlings of C. picta exhibited an extraordinary capacity for supercooling (temperature of crystallization, −16° to −20°C) when cooled in isolation from external ice nuclei. However, hatchlings of these four species were equally susceptible to inoculation by suspensions of the ice-nucleating bacterium, Pseudomonas syringae. Indirect evidence suggests that the soil nuclei are associated with such microbes. Nucleating activity was higher in soil collected within nests than in soil collected at the same depth, adjacent to these nests. Differences in the activities of ice nuclei in nesting soils may account for geographic and local variation in winter survival of hatchling C. picta. Our finding that similar agents occur in various other terrestrial habitats in central North America suggests that such nuclei may pose a formidable challenge to the overwintering survival of ectothermic animals that rely on supercooling to withstand frost exposure.