Ice nucleation studies of two beetles from sub-antarctic South Georgia

摘要

Supercooling points of adults and larvae of the coleopteransHydromedion sparsutumandPerimylops antarcticusat South Georgia ranged from -3.0 to -5.4°C withPerimylopsfreezing atc.1.6°C lower thanHydromedion. Intact excised guts from adults of both species frozec. 1°C lower than the adult insects. Ice nucleating activity of homogenized faeces from larvae and adults of both species and excised guts were compared with three potential food plants using an ice nucleation spectrometer. Mean supercooling points of the insect materials at four concentrations in distilled water (range from 0.01 to 10 g 1−1) were significantly different (P<0.01) within species, and within life stages between species. Differences in the supercooling points of suspensions ofPolytrichum alpinum(moss) andUsnea fasciata(lichen) were not significant. In general, differences between supercooling points were greater at the higher concentrations. Histograms of the supercooling points showed unimodal distributions particularly at high concentrations and greater dispersion with increased dilution. Spectra showing the concentration of active ice nucleators over the temperature range 0 to -20°C were developed. These showed that nucleation occurred as high as -2°C in faecal material and all insect samples nucleated above -3°C, whereas the plant materials nucleated between -4 and -5°C. The calculated number of ice nucleators for each material in suspension revealed low values (5.3 to 5.8 × 103) for the plants, but a greater abundance (1.3 × 105to 1.3 × 106) in the insect samples. It is concluded thatc.1000 active nucleators g−1are required for ice nucleation to occur in these suspensions. Ice nucleator activity of a suspension ofHydromedionfaeces was much reduced by heating to 75°C, suggesting a proteinaceous structure. These results are discussed in relation to ice nucleation in other insects, and it is concluded that bacteria may be responsible for the high nucleation temperatures, and hence poor supercooling, in these South Georgia insects. An empirical model is developed for ice nucleation spectra based