Ice-nucleating bacteria from the guts of two sub-Antarctic beetles, Hydromedion sparsutum and Perimylops antarcticus (Perimylopidae)

The site of ice nucleation in the freeze-tolerant, sub-Antarctic beetle Hyd romedion sparsutum has been investigated. Ice(+) bacteria, active at above -2.0 degrees C, were isolated from the guts of beetles and identified as a fluorescent Pseudomonas species. Other possible sites of nucleation, includ ing the hemolymph, were examined but had a lower activity. Ice+ bacteria we re isolated from mixed populations, isolated from the guts of adult beetles , and grown on nutrient agar plates and in nutrient broth. Nucleation activ ity of the broth culture peaked after only 2 days although the number of li ve cells continued to increase until day 6. These cultures were used to det ermine the maximum nucleation activity of a bacterial suspension in sterile distilled water (-3.4 degrees C) and the dilution factor required to cause a 50% reduction in activity (10(4)). The original bacterial suspension had an absorbance of 0.5 measured at 660 nm and contained 6 x 10(11) bacteria per milliliter. From this it is estimated that only I in 10(6) bacteria pos sessed the highest levels of ice-nucleating activity. Other insect species, including Perimylops antarcticus, which are found in habitats similar to t hat of H. sparsutum, were examined for the presence of ice(+) bacteria. All contained ice-nucleating bacteria in their guts but with a lower level of activity than in H. sparsutum. (C) 1999 Academic Press.