Release of cell‐free ice nuclei from Halomonas elongata expressing the ice nucleation gene inaZ of Pseudomonas syringae

Release of ice nuclei in the growth medium of recombinant Halomonas elongata cells expressing the inaZ gene of Pseudomonas syringae was studied in an attempt to produce cell‐free active ice nuclei for biotechnological applications. Cell‐free ice nuclei were not retained by cellulose acetate filters of 0·2 µm pore size. Highest activity of cell‐free ice nuclei was obtained when cells were grown in low salinity (0·5–5% NaCl, w/v). Freezing temperature threshold, estimated to be below − 7 °C indicating class C nuclei, was not affected by medium salinity. Their density, as estimated by Percoll density centrifugation, was 1·018 ± 0·002 g ml −1 and they were found to be free of lipids. Ice nuclei are released in the growth medium of recombinant H. elongata cells probably because of inefficient anchoring of the ice‐nucleation protein aggregates in the outer membrane. The ice + recombinant H. elongata cells could be useful for future use as a source of active cell‐free ice nucleation protein.