Looking back: An account of how ice nucleation by bacteria was discovered; 1963 to about mid-1980s. Part 1. The basics

Abstract An overview is given of the path of research that led from asking how hailstones originate to the discovery that ice nucleation can be initiated by bacteria and other microorganisms at temperatures as high as −2°C. The major steps along that path were finding exceptionally effective ice nucleators in soils of high content of decayed vegetative matter, then in decaying tree leaves, then in plankton-laden ocean water. Eventually, it was shown that Pseudomonas syringae bacteria were responsible for the most of the observed activity. That identification coincided with the demonstration that the same bacteria cause frost damage on plants. Ice nucleation by bacteria meant an unexpected turn in the understanding of ice nucleation and of ice formation in the atmosphere. Subsequent research confirmed the unique effectiveness of ice nucleating particles of biological origin, referred to as bio-INPs, so that bio-INPs are now considered to be important elements of lower-tropospheric cloud processes. Nonetheless, some of the questions which originally motivated the research are still unresolved, so that revisiting the early work may be helpful to current endeavors. Part 1 of this manuscript summarizes how the discovery progressed. Part 2, (Schnell and Vali, 2024; SV24) shows the relationship between bio-INPs in soils and in precipitation with climate, and other findings. The online Supplemental Material contains a bibliography of recent work about bio-INPs.