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Ice nuclei (IN) that catalyze ice nucleation in the atmosphere are considered to be vital to the initiation of ice formation in clouds, which in turn impact precipitation and climate. Some bacterial ice-nucleating particles are presumed to speed up ice formation at relatively warm temperatures (above –10°C), and may thus contribute to the induction of precipitation. In this study, nine rainwater samples were collected from forest ecosystems located in the Changbai, Wuling, and Dinghu mountains in eastern China, and the microbial community compositions were determined. Species of the genus Pseudomonas are considered to be the most efficient ice nucleation-active bacteria; however, Pseudomonas spp. were only observed in two of the rainwater samples from two months (June and September) at the Wuling Mountain site (2% of total Sanger clones sets). The median freezing temperature (T50) of crude rain droplets ranged from –11.2°C to –18.6°C based on immersion-mode freezing experiments, and their cumulative IN spectrum revealed a very low or near-zero frequency of bacterial IN at –10°C, which was used as the temperature cutoff to define ice nucleators of biological origin. The T50 of the filtrate (< 0.22 µm) was between –16.0°C and –20.8°C. The frequency of IN was higher at –10°C from the particle (≥ 0.22 µm) suspension of rainwater collected in 2013, with an onset freezing temperature of approximately –6°C or warmer, and a T50 value from –8.2 to –14.0°C. Moreover, ice nucleation was significantly deactivated by heat treatment (to disrupt the structure of membrane-bound proteins) at –10°C, with an average inhibition of 85%. Our results indicate that bacterial IN are present but play a minor role in ice nucleation. Further studies evaluating the concentration and physical chemistry of bacteria in the atmosphere are needed to confirm these results.

The Diversity and Role of Bacterial Ice Nuclei in Rainwater from Mountain Sites in China