Hail is a common phenomenon in the world and in Slovenia, however it is poorly described in the literature. The hailstones sampled in this study were 10-12 cm in diameter, representing extremely rare samples due to the record size, never before reported in Slovenia. The goal of our research was to identify abiotic and biotic factors that could act as ice nucleation in hailstone formation. With metagenomic analysis of hail using the 16S rRNA gene high-throughput sequencing we identified a high diversity of bacteria in the hailstone. Bacteria belong to 25 phyla, 44 classes, 88 orders, 231 families, 315 genera and 505 species. The vast majority of these isolates are frequent in soil, followed by aquatic environments and plant material. We also identified some plant symbionts and plant pathogenic species. Using scanning electron microscopy we additionally observed various fungal spores, diatoms and pollen. Analysis of the hail content resulted in the first report of microplastic fibers. Micro-FTIR and Raman spectroscopy were used for the characterisation of the microplastic. In the total volume of hail stone, over 450 microplastic fibers, 43 coloured, were quantified. Most fibers were in the interior of the hailstone, strongly impliying their incorporation at high altitudes of the atmosphere. In addition we detected also a high amount of mineral particles, probably desert sand.
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