Various sparse Proteobacteria assemblages in the deep, close-to-bedrock Elbrus glacier could indicate local climate changes as well as volcanic activity in the past

The study's objective was to search for microbial communities in the deepest layers of the Elbrus glacier, close to the bedrock. The study aimed to test the hypothesis that microbial assemblages deposited in high mountain environments could provide information about past local climate variations. The ice coring was carried out on the Western plateau of Mt Elbrus, 5115m above sea level) glacier in the Caucasus, Russia (2009). The deepest ice sample (closest to bedrock), 182 meters deep, was selected for the study. The core age could be estimated at 1670+/- 400 years. The temperature in situ (volcanic caldera) is expected to fluctuate around 0oC. This ice sample contained small-size mineral inclusions and underwent thorough decontamination and processing in clean rooms. Two sub-segments, 182up, and 182bottom, separated by 6.5cm, were treated individually. Genomic DNA was isolated from the concentrated meltwater and amplified using universal degenerate primers targeting the v3-v4 region of bacterial 16S rRNA genes. Initially, the amplicons were sequenced using a common Sanger sequencing approach, but no significant findings were obtained due to contamination and the limited number of clones investigated. Subsequently, the identical amplicons were sequenced using high-throughput Oxford nanopore technology. The sequencing revealed that most reads were contaminants, but around 1% passed all contamination criteria and belonged to the Proteobacteria (phylum Pseudomonadota).Interestingly, the two subsegments of ice contained unique non-overlapping taxa of Proteobacteria, suggesting that the ice in these segments may have been deposited under different local climatic conditions. Thus, the upper-most ice segment showed presence of Methylobacterium tardum, Sphingomonas gilva, Brevundimonas naejangsanensis, Comamonas aquatica, Luteimonas pelagia, and Halomonas (Vreelandella) azerica, while the bottom segment - Paracoccus gahaiensis, Delftia tsuruhatensis, Stenotrophomonas (Pseudomonas) geniculata, and the phylotype related to Acidibacter ferrireducens. Notably, Halomonas azerica, an indicator for cold period depositions (Bulat et al., 2024a, 2024b), was detected in the top 182up sample, indicating that the enclosed snow was deposited during a cold period. This finding suggests that deep terrestrial glaciers may contain diverse Eolian microbial assemblages, which could serve as proxies for local climate changes in the past.