Global warming poses a significant threat to lake ecosystems, with high-mountain lakes being among the earliest and most severely impacted. However, the processes affecting water ecology under climate change remain poorly understood. This study investigates, for the first time, the effects of regional warming on three high-mountain lakes, Sulzata, Okoto and Bubreka, located in the Rila Mountains, Bulgaria, by examining shifts in bacterial metabolic capacity in relation to the rate and range of utilizable carbon sources using the Biolog EcoPlate™ assay. Over the last decade, ice-free water temperatures in the lakes have risen by an average of 2.6 °C, leading to increased nutrient concentrations and enhanced primary productivity, particularly in the shallowest lake. Bacterial communities responded to these changes by increasing their metabolic rates and shifting substrate preferences from carbohydrates to carboxylic acids. While the utilization rates of some carbon sources remained stable, others showed significant changes—some increased (e.g., D-galactonic acid γ-lactone and itaconic acid), while others decreased (e.g., α-D-lactose and D-xylose). The most pronounced effects of warming were observed in June, coinciding with the onset of the growing season. These findings suggest that rising temperatures may substantially alter bacterial metabolic potential, contributing to a long-term positive feedback loop between lake nutrient cycling and climate change.

High-altitude lakes are sensitive sentinels of climate change, and yet the microbial processes driving biogeochemical cycles under warming remain poorly understood. Seasonal dynamics of sediment microbial communities in four glacial lakes within the cirques of the Seven Rila Lakes (Rila Mountains, Bulgaria) were investigated during June, August, and October 2024. Environmental monitoring showed pronounced seasonal variability with summer peaks in temperature, dissolved organic carbon and nitrogen, and increased inputs of labile organic matter. Multivariate analyses revealed strong seasonal structuring of microbial assemblages, with the largest shifts occurring between the early ice-free season and summer. Archaeal communities were dominated by Halobacteriota over time, whereas lake warming corresponded to a shift from early-season dominance of Actinomycetota and Bacillota to the summer prevalence of Pseudomonadota and Cyanobacteriota, as well as a proliferation of minor phyla Gemmatimonadota. At the genus level, summer enrichment of archaeal methanogen Methanosaeta and bacterial genera Paucibacter, Ca. Accumulibacter, Methylibium, Crenothrix, and Pseudanabaena was observed. Canonical correspondence analysis identified temperature and nutrient availability as the primary drivers of microbial community restructuring. Our results provide empirical evidence that warming and associated changes in organic matter inputs drive shifts in sediment microbial communities, with direct implications for carbon cycling and methane production. Additionally, the results highlight the sensitivity of high-altitude lakes to global warming and emphasize the critical role of microbial communities in modulating ecosystem responses to climate change.

The main scientific interest for decades remained focused primarily on physical and chemical air pollutants, which severely limited our knowledge of microbial diversity in the atmosphere. The ecology of airborne microorganisms is poorly understood, even though they are a constant element of the air we breathe. Their exploration is very important for understanding health and ecological impacts, mainly in the highly urbanized cities. The main purpose of this study is to explore the biodiversity of microorganisms in the atmosphere over the densely populated centre of Sofia (Bulgaria) during spring and winter by Next-generation sequencing. The results showed that the most prevailing bacteria in the urban bioaerosols were representatives of phyla Firmicutes, Proteobacteria, Bacteroides, Actinobacteria. Many of the identified species were soil and leaf surface dwellers. Most of the detected fungal sequences belong to the phylum Ascomycota. Some pathogenic bacterial and fungal taxa were also found. This study provides the first information about microbial diversity in the outdoor air above Sofia and contributes to better understanding of urban airborne microbiota.

Glacial lakes are harsh ecosystems harbouring unique microbiota. In this study, we investigated the distribution of bacterial families in the water and sediment of three glacial lakes located in Rila Mountain, Bulgaria. Our results revealed high bacterial diversity and pronounced habitat preferences, as evidenced by the presence of 310 bacterial families with distinct distribution patterns between lake water and sediments. Taxonomic dissimilarities between water and sediment bacterial communities ranged from 74% to 100%. In general, water communities exhibited a common set of bacterial families with relatively consistent abundances across lakes, whereas sediment communities were more lake specific. Dominant families in sediments included Bacteroidaceae, Veillonellaceae, and Nitrosomonadaceae, whereas Comamonadaceae, Mycobacteriaceae, and Sporichthyaceae were prevalent in water samples. These findings highlight a clear bacterial partitioning between lake habitats and provide new insights for glacial lake conservation in a changing environment, particularly in the context of global warming.

In terms of air quality, Sofia, Bulgaria, is one of Europe's most polluted capitals. The city represents a unique pollution challenge due to the combination of specific environmental and orographic features. Until now, no studies have been conducted to monitor outdoor bioaerosol concentrations. This pioneering research presents a systematic study of the seasonal cycle of bacterial and fungal aerosol concentrations in Sofia. Although the overall study extends over a full year, this paper focuses specifically on results from the spring and summer seasons. Airborne microbial samples were collected from the 1st of March 2024 to the 31st of August 2024 at an open-air location situated in a central urban area. The findings reveal a clear seasonal distribution pattern for bacterial and fungal aerosols. The mean bioaerosol concentrations during the spring-summer period were ranked in descending order as follows: a) bacterial aerosols: summer (258.0 CFU/m3) > spring (197.0 CFU/m3); (b) fungal aerosols: summer (987.0 CFU/m3) > spring (228.0 CFU/m3). This data on bioaerosol levels can be used to support future aeromicrobiology studies, policymaking, and urban planning in Sofia.

High-mountain lakes are biodiversity hotspots sensitive to increasing regional and global climate warming. However, their microbial communities remain insufficiently characterized due to their remoteness and limited accessibility. This study aimed to determine how seasonal environmental parameters shape the composition, structure and diversity of littoral bacterial communities in three glacial lakes in Rila Mountains (Bulgaria). Water samples were collected during ice-free periods in 2023 and 2024, and bacterial taxonomic composition was analysed by Next-generation sequencing. A total of 1158 bacterial OTUs were identified encompassing 18 phyla and 165 families. Actinomycetota, Pseudomonadota, and Bacteroidota were dominant at the phylum level, and Sporichthyaceae, Comamonadaceae, Chitinophagaceae and Mycobacteriaceae were most abundant among the families. Community richness and diversity peaked in June, immediately after ice melting, particularly in the highest-altitude lake (Sulzata Lake), and declined during the warm season (August), when the relative abundances of Sporichthyaceae and Mycobacteriaceae (Actinomycetota) increased. Seasonal restructuring occurred across phyla and families even in a single taxon, with water temperature and organic carbon availability identified as the main environmental drivers. The findings have improved our understanding of temperature-driven bacterial responses. They have also highlighted the vulnerability of cold-adapted taxa to regional climate warming which may contribute to more effective biodiversity conservation strategies for these unique ecosystems.