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.

The submerged petrified forest in Sozopol Bay, located along Bulgaria’s southeastern coast in the Black Sea, is an extraordinarily rare natural phenomenon that has remained unexplored in terms of microbial diversity until now. This study focuses on characterizing the microbial communities associated with this unique habitat. Ancient petrified tree remnants located at depths of 18–20 m were sampled in August–September 2024, targeting four tree trunks from different sites within the bay. The quantitative assessment of selected bacterial groups, essential for nutrient cycling, organic matter degradation, and marine ecosystem health, revealed distinct community profiles. 16S rDNA sequencing of cultivated isolates identified a diverse microbial community predominantly composed of γ-Proteobacteria, with key representatives such as Vibrio aestuarianus, Vibrio orientalis, Pseudoalteromonas, and Cobetia sp. The culture-independent approach confirmed the dominance of Proteobacteria, along with other prevalent phyla like Bacteroidetes, Planctomycetes, and Actinobacteria. The most abundant taxa included Woeseia oceani, Ilumatobacter coccineus, Halioglobus maricola, and Vibrio breoganii. Archaea made up about 3% of classified reads. Fungal sequences accounted for less than 2% of the total reads, indicating a low fungal prevalence. These results provide essential baseline data for future monitoring and the conservation of this unique habitat and its diverse microbial communities.