Ivan Traykov

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.

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.

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.

Macrophyte overgrowth in freshwater ecosystems is a worldwide problem, and a common management practice for these ecosystems is to remove the aquatic vegetation. In the study, the short-term effects of macrophyte harvesting in a mountain lake (Trevnoto Lake, Bulgaria) on both bacterial metabolic activity and fingerprints were assessed using Biolog Ecoplate™ assay. Bacterial responses to human activity were manifested by: 1) increased bacterial metabolic activity by 15%, and 2) shifts in bacterial metabolic fingerprints. The most affected carbon sources were those with low availability for aquatic bacteria – 85% of them were influenced and the rates of their utilizations changed from a decrease of 96% to an increase of 681%. In general, a clear trend was formed that the greater the bioavailability of the carbon sources, the less susceptible is their utilization to the environmental change. This preliminary study indicates an immediate impact of macrophyte harvesting on lake bacteria. In this aspect, Biolog Ecoplate™ assay could be useful in lake monitoring programmes in order to develop better sustainable management practices for restoration.

Upland rivers across Europe still exhibit undisturbed conditions and represent a treasure that we cannot afford to lose. We hypothesize that the combination of pristine and modified conditions could demonstrate biological responses along the stressor gradients. Thus, the response of aquatic macrophyte communities to anthropogenic stressors along upland rivers in Bulgaria was studied. Six stressors were selected out of 36 parameters grouped into hydromorphological, chemical variables and combined drivers (catchment land use). The stressors strongly affected species richness on the basis of biological type (bryophytes vs. vascular plants) and ecomorphological type (hydrophytes vs. helophytes). Hydrological alteration expressed by the change of the river’s base flow and altered riparian habitats has led to a suppression of bryophytes and a dominance of riverbank plant communities. Seventy-five percent of mountain sites were lacking bryophytes, and the vegetation at semi-mountainous sites was dominated by vascular plants. It can be concluded that hydropeaking, organic and inorganic pollution, and discontinuous urban structures caused important modifications in the aquatic macrophyte assemblages. Macrophyte abundance and the biological and ecomorphological type of aquatic macrophytes reflect multi-stressor effects in upland rivers.

Over the past decade, efforts to value and protect ecosystem services have been promoted by many as the last, best hope for making conservation mainstream. Here, we present the results from the evaluation of the maintenance ecosystem services, provided by the agroecosystems in Bulgaria. The evaluation was conducted on a range of national, European and international databases following the methodology of MAES and classification system of CICES adapted by the Bulgarian Ministry of Environment and Water. During the study, 213857 agroecosystems were evaluated referring to their capacity to maintain pollinators, natural pest control, soil quality and atmospheric carbon sequestration. The capacity of agroecosystems to provide the above mentioned ecosystem services was ranged from moderate to very high, tending the mountainous agroecosystems to be more beneficial than that located in the plains and lowlands. The intensive management of the latter in order to achieve perceived efficiencies in the production of agricultural goods reduces their importance for local and regional ecological processes.

Ore mining and processing have greatly altered ecosystems, often limiting their capacity to provide ecosystem services critical to our survival. The soil environments of two abandoned uranium mines were chosen to analyze the effects of long-term uranium and heavy metal contamination on soil microbial communities using dehydrogenase and phosphatase activities as indicators of metal stress. The levels of soil contamination were low, ranging from ‘precaution’ to ‘moderate’, calculated as Nemerow index. Multivariate analyses of enzyme activities revealed the following: (i) spatial pattern of microbial endpoints where the more contaminated soils had higher dehydrogenase and phosphatase activities, (ii) biological grouping of soils depended on both the level of soil contamination and management practice, (iii) significant correlations between both dehydrogenase and alkaline phosphatase activities and soil organic matter and metals (Cd, Co, Cr, and Zn, but not U), and (iv) multiple relationships between the alkaline than the acid phosphatase and the environmental factors. The results showed an evidence of microbial tolerance and adaptation to the soil contamination established during the long-term metal exposure and the key role of soil organic matter in maintaining high microbial enzyme activities and mitigating the metal toxicity. Additionally, the results suggested that the soil microbial communities are able to reduce the metal stress by intensive phosphatase synthesis, benefiting a passive environmental remediation and provision of vital ecosystem services.