Urban biodiversity supports crucial ecosystem services benefiting human well-being, including climate amelioration, soil erosion control, and air pollution reduction. However, urbanization degrades biodiversity, negatively impacting these services and potentially human health through reduced contact with natural environments. Fungi, a megadiverse kingdom, play key roles in ecosystem processes. Lichens fix nitrogen, mycorrhizal fungi enhance plant growth, endophytic fungi act as biocontrols, and wood-decaying fungi are essential for nutrient cycling. While lichenized fungi are known bioindicators of air quality and mycorrhizal fungi are sensitive to soil pollutants, understanding the overall impact of urbanization on fungal diversity remains limited due to methodological challenges. This study addresses this gap by using a novel approach combining aerial and soil sampling with molecular identification techniques to assess fungal community changes along an urbanization gradient in five Finnish cities.

The introduction cites existing literature highlighting the importance of urban biodiversity for human well-being and the negative impacts of urbanization on biodiversity and ecosystem services. It references studies demonstrating the links between biodiversity loss and human health issues. The role of fungi in various ecosystem processes, such as nitrogen fixation, nutrient cycling, and biocontrol, is also discussed, along with the use of specific fungal groups as bioindicators of environmental quality. The authors point out the lack of comprehensive studies on the effects of urbanization on fungal diversity, largely due to methodological limitations in surveying this diverse kingdom. The introduction positions the current study as addressing this gap by utilizing recent advances in aerial fungal sampling and molecular identification techniques.

The study employed a sampling design across five Finnish cities, establishing six plots per city (three urban, three natural) encompassing core and edge areas. Soil samples (5 cm depth) and air samples (24-hour collection using a cyclone sampler) were collected from each plot in triplicate. Samples were processed at the University of Helsinki and sent to the University of Guelph for DNA extraction, PCR amplification targeting the ITS region (a fungal barcode), and Illumina MiSeq sequencing. Synthetic DNA was added as a positive control to quantify fungal DNA. Bioinformatic analyses involved sequence processing, OTU clustering (99.5% similarity threshold), taxonomic placement using PROTAX (reliability threshold >90%), and constrained clustering of non-reliably classified sequences. Statistical analyses included PERMANOVA to assess the impact of urbanization and habitat type on community composition, and generalized linear mixed models to analyze DNA abundance and OTU richness.

The study revealed a significant decline in both fungal species richness and DNA abundance in urban areas compared to natural areas. This decline was observed at a spatial scale of just 1 km from the edge of natural habitats into urban areas, with a fivefold reduction in fungal DNA abundance in both air and soil samples. Contradicting the initial hypothesis, the reduction in fungal diversity was more pronounced in the air than in the soil. Analysis showed that a larger proportion of air-detectable fungi are specialists of natural habitats, while soil communities contain a higher proportion of habitat generalists. This finding emphasizes the sensitivity of the aerial fungal community to anthropogenic disturbances, highlighting the effectiveness of air sampling as a biomonitoring tool in urban settings.

The findings demonstrate the significant negative impact of urbanization on fungal biodiversity, even at relatively small spatial scales. The stronger effect of urbanization on aerial fungal communities compared to soil communities underscores the importance of airborne dispersal in shaping fungal diversity patterns. The observed differences in habitat specialization between aerial and soil fungi contribute to the distinct responses to urbanization observed in the two environments. The study highlights the value of air sampling as a practical and efficient method for biomonitoring fungal diversity and ecosystem health in urban areas, offering a potentially more sensitive and cost-effective alternative to traditional soil-based methods. Further research could explore the specific mechanisms driving the observed changes in fungal communities and the long-term consequences of these changes for urban ecosystem functioning.

This study provides quantitative evidence of the impact of urbanization on fungal diversity, showing a substantial decrease in both richness and abundance in urban environments compared to natural areas. The greater sensitivity of aerial fungal communities to urbanization highlights the potential of air sampling as a valuable biomonitoring tool. Future research should investigate the functional consequences of these biodiversity changes and explore the effectiveness of different urban planning strategies in mitigating the negative impacts of urbanization on fungal communities.

While the study used rigorous methodologies, there are some limitations. The sampling period was relatively short (8 days), potentially limiting the representation of seasonal variation in fungal communities. The study focused on five cities in Finland, limiting the generalizability of the findings to other regions or geographical contexts. The reliance on DNA-based methods may not fully capture the functional diversity of fungal communities. Future research could address these limitations by employing longer-term sampling strategies, broader geographical coverage, and incorporating functional trait analysis.