Spatiotemporal dynamics of benthic Microcoleus in a Swiss lotic system

Toxic species within the benthic Microcoleus cyanobacterial genus pose significant ecological and public health risks due to the production of the potent neurotoxin anatoxin. In Switzerland, Microcoleus anatoxicus, a known producer of anatoxin derivatives, has caused several dog fatalities causing alarm among the public and authorities. Similar incidents worldwide have been associated with various Oscillatoriales cyanobacteria. While previous studies have explored the microbial diversity within Microcoleus mats, annual spatiotemporal dynamics in lotic systems remain poorly understood. In this study, sediment samples at defined locations along the Areuse river (Neuchâtel) were collected for a year. Together with measurements of the local environmental parameters, the associated bacterial and eukaryotic communities were characterized using the 16S and 18S rRNA genes, respectively. Anatoxin-producing capacity was inferred using PCR of the anaC gene. The results show that while environmental parameters differed significantly over time, microbial communities differed significantly between sites. Bacterial communities showed relatively high abundances of cyanobacteria, specifically Microcoleus, which corresponded to up to 56% of all cyanobacteria. Furthermore, anaC PCR results were negative suggesting the dominance of non-toxic Microcoleus. Relative abundance of Microcoleus was highest between spring and summer, corresponding to up to 37% of the total bacterial communities. Significant positive correlations (approximately 64%) were observed between Microcoleus and other organisms such as rotifers, nematodes and chytrids. These positive relationships might highlight potential interactions between Microcoleus and predators. This is the first study of benthic microbial populations in a Swiss river. This study advances our understanding of benthic cyanobacterial ecology and provides insights into natural mechanisms that may mitigate toxin-related risks in freshwater environments. Moreover, the results highlight the prevalence of non-toxic Microcoleus on epilithic substrates suggesting its role in the establishment of microbial communities in the benthos.