Cyanobacterial harmful algal blooms (cyanoHABs), which are a form of microbial dysbiosis in freshwater environments, are an emerging environmental and public health concern. Additionally, the freshwater environment serves as a reservoir of antibiotic resistance genes (ARGs), which pose a risk of transmission during microbial dysbiosis, such as cyanoHABs. However, the interactions between potential synergistic pollutants, cyanoHABs, and ARGs remain poorly understood. During cyanoHABs, Microcystis and high microcystin levels were dominant in all the nine regions of the river sampled. The resistome, mobilome, and microbiome were interrelated and linked to the physicochemical properties of freshwater. Planktothrix and Pseudanabaena competed with Actinobacteriota and Proteobacteria during cyanoHABs. Forty two ARG carriers were identified, most of which belonged to Actinobacteriota and Proteobacteria. ARG carriers showed a strong correlation with ARGs density, which decreased with the severity of cyanoHAB. Although ARGs decreased due to a reduction of ARG carriers during cyanoHABs, mobile gene elements (MGEs) and virulence factors (VFs) genes increased. We explored the relationship between cyanoHABs and ARGs for potential synergistic interaction. Our findings demonstrated that cyanobacteria compete with freshwater commensal bacteria such as Actinobacteriota and Proteobacteria, which carry ARGs in freshwater, resulting in a reduction of ARGs levels. Moreover, cyanoHABs generate biotic and abiotic stress in the freshwater microbiome, which may lead to an increase in MGEs and VFs. Exploration of the intricate interplays between microbiome, resistome, mobilome, and pathobiome during cyanoHABs not only revealed that the mechanisms underlying the dynamics of microbial dysbiosis but also emphasizes the need to prioritize the prevention of microbial dysbiosis in the risk management of ARGs.

Harmful algal blooms (HABs) represent major ecological disturbances that can alter microbial community composition and reduce ecosystem resilience. Here, we investigated bacterial and microeukaryotic assemblage transitions during a bloom-affected period in the Nakdong River, a regulated river system in South Korea. By integrating microcystin (MC) measurements, cyanobacterial dominance, water-quality parameters, and amplicon-based community profiling (16S rRNA gene and ITS2), we characterized how microbial communities reorganized along gradients of bloom intensity. MC concentrations increased markedly across the sampling period, and early-phase qPCR data showed concurrent increases in mcyE gene abundance through 6 August 2021. Bacterial communities shifted from Actinobacteriota- and Proteobacteria-dominated assemblages toward Microcystaceae-enriched communities. Microeukaryotic communities similarly transitioned from diverse Chlorophyta, fungi, and ciliates to Chlamydomonadaceae- and ciliate-dominated assemblages under high-MC conditions. Both domains exhibited significant reductions in alpha diversity with increasing bloom intensity, although microeukaryotic responses were comparatively moderate. Correlation analyses revealed strong associations of MC concentration and Microcystis abundance with declining pH, elevated BOD, and the enrichment of bloom-associated taxa. These results indicate that bloom-associated conditions in this regulated river system were accompanied by parallel restructuring of bacterial and microeukaryotic assemblages. The integration of environmental and microbial indicators presented here provides a framework for understanding HAB-associated microbial reorganization and for informing monitoring strategies in hydrologically altered freshwater ecosystems.

Chlorophyll-a monitoring characterizes algal biomass but does not directly measure cyanotoxins or human exposure. We examined ecological spatial associations between chlorophyll-a and population-weighted estimates of Alzheimer’s disease (AD) and Parkinson’s disease (PD) patient counts in the Nakdong River basin, South Korea. Data for 2010–2022 combined chlorophyll-a from a nationwide 939-site monitoring network, older-adult populations, and administrative patient counts across 33,049 1 km grids. Geographically weighted regression described local associations. Higher chlorophyll-a occurred along river reaches and downstream areas; positive local coefficients were strongest in selected inland districts, including Sangju-si and Jinju-si, and weaker in metropolitan areas. Administrative sensitivity analyses showed that association direction and magnitude depended on population adjustment and model specification. Supplementary bandwidth analyses also identified scale dependence and persistent residual spatial autocorrelation. Because the grid outcomes were constructed from administrative counts, interpolation accuracy was unquantified, and source-specific clinical definitions were unverified; these findings indicate exploratory ecological covariation rather than a toxicological effect or causal risk. The mapped contrasts may guide targeted water-quality and cyanotoxin monitoring and epidemiological validation.