Lazarcik, J. | Search bioRxiv

bioRxiv2026

Time, Not fungicide treatment, influences the resistome of the turf phyllosphere

Antibiotic resistance genes (ARGs) are an emerging class of environmental contaminants with significant implications for public health. Previous studies have linked fungicide exposure to elevated levels of ARGs in soil microbiomes, but research investigating the impacts of fungicide on ARGs within phyllosphere bacterial communities is limited. To address this, creeping bentgrass was treated with the fungicide active ingredients chlorothalonil, fluxapyroxad, and propiconazole and sampled at 4 hours, 96 hours, and one week post-application. Quantitative PCR (qPCR) was performed to quantify the abundance of ARGs and a metal resistance genes (MRG) abundance relative to 16s. Additionally, 16S rRNA gene sequencing was performed to characterize bacterial community composition. Results indicated that fungicide treatments did not significantly alter the relative abundance of ARGs or an MRG within bent grass bacterial communities. However, significant changes were observed over time, with changes in ARG and MRG abundance mirroring temporal shifts in bacterial beta diversity. ARGs and the MRG relative abundance had significant correlations with Proteobacteria, Actinobacteria, Bacteriadota, and Firmicutes, including with genera Pseudoxanthomas, and Dyandobacter, which contain opportunistic human pathogens. This study demonstrates that fungicides have limited influence on the abundance of ARGs and MRGs in the phyllosphere and helps guide further investigations aiming to mitigate the spread of antibiotic resistance. ImportanceAntibiotic resistance makes it harder to treat bacterial infections. Recent evidence suggests that fungicides may increase the abundance of antibiotic resistance genes (ARGs) in soil. Leaves are also treated with fungicides, but it is unclear if there will be increases of antibiotic resistance in this environment because bacteria on leaves face different physiological stresses. Additionally, plants may serve as a route of infection to humans or animals with antibiotic resistant bacteria making it a critical micro-environment to investigate. The purpose of this study was to determine whether the prevalence of antibiotic resistance on plant surfaces changes after short-term exposure to fungicides. This study shows that over short-term application periods, time has a greater effect on the abundance of genes that cause antibiotic resistance than treatment with the fungicides chlorothalonil, fluxapyroxad, and propiconazole on plants. This work helps inform future efforts to mitigate the spread of antibiotic resistance.

microbiology↗

bioRxiv2024

Cryptic cycling by electroactive bacterioplankton in Trout Bog Lake

The genetic potential for extracellular electron transfer (EET)-based metabolism has been shown to be a prevailing feature of humic lakes where bacterioplankton may be able to use EET to cycle dissolved organic matter (DOM) extracellularly between oxidized and reduced states, but measurable abiotic features resulting from this phenomenon have yet to be demonstrated. We observed an anoxygenic photosynthetic Chlorobium sp. bloom each summer in Trout Bog Lake in northern WI, USA. Given this blooms characteristics, we hypothesized that EET-based metabolisms of Chlorobium sp. and accompanying bacteria cycle DOM between oxidized and reduced states with seasonal or diel-timescale oscillations; therefore, we anticipated this could be measured by weekly and subdaily sampling. We collected vertical profiles on these timescales using a multiparameter sonde, including oxidation-reduction potential measurement, and we assayed for inorganic electron donors. We also developed and deployed a buoy to measure electric current flow between many pairs of electrodes simultaneously. Using metagenomics analyses, we examined the EET genes and other oxidoreductases of bacteria from water column samples at select depths and from biofilms that developed on electrodes at similar depths. Our results indicate the occurrence of diel electron cycling between phototrophic oxidation (electrotrophic metabolism) and anaerobic respiration (electrogenic metabolism), likely involving DOM. We also observed a gradual seasonal increase in hypolimnion oxidation-reduction potential. These diel and seasonal patterns have implications for carbon emissions and the ecology of electroactive bacteria in lakes. IMPORTANCEWe investigated the physical, chemical, and redox characteristics of a bog lake and electrodes hung therein to test the hypothesis that dissolved organic matter is being cycled between oxidized and reduced states by electroactive bacterioplankton powered by phototrophy. To do so we performed field-based analyses on multiple timescales using both established and novel instrumentation. We paired these analyses with recently developed bioinformatics pipelines for metagenomics data to investigate genes that enable electroactive metabolism and accompanying metabolisms. Our results are consistent with our hypothesis and yet upend some of our other expectations. Our findings have implications for understanding greenhouse gas emissions from lakes, including electroactivity as an integral part of lake metabolism throughout more of the anoxic parts of lakes and for a longer portion of the summer than expected. Our results also give a sense of what electroactivity occurs at given depths and provide a strong basis for future studies.

microbiology↗