Cryptic cycling by electroactive bacterioplankton in Trout Bog Lake
Charles N Olmsted1,2,3, Mark Gahler2, Eric Roden4
1Department of Molecular and Environmental Toxicology, Universities of Wisconsin, Madison, Wisconsin, USA.
Applied and Environmental Microbiology
|June 20, 2025
Summary
Bacterial communities in humic lakes use light-driven extracellular electron transfer (EET) to cycle dissolved organic matter (DOM). This process influences lake metabolism and methane emissions, with diel and seasonal patterns observed.
Area of Science:
- Environmental Microbiology
- Biogeochemistry
- Aquatic Ecology
Background:
- Extracellular electron transfer (EET) is a common genetic trait in humic lake bacterioplankton, but its ecological significance is not well understood.
- Previous research predicted a substantial ecological role for EET, yet direct evidence has been limited.
- Dissolved organic matter (DOM) cycling is crucial in aquatic ecosystems, influencing redox conditions and microbial metabolism.
Purpose of the Study:
- To investigate the hypothesis that anoxygenic phototrophic electrotrophs and heterotrophic electrogens cycle DOM between oxidized and reduced states in humic lakes.
- To explore the diel-scale oscillations in microbial activity driven by light-dependent photosynthesis and associated electrogenic processes.
- To determine the ecological contribution of EET to humic lake bacterioplankton metabolism and its impact on biogeochemical cycles.
Main Methods:
- Field-based analyses in Trout Bog Lake, Wisconsin, USA, including depth-discrete metagenomic, physiochemical, and electrochemical profiling.
- Development and deployment of an automated buoy for simultaneous measurement of electric current flow between multiple electrode pairs.
- Integration of field data with published metatranscriptomic analyses to identify electroactive microbial communities and metabolic pathways.
Main Results:
- Observed variations in oxidation-reduction potential (ORP) correlated with sunlight, initiating at depths with anoxygenic phototrophs possessing EET genes.
- Measured electric current flow showed a correlation with sunlight, indicating light-driven electron consumption.
- Evidence of electron cycling between phototrophic oxidation by *Chlorobium* and anaerobic respiration by *Geothrix* involving DOM was found, alongside seasonal ORP increases in the hypolimnion.
Conclusions:
- Electroactive DOM mediates the ecology of electroactive bacteria in humic lakes, driving diel and seasonal redox patterns.
- EET plays an integral role in lake metabolism, extending into anoxic zones and persisting longer into summer than previously expected.
- These findings have significant implications for understanding and managing greenhouse gas emissions, particularly methane, from humic lakes.
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