Understanding Ferruginous Versus Euxinic Conditions by Simulating Microbial Conditions in Meromictic Lakes
Vanessa M Hawkins1, Cody S Sheik2,3, Sergei Katsev1,3
1Department of Physics and Astronomy, University of Minnesota Duluth, Duluth, Minnesota, USA.
Geobiology
|October 20, 2025
Summary
Ferruginous (iron-rich) and euxinic (sulfide-rich) conditions in anoxic lakes depend on sulfate levels. Low sulfate (<100 μM) favors ferruginous environments, while higher sulfate can lead to euxinic conditions or accumulation in water columns.
Area of Science:
- Biogeochemistry
- Environmental Microbiology
- Geochemistry
Background:
- Ferruginous (iron-rich) conditions were common in Earth's oceans but are now rare, found mainly in stratified lakes.
- Microbial iron reduction competes with sulfate reduction in anoxic environments, influencing whether conditions become ferruginous or euxinic (sulfide-rich).
- Understanding this competition is crucial for interpreting ancient and modern anoxic ecosystems and methanogenic fermentation.
Purpose of the Study:
- To model and understand the biogeochemical factors controlling ferruginous versus euxinic conditions in anoxic lakes.
- To investigate the role of microbial metabolism and physical transport in shaping these environments.
- To assess the applicability of a unified model across diverse anoxic lake systems.
Main Methods:
- Utilized a biomass-explicit reaction-transport model to simulate biogeochemical distributions.
- Employed a fixed set of metabolism-specific microbial parameters across multiple anoxic, low-sulfate, meromictic lakes.
- Analyzed the influence of varying sulfate concentrations and physical transport rates on microbial abundances and geochemical patterns.
Main Results:
- Sulfate reduction and methanogenesis are widespread in iron-rich systems, confirmed by microbial surveys.
- Ferruginous conditions prevail when surface sulfate is below approximately 100 μM.
- Higher sulfate concentrations (mM) can accumulate in water columns when sulfur burial is iron-limited; physical transport significantly impacts biogeochemical distributions.
Conclusions:
- The balance between iron and sulfate reduction, influenced by sulfate concentration and physical mixing, dictates ferruginous versus euxinic conditions.
- A unified biogeochemical model based on fundamental thermodynamic and kinetic principles can describe geochemical patterns in diverse anoxic systems.
- This research provides insights into both modern lake ecosystems and early Earth ocean conditions.
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