Groundwater mixing at fracture intersections triggers massive iron-rich microbial mats

Abstract : While most freshwater on Earth resides and flows in groundwater systems, these deep subsurface environments are often assumed to have little biogeochemical activity compared to surface environments. Here we report a massive microbial mat of iron-oxidizing bacteria, flourishing 60 meters below the surface, far below the mixing zone where most microbial activity is believed to occur. The abundance of microtubular structures in the mat hinted at the prevalence of of Leptothrix ochracea, but metagenomic analysis revealed a diverse consortium of iron-oxidizing bacteria dominated by unknown members of the Gallionellaceae family. This deep biogeochemical hot spot formed at the intersection of bedrock fractures, which maintain redox gradients by mixing water with different residence times and chemical compositions. Using measured fracture properties and hydrological conditions we developed a quantitative model to simulate the reactive zone where such deep hot spots could occur. While seasonal fluctuations are generally thought to decrease with depth, we found that meter-scale changes in water table level moved the depth of the reactive zone hundreds of meters because the microaerophilic threshold for ironoxidizers is highly sensitive to changes in mixing rates at fracture intersections. These results demonstrate that dynamic microbial communities can be sustained deep below the surface in bedrock fractures. Given the ubiquity of fractures at multiple scales in Earth’s subsurface, such deep hot spots may strongly influence global biogeochemical cycles.
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Submitted on : Wednesday, March 14, 2018 - 2:23:53 PM
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  • HAL Id : insu-01731725, version 1


Olivier Bochet, Tanguy Le Borgne, Lorine Bethencourt, Luc Aquilina, Alexis Dufresne, et al.. Groundwater mixing at fracture intersections triggers massive iron-rich microbial mats . American Geophysical Union Fall Meeting 2017, Dec 2017, New Orleans, United States. pp.H31F-1570. ⟨insu-01731725⟩



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