Diffusive Controls and Thermodynamics Constraints On Reductive Biogeochemical Reactions.

For any favorable biogeochemical reaction in soils and sediments, microorganisms exploit thermodynamic disequilibrium; as a community, they are generally perceived to select and deplete electron acceptors that yield the greatest energy return until the next acceptor becomes more favorable. This general pattern is often apparent in, or at least perceived for, soils and sediments transitioning from aerobic to anaerobic conditions, leading to progressive reduction of O₂, NO₃^-, Fe(III), SO₄^2-, and CO₂. Owing to the physical heterogeneity of most natural systems, extreme concentration gradients can be established within diffusively controlled domains that thus alters the expected redox sequence and leads to microscale variation in operative biogeochemical processes.  Iron(III) minerals impart a particularly important variable to the thermodynamic favorability of anaerobic electron acceptors: their redox potentials vary appreciably.  Owing to differences in reduction potential, ferrihydrite should be preferentially reduced relative to SO₄^2- or CO₂ (methanogenesis) but not goethite or hematite.  Moreover, the selection of electron acceptors is dependent on the electron donor.  Here we provide a detailed account of the thermodynamic variability for anaerobic soils and sediments, revealing the expected preference of electron acceptors—results not always consistent with commonly presumed redox ladders.  In support of the thermodynamic analysis, we conducted experiments utilizing reaction cells designed to mimic the interface between diffusive and advective domains within structured soil/sediment media. Diffusive domains were packed with ferrihydrite or hematite-coated gibbsite inoculated with a microbial community collected from a California (USA) rice paddy (capable of Fe(III)_(s), SO₄^2- and CO₂ reduction) and overlain with an advective flow channel consisting of quartz sand. Our results demonstrate that the segregation of biogeochemical processes within diffusive domains is highly dependent on the thermodynamic favorability of the Fe (hydr)oxide present, and that thermodynamic disequilibrium results in the co-occurance of sulfate reduction, methanogenesis, and reduction of hematite.