Thermodynamic energy of anaerobic microbial redox reactions couples elemental biogeochemical cycles

The thermodynamic energy of redox reactions affects the distribution of microbial redox reactions and cyclic transformation of elements in various anaerobic ecosystems. The principle of thermodynamics is of dramatic significance in understanding the energetics of metabolic processes, the biogeochemical behavior of microorganisms, and mass and energy cycles. The purpose of this paper is to relate the distribution of the coupling reactions between C, N, Fe, and S, the most important elements involved in microbially mediated redox reactions, with their thermodynamic feasibility to provide theoretical foundation of their occurrence. Anaerobic microorganisms catalyze diverse redox reactions in anoxic environments, driving elemental biogeochemical cycles on the earth. They capture energy from catalyzing these redox reactions in order to support life. The thermodynamic feasibility of these microbe-driven redox reactions is controlled by their energy yields which depend on environmental conditions. Anaerobic microorganisms can oxidize organic carbon with diverse inorganic compounds including nitrate/nitrite, ferric iron, and sulfate as electron acceptors in various anoxic environments which is referred to anaerobic respiration of organic matter; reversely, inorganic carbon can be reduced to synthesize cell material with ferrous iron and sulfide as an alternative electron donor by phototrophs under different sets of circumstances. Nitrate/nitrate can be microbically reduced by inorganic compounds such as ferrous iron and sulfide under some specific situations; the coupling of anaerobic anammox oxidation and reduction of nitrite (anammox), ferric iron (feammox), and sulfate (suramox) driven by anaerobes occurs in other particular systems. Although there are increasing researches investigating the anaerobe-driven coupling of pairs of elements such as C-N, C-Fe, C-S, N-Fe, N-S, and Fe-S, much more intricate situations associating the coupling of multiple elements are still not comprehensively understood. A great many reactions which are thermodynamically feasible have not yet been identified in natural environments or laboratories. Further work focusing on the metabolic pathways from a genetic and enzymatic perspective and the factors controlling the feasibility of the reactions by using updated technical tools and methods is required.