Abstract
Microorganisms are found in nearly every surface and near-surface environment, where they gain energy by catalyzing reactions among a wide variety of chemical compounds. The discovery of new catabolic strategies and microbial habitats can therefore be guided by determining which redox reactions can supply energy under environmentally-relevant conditions. In this study, we have explored the thermodynamic potential of redox reactions involving manganese, one of the most abundant transition metals in the Earth’s crust. In particular, we have assessed the Gibbs energies of comproportionation and disproportionation reactions involving Mn2+ and several Mn-bearing oxide and oxyhydroxide minerals containing Mn in the +II, +III, and +IV oxidation states as a function of temperature (0–100°C) and pH (1–13). In addition, we also calculated the energetic potential of Mn2+ oxidation coupled to O2, NO2–, NO3–, and FeOOH. Results show that these reactions—none of which, except O2 + Mn2+, are known catabolisms—can provide energy to microorganisms, particularly at higher pH values and temperatures. Comproportionation between Mn2+ and pyrolusite, for example, can yield 10 s of kJ (mol Mn)–1. Disproportionation of Mn3+ can yield more than 100 kJ (mol Mn)–1 at conditions relevant to natural settings such as sediments, ferromanganese nodules and crusts, bioreactors and suboxic portions of the water column. Of the Mn2+ oxidation reactions, the one with nitrite as the electron acceptor is most energy yielding under most combinations of pH and temperature. We posit that several Mn redox reactions represent heretofore unknown microbial metabolisms.
Highlights
Identifying the catabolic reactions that microorganisms catalyze in nature is critical to understanding the flows of energy and matter in ecosystems
Similar types of Gibbs energy calculations have been used to predict the existence of novel catabolic strategies that were later found in natural systems and built environments, such as anaerobic ammonia oxidation (Broda, 1977; van de Graaf et al, 1995; Kuypers et al, 2003), the anaerobic oxidation of methane (AOM) (Barnes and Goldberg, 1976; Hinrichs et al, 1999; Boetius et al, 2000; Orphan et al, 2001) and complete ammonia oxidation (Costa et al, 2006; Daims et al, 2015; van Kessel et al, 2015)
We have explored the energetic potential of a variety of undiscovered manganese-based microbial metabolisms including comproportionation, disproportionation, and oxidation by several electron acceptors including O2, NO2−, NO3−, and FeOOH, summarized schematically in Figure 1, as a function of temperature and pH
Summary
Identifying the catabolic reactions that microorganisms catalyze in nature is critical to understanding the flows of energy and matter in ecosystems. Similar types of Gibbs energy calculations have been used to predict the existence of novel catabolic strategies that were later found in natural systems and built environments, such as anaerobic ammonia oxidation (anammox) (Broda, 1977; van de Graaf et al, 1995; Kuypers et al, 2003), the anaerobic oxidation of methane (AOM) (Barnes and Goldberg, 1976; Hinrichs et al, 1999; Boetius et al, 2000; Orphan et al, 2001) and complete ammonia oxidation (comammox) (Costa et al, 2006; Daims et al, 2015; van Kessel et al, 2015) Motivated by these successful thermodynamic prognostications, sulfur comproportionation, a heretofore undiscovered catabolic pathway, has recently been predicted to exist in ecosystems with acidic pH over a broad range of temperatures (Amend et al, 2020). Nodules from the NE Equatorial Pacific were revealed to have connected pore space and molecular data showed that the microbial community was dominated by nodule-specific Mn(IV)-reducing and Mn(II)oxidizing bacteria that were not found in the surrounding environment (Blöthe et al, 2015)
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