Читать книгу Wetland Carbon and Environmental Management - Группа авторов - Страница 54

Where O₂ is depleted, a suite of anaerobic pathways can be used by microbes to mineralize organic carbon to CO₂ and/or CH₄. Thermodynamics dictates that the energy yield from the use of alternate electron acceptors proceeds in the order NO₃^‐ (denitrification), Mn(III, IV) (manganese reduction), Fe(III) (iron reduction), humic acids (humic acid reduction), SO₄^2– (sulfate reduction), and CO₂ (methanogenesis). In an idealized wetland soil, aerobic respiration would occur in surface soils to the depth where O₂ becomes depleted, whereupon denitrification would occur in deeper soils where NO₃^– was available, followed next by the reduction of Mn oxides, and so on, following the thermodynamic order presented above. The same sequence of processes would be expected as one moves laterally away from a wetland plant root. In reality, multiple respiratory pathways can coexist within the same volume of soil due to microscale variations in the availability of electron acceptors and electron donors (Angle et al., 2017; Oremland et al., 1982).

The availability of electron acceptors is important in determining which metabolic pathways are most important in any given wetland. Regardless of the thermodynamics at standard conditions, a reaction will not proceed at appreciable rates if its electron acceptor is present at low concentrations. Thus, low concentrations of NO₃^– mean that denitrification often accounts for ≤1% of anaerobic carbon mineralization in wetland soils (e.g., Keller & Bridgham, 2007; Kristensen et al., 2011; Tobias & Neubauer, 2019). Similarly, differences in the abundance of Fe(III) explain why rates of Fe(III) reduction are trivial in peatland soils (Keller & Bridgham, 2007) but can account for the majority of anaerobic carbon turnover in soils with more mineral matter (Kostka et al., 2002; Neubauer, Givler et al., 2005; Roden & Wetzel, 1996; Yao et al., 1999). Likewise, SO₄^2– limitation causes methanogenesis to be more important than SO₄^2– reduction in freshwater wetlands (Neubauer, Givler et al., 2005; Weston et al., 2014), with the relative importance of these processes switching in brackish and saline wetlands as SO₄^2– availability increases (Poffenbarger et al., 2011).

The resupply and regeneration of electron acceptors is necessary to maintain rates of soil metabolism. Soluble electron acceptors such as NO₃^– and SO₄^2– can diffuse into the anaerobic zone or be resupplied by the advective movement of water through the soil. In contrast, solid‐phase electron acceptors such as Mn(III, IV) oxides and Fe(III) (oxyhydr)oxides are effectively regenerated in situ at aerobic–anaerobic interfaces, including the rhizosphere and walls of infaunal burrows (Gribsholt et al., 2003; Luo et al., 2018; Neubauer, Givler et al., 2005) or where moving water delivers O₂ to subsurface soils (Roychoudhury et al., 2003). Lastly, electron acceptors including Fe(III) and SO₄^2– can be regenerated in wetlands through anaerobic chemoautotrophic reactions where the oxidation of Fe²⁺ is coupled with the reduction of NO₃^– (Straub et al., 2001) and the oxidation of reduced sulfur compounds proceeds with NO₃^–, MnO₂, or Fe(III) serving as the oxidant (Schippers & Jørgensen, 2002). The contribution of these chemoautotrophic reactions to anaerobic carbon cycling is largely unknown (Burgin & Hamilton, 2008; Carey & Taillefert, 2005; but see Schippers & Jørgensen, 2002).

The supply of electron donors is as important as the resupply/regeneration of electron acceptors in regulating anaerobic metabolism. The energetic potential of an electron donor (that is, its ability to give up electrons to an electron acceptor) can be summarized in thermodynamic concepts such as the nominal oxidation state of carbon (NOSC: LaRowe & Van Cappellen, 2011) and the oxidation state of organic carbon (C_ox; Masiello et al., 2008). For uncharged molecules, the difference between NOSC and C_ox values is negligible (Hockaday et al., 2009) and we treat these terms as synonymous. Thermodynamic calculations and experimental culture work indicate that aerobic microbes can use a wide range of organic carbon molecules as electron donors, but anaerobic decomposers can use fewer substrates due to thermodynamic limitations (Keiluweit et al., 2016; LaRowe & Van Cappellen, 2011). As a group, NO₃^– and metal reducers can use many organic molecules as electron donors, including amino acids, short‐ and long‐chain fatty acids, some aromatic compounds, the monomers (e.g., glucose) resulting from extracellular enzymatic hydrolysis of polymers, and products of fermentation such as H₂, acetate, lactate, and pyruvate (Küsel et al., 1999; Megonigal et al., 2004; Reddy & DeLaune, 2008). Sulfate reducers are able to use many of the same electron donors (Christensen, 1984; Parkes et al., 1989; Sørensen et al., 1981), but some cannot use glucose and other monomers and thus are largely dependent on the activities of fermenters for electron donors (Reddy & DeLaune, 2008). The denitrifiers, metal reducers, and SO₄^2– reducers can oxidize electron donors all the way to CO₂ (or to H₂O, when H₂ is the electron donor) or they can ferment larger molecules to acetate (Megonigal et al., 2004; Reddy & DeLaune, 2008). The thermodynamics of using CO₂ as an electron acceptor means that methanogens can use the smallest number of electron donors. Hydrogenotrophic methanogens use H₂ (and sometimes formate) as the electron donor while acetoclastic methanogens use acetate as both electron acceptor and electron donor, with some also able to use methanol, methylated amines, and methylated sulfur compounds (Bridgham et al., 2013).