7.6: Anaerobic and aerobic metabolic pathways
Microbial metabolism in anaerobic sediments, such as waterlogged wetlands, plays a crucial role in regulating several essential biogeochemical cycles, i.e., carbon, nitrogen, phosphorus, and sulfur (Inglett et al., 2005). Flooded soils support intense heterotrophic activity and many transformations are directly mediated by aerobic and anaerobic microorganisms (D’Angelo & Reddy, 1999). The reduction of submerged soil and sediment is a consequence of the anaerobic respiration of bacteria (Reddy & DeLaune, 2008). During anaerobic respiration, organic matter is oxidized, and the components of the medium are reduced (Boon, 2006; Madsen, 2015; Reddy & DeLaune, 2008; Hanson et al., 2013). Examples include: (i) denitrification (Equation \(\PageIndex{1}\)); (ii) the (dissimilatory) reduction of nitrate and nitrite to ammonium (Equations \(\PageIndex{2}\) and \(\PageIndex{3}\)) (iii) the reductions of manganese dioxide (Equations \(\PageIndex{4}\) and \(\PageIndex{5}\)); (iv) the reduction of ferric hydroxide (Equations \(\PageIndex{6}\) and \(\PageIndex{7}\); (v) sulfate reduction (Equations \(\PageIndex{8}\) to \(\PageIndex{10}\)) and (vi) carbon dioxide reduction (Equation \(\PageIndex{11}\) ):
\[\begin{array}{c}\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{NO}_3^{-} \rightarrow 2 \mathrm{~N}_2+2 \mathrm{NO}_2^{-}+4 \mathrm{CO}_2+2 \mathrm{CO}_3{ }^{2-} \\ +6 \mathrm{H}_2 \mathrm{O}+\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-590 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}4 \mathrm{CH}_2 \mathrm{O}_2+\mathrm{NO}_3{ }^{-}+2 \mathrm{H}^{+} \rightarrow 4 \mathrm{CO}_2+\mathrm{NH}_4{ }^{+}+3 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-226 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}3 \mathrm{CH}_2 \mathrm{O}_2+\mathrm{NO}_2^{-}+2 \mathrm{H}^{+} \rightarrow 3 \mathrm{CO}_2+\mathrm{NH}_4{ }^{+}+2 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-166 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{aligned} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+ & 12 \mathrm{MnO}_2+24 \mathrm{H}^{+} \rightarrow 6 \mathrm{CO}_2+12 \mathrm{Mn}^{+}+18 \mathrm{H}_2 \mathrm{O} \\ + & \operatorname{Energy}\left(\Delta G_0{ }_0=-484 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{aligned}\]
\[\begin{array}{c}\mathrm{CH}_3 \mathrm{COO}^{-}+4 \mathrm{MnO}_2+9 \mathrm{H}^{+} \rightarrow 2 \mathrm{CO}_2+4 \mathrm{Mn}^{+}+6 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-137 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+24 \mathrm{Fe}(\mathrm{OH})_3+48 \mathrm{H}^{+} \rightarrow 6 \mathrm{CO}_2+24 \mathrm{Fe}^{2+}+66 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta G_0^{\prime}=-40 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}\mathrm{CH}_3 \mathrm{COO}^{-}+ \\ 8 \mathrm{Fe}(\mathrm{OH})_3+17 \mathrm{H}^{+} \rightarrow 2 \mathrm{CO}_2+8 \mathrm{Fe}^{2+}+22 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta G_0^{\prime}=-11 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}2\left(\mathrm{CH}_2 \mathrm{O}\right)+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow 2 \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}+\mathrm{H}_2 \mathrm{~S} \\ + \text { Energy }\left(\Delta G_0{ }_0=-21 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}\mathrm{CH}_4+\mathrm{SO}_4{ }^{2-} \rightarrow \mathrm{HCO}_3{ }^{-}+\mathrm{HS}^{-}+\mathrm{H}_2 \mathrm{O} \\ + \text { Energy }\left(\Delta G_0{ }_0=-20 \text { to }-40 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}\mathrm{CH}_3 \mathrm{COO}^{-}+\mathrm{SO}_4{ }^{2-}+3 \mathrm{H}^{+} \rightarrow 2 \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}+\mathrm{H}_2 \mathrm{~S} \\ +\operatorname{Energy}\left(\Delta G_0^{\prime}=-13.7 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}2 \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}+\mathrm{CO}_2+17 \mathrm{H}^{+} \rightarrow \mathrm{CH}_4+2 \mathrm{CH}_3 \mathrm{COO}^{-}+2 \mathrm{H}^{+} \\ + \text {Energy }\left(\Delta \mathrm{G}_0{ }_0=-24 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
In addition to anaerobic respiration, chemosynthesis also occurs; instead of organic matter, bacteria oxidize reduced inorganic compounds, such as ammonium (Equation \(\PageIndex{12}\) ; Madsen, 2015), sulfides, elemental sulfur (Equation (\PageIndex{12}\)) and other reduced sulfur compounds, such as thiosulphate (Wetzel, 2001).
\[\begin{array}{c}\mathrm{NH}_4{ }^{+}+\mathrm{NO}_2^{-} \rightarrow \mathrm{N}_2+2 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-86.2 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{c}5 \mathrm{~S}^{\mathrm{o}}+6 \mathrm{NO}_3{ }^{-}+2 \mathrm{CO}_3{ }^{2-} \rightarrow 5 \mathrm{SO}_4{ }^{2-}+2 \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}+3 \mathrm{~N}_2 \\ +\operatorname{Energy}\left(\Delta G_0^{\prime}=-179 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
In anaerobic environments, such as water-saturated wetlands soils, fermentations occur when the electron acceptors are organic compounds (Schlegel, 1997). In this case, the oxidation may be incomplete, generating intermediate organic products (such as lactic acid and ethanol; Equations \(\PageIndex{14}\) and \(\PageIndex{15}\), respectively; Conn et al., 1987). Substrate catabolism may be incomplete (Denef et al., 2009), and the products (organic or inorganic) enter the anabolic routes of the decomposers and, consequently, are resynthesized and incorporated by these organisms (Fenchel, 2008). Other products are incorporated and/or converted into the class of non-cellular organic compounds, such as humic substances (Assunção et al., 2017).
\[\begin{array}{c}\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6 \rightarrow 2 \mathrm{CH}_3 \mathrm{CHOHCOOH}+2 \mathrm{H}_2 \mathrm{O} \\ + \text { Energy }\left(\Delta G_0^{\prime}=-32.4 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
\[\begin{array}{l}\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6 \rightarrow 2 \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}+2 \mathrm{CO}_2 \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-25.4 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
The oxidation of organic matter (e.g., glucose, palmitic acid, and lactic acid; Equations \(\PageIndex{16}\), \(\PageIndex{17}\) and \(\PageIndex{18}\), respectively) is interrupted by soil submersion, and the aerobic microorganisms consume the oxygen in the soil and become quiescent or die. Facultative and obligate anaerobes then proliferate, using carbon compounds and oxidized components of the soil as substrate and dissimilation products of organic matter as electron acceptors in respiration (Sahrawat, 2004).
\begin{array}{l}\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 \rightarrow 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-686 \mathrm{kcal} \mathrm{mol}^{-1}\right) \end{array}
\begin{array}\mathrm{C}_{16} \mathrm{H}_{32} \mathrm{O}_2+23 \mathrm{O}_2 \rightarrow 16 \mathrm{CO}_2+16 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0=-2338 \mathrm{kcal} \mathrm{mol}^{-1}\right) \end{array}
\begin{array}\mathrm{CH}_3 \mathrm{CHOHCOOH}+3 \mathrm{O}_2 \rightarrow 3 \mathrm{CO}_2+3 \mathrm{H}_2 \mathrm{O} \\ \quad+\text { Energy }\left(\Delta \mathrm{G}_0=-319.5 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}
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Excerpted from
Cunha-Santino, M. B. D., & Bianchini Júnior, I. (2023). Reviewing the organic matter processing by wetlands. Acta Limnologica Brasiliensia , 35 , e19. Accessed December 2023 from https://www.scielo.br/j/alb/a/ypwb635W6PGrxXSZWPtgt4S CC-BY