7.5: Microbial metabolism in saturated sediments
The requirements for organic matter reduction soil are: (i) the absence of oxygen; (ii) the presence of organic matter, and (iii) anaerobic bacterial activity (Sigee, 2005). Metabolic routes, reaction rates and the degree of organic matter reduction are influenced by the nature and content of organic matter, temperature, nature and content of electron acceptors, and pH (Wilson et al., 2011; Szafranek-Nakonieczna & Stepniewska, 2014; Boye et al., 2018; Li et al., 2021b). Growth rates (i.e., anabolism) are not invariant properties of organisms; they are markedly influenced by the prevailing environmental conditions, particularly by the complexity of the medium (substrate) and by the nature of the primary source of carbon and energy (Mandelstam et al., 1982).
Usually, heterotrophic bacteria grow faster in a medium containing nutrients and complex organic compounds (e.g., amino acids, purines, pyrimidines, and vitamins). Facultative anaerobic bacteria growth in the presence of oxygen is generally faster than in its absence. Presumably, this reflects the greater efficiency with which organisms can generate ATP in an aerobic environment; comparisons of the energy yields of aerobic oxidations (e.g., Conn et al., 1987) with those of anaerobic respirations and fermentations illustrate these processes. Substances such as nitrate, whose assimilation requires extra energy demands on organisms, generally support growth at a lower rate than those such as ammonia, whose assimilation requires less energy input (Mandelstam et al., 1982).
Microorganisms can use a wide variety of organic compounds as a source of energy. The catabolism of these substances leads to the production of intermediate compounds that act as raw material suppliers for biosynthetic reactions and energy production (Williams & del Giogio, 2005). The CO 2 molecules produced in the decomposition (anaerobic: or aerobic decay: ; Figure 2) associate with water molecules and form carbonic acid; carbonic acid dissociations produce bicarbonates and carbonates; such dissociations generate H+ (Equation \(\PageIndex{1}\)), thus acidify the medium (Langmuir, 1997).
In water-saturated soils, typical of wetlands, the degree of acidity partially depends on the amount of decomposed organic material and the degradation velocity. The degree of acidity is mainly proportional to the production of CO 2 . Other processes also contribute to the increase in the acidity are: (i) leaching (dissolutions) of detritus and subsequent dissociation of organic acids; (ii) nitrification (Equation \(\PageIndex{2}\)); (iii) oxidations of sulfur compounds (Equations \(\PageIndex{3}\) to \(\PageIndex{5}\)) and (iv) cation hydrolysis (Mihelcic, 1999; Cunha-Santino & Bianchini Júnior, 2002; Konhauser, 2007).
Increases in water electrical conductivity values due to soil submersion are related to releasing ions and compounds from leaching and mineralization processes. Changes in the color of water result from the formation and dissolution of inorganic colloids (e.g., iron and sulfur salts; Ciminelli et al., 2014; Li et al., 2021a; Equations \(\PageIndex{6}\) and \(\PageIndex{7}\)), the release of pigments from plants detritus (Killops & Killops, 2013) and the formation of organic colloids (e.g., humic substances; Cunha-Santino et al., 2013).
\begin{array}{l}\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}^{+} \\ \quad+\mathrm{HCO}_3{ }^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}^{+}+\mathrm{CO}_3{ }^2 \end{array}
\begin{array}{l} \mathrm{NH}_4{ }^{+}+2 \mathrm{O}_2 \rightarrow \mathrm{NO}_3{ }^{-}+\mathrm{H}_2 \mathrm{O}+2 \mathrm{H}^{+} \end{array}
\begin{array}{l} \mathrm{H}_2 \mathrm{~S}+2 \mathrm{O}_2 \rightarrow \mathrm{SO}_4{ }^{2-}+2 \mathrm{H}^{+} \end{array}
\begin{array}{l} \mathrm{S}^0+\mathrm{H}_2 \mathrm{O}+1 \frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{SO}_4{ }^{2-}+2 \mathrm{H}^{+} \end{array}
\begin{array}{l} \mathrm{S}_2 \mathrm{O}_3{ }^{2-}+\mathrm{H}_2 \mathrm{O}+2 \mathrm{O}_2 \rightarrow 2 \mathrm{SO}_4{ }^{2-}+2 \mathrm{H}^{+} \end{array}
\begin{array}{l} \mathrm{CH}_2 \mathrm{O}+8 \mathrm{H}^{+}+4 \mathrm{FeOOH} \rightarrow \mathrm{CO}_2+4 \mathrm{Fe}^{2+}+7 \mathrm{H}_2 \mathrm{O} \end{array}
\begin{array}{l} 3 \mathrm{H}_2 \mathrm{~S}+2 \mathrm{FeOOH} \rightarrow 2 \mathrm{FeS}+\mathrm{S}^0+4 \mathrm{H}_2 \mathrm{O}\end{array}
In anaerobic environments (e.g., sediments and saturated soils), the rates of detritus mineralization (which are derived mainly from the growth rates of microorganisms) usually are lower than those observed under aerobic conditions (Wetzel, 2001; Kirk, 2004; Mitsch & Gosselink, 2015). The tendency is for a greater stock of organic matter to occur in sediments, submerged soils, and hydrosols. The accumulation of plant residues in soils and sediments generally results from a long time of degradation of plant resources compared to the rates of input of allochthonous detritus and those from autochthonous primary production, i.e., chemosynthesis, oxygenic (Equation \(\PageIndex{8}\)) or anoxygenic (Equations \(\PageIndex{9}\) to \(\PageIndex{10}\)) (Conn et al., 1987). The lack of oxygen has drastic consequences for the decomposition of organic material. Obligate aerobic decomposers are limited to the more aerated patches of sediment. Fungi and bacteria that can break down refractory organic molecules mainly belong to this group. Although the half-life of such substances increases strongly in anaerobic soils, the more easily degradable organic matter will be transformed by anaerobes (facultative or obligate). These organisms use other electron acceptors to replace oxygen, which leads to interactions of the carbon cycle with those of nitrogen, manganese, iron, and sulfur (Verhoeven, 2009), as demonstrated by Equations 1 to 10.
\begin{array}{l}6 \mathrm{CO}_2+12 \mathrm{H}_2 \mathrm{O}+\operatorname{Energy}\left(\Delta G_0{ }_0=+686 \mathrm{kcal} \mathrm{mol}^{-1}\right) \\ \rightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O}+6 \mathrm{O}_2 \end{array}
\begin{array}\mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{~S}+\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=+686 \mathrm{kcal} \mathrm{mol}^{-1}\right) \\ \rightarrow \mathrm{CH}_2 \mathrm{O}+2 \mathrm{~S}^{\mathrm{o}}+\mathrm{H}_2 \mathrm{O} \end{array}
\begin{array} 2 \mathrm{CO}_2+\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3+5 \mathrm{H}_2 \mathrm{O}+\operatorname{Energy}\left(\Delta G_0^{\prime}=+686 \mathrm{kcal} \mathrm{mol}^{-1}\right) \\ \rightarrow \mathrm{CH}_2 \mathrm{O}+2 \mathrm{H}_2 \mathrm{O}+2 \mathrm{NaHSO}_4 \end{array}
\begin{array} \mathrm{CO}_2+2 \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}+\operatorname{Energy}\left(\Delta G_0{ }_0=+686 \mathrm{kcal} \mathrm{mol}^{-1}\right) \\ \rightarrow \mathrm{CH}_2 \mathrm{O}+2 \mathrm{CH}_3 \mathrm{CHO}+\mathrm{H}_2 \mathrm{O} \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