Chemoorganotrophy

Where O2 is present, aerobic respiration mediates the oxidation of an amazingly wide range of organic compounds, from the carbohydrate, lipid, and protein components of eukaryotic and prokaryotic cells, to complex polymeric components (e.g., cellulose and lignin) of plant tissues, to the straight-chain, branched-chain, and aromatic ring structures of hydrocarbons. In all cases, the activity of one or more aerobic organisms leads to direct oxidation ofthe organic compound to CO2 and H2O, with little or no accumulation of extracellular intermediates (Figure 7, red text and arrows). In addition, many aerobic microorganisms, when faced with a temporary shortage of O2, can substitute nitrate (NOf) in place of O2 at the end of their electron-transport chain. This process is known as denitrification (see Denitrification), and results in the production of dinitrogen gas (N2).

Virtually all other forms of chemoorganotrophic anaerobic respiration take place by a fundamentally different process, whereby fermentative microorganisms convert complex organic compounds to intermediates such as two-to four-carbon organic acids (e.g., acetic, propionic, and butryric acid) and dihydrogen gas (H2) (see Fermentation). These intermediates then serve as the fuel for additional anaerobic respiratory organisms (Figure 7, blue text and arrows). The reason for the fundamental difference in the pathway for organic matter decay in aerobic (oxidation to

Figure 7 Conceptual diagram of organic matter oxidation through aerobic (red) versus anaerobic (blue) respiratory pathways. Virtually all forms of complex organic matter can be oxidized directly back to CO2 by aerobic microorganisms. When oxygen (O2) is temporarily depleted (see The Significance of O2 for Biology), many aerobic microorganisms can utilize nitrate (NO3) as an alternative electron acceptor in a process called denitrification (see Denitrification). Under long-term anaerobic conditions, a characteristic assemblage of fermentative (see Fermentation) and anaerobic respiratory organisms, referred to collectively as the 'anaerobic microbial food chain' (indicated by the dashed polygon), is responsible for organic matter decomposition. Nonrespiratory 'electron dumping' refers to the transfer of small amounts of electron equivalents to certain electron acceptors for the purpose of NADH oxidation during fermentation, as opposed to energy generation coupled to respiration. Typical organic acids produced during fermentation include acetic (two carbons), propionic (three carbons), and butyric (four carbons) acid. Anaerobic respiration refers to a process whereby organisms gain energy from electron-transport-driven ATP production coupled to reduction of electron acceptors such as NO3, manganese and iron oxides (MnO2, FeOOH), sulfate (SO23), and CO2. These processes result in the production of reduced inorganic compounds (NH4, Mn2+, Fe2+, H2S) and methane (CH4), which may in turn serve as energy sources for chemolithotrophic respiration (see Figure 5 and text).

Figure 7 Conceptual diagram of organic matter oxidation through aerobic (red) versus anaerobic (blue) respiratory pathways. Virtually all forms of complex organic matter can be oxidized directly back to CO2 by aerobic microorganisms. When oxygen (O2) is temporarily depleted (see The Significance of O2 for Biology), many aerobic microorganisms can utilize nitrate (NO3) as an alternative electron acceptor in a process called denitrification (see Denitrification). Under long-term anaerobic conditions, a characteristic assemblage of fermentative (see Fermentation) and anaerobic respiratory organisms, referred to collectively as the 'anaerobic microbial food chain' (indicated by the dashed polygon), is responsible for organic matter decomposition. Nonrespiratory 'electron dumping' refers to the transfer of small amounts of electron equivalents to certain electron acceptors for the purpose of NADH oxidation during fermentation, as opposed to energy generation coupled to respiration. Typical organic acids produced during fermentation include acetic (two carbons), propionic (three carbons), and butyric (four carbons) acid. Anaerobic respiration refers to a process whereby organisms gain energy from electron-transport-driven ATP production coupled to reduction of electron acceptors such as NO3, manganese and iron oxides (MnO2, FeOOH), sulfate (SO23), and CO2. These processes result in the production of reduced inorganic compounds (NH4, Mn2+, Fe2+, H2S) and methane (CH4), which may in turn serve as energy sources for chemolithotrophic respiration (see Figure 5 and text).

CO2 by a single organism) versus anaerobic environments (complete oxidation requiring passage through multiple species) is not well understood.

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